Bone and Soft-Tissue Tumors

By admin Last updated Jul 8, 2023

14 – Bone and Soft-Tissue Tumors

Chapter 14

Miriam A. Bredella

David W. Stoller

James O. Johnston

The superior contrast discrimination provided by magnetic resonance (MR) imaging has proved extremely valuable in the assessment of bone and soft-tissue tumors.¹^,²^,³^,⁴^,⁵^,⁶^,⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵^,¹⁶^,¹⁷ T1 and T2 relaxation times are prolonged in a variety of malignant tissue types; therefore, malignant change tends to demonstrate low to intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Although computed tomography (CT) may provide superior cortical detail, MR imaging is much more sensitive to marrow involvement by edema or tumor and provides soft-tissue definition of the surrounding musculature, fascial planes, and neurovascular bundles without artifact from cortical bone.

Imaging Techniques

Conventional radiographs remain the initial imaging examination in evaluating bone and some soft-tissue tumors and are almost invariably the most diagnostic. Although other imaging modalities such as MR imaging are superior to radiographs in staging musculoskeletal neoplasms, radiographs remain the best modality for establishing a diagnosis and assessing the activity (benign or aggressive) of a lesion.¹⁸^,¹⁹

For MR imaging, a combination of both T1 and T2 weighting, fat-suppressed T2-weighted or proton density-weighted fast spin-echo, or gradient-echo techniques is essential in the evaluation of musculoskeletal neoplasms. With short TI inversion recovery (STIR) imaging sequences, T1 and T2 contrast are additive.²⁰^,²¹

FIGURE 14-1 ● T1-weighted images (A) before and (B) after intravenous gadolinium administration show malignant fibrous histiocytoma. The tumor enhances after intravenous injection of gadolinium. Central tumor necrosis remains unenhanced (arrow).

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The MR protocols most useful in tumor imaging include the following:

T1-weighted images provide excellent contrast for identification of marrow, cortical, and soft-tissue involvement. In particular, T1-weighted images allow differentiation between fat and tumor, as well as definition of muscle planes and separate anatomic compartments.
Gadolinium has been used to enhance contrast on T1-weighted images to better characterize osseous and soft-tissue tumor involvement.²^,⁷^,¹⁵^,²²^,²³^,²⁴ On gadolinium-enhanced images, nonenhancing regions are thought to represent areas of nonviable tumor or necrosis (Fig. 14-1). Gadolinium-enhanced images may also be useful for differentiating peritumoral edema from underlying tumor and recurrent tumor from scar or fibrosis.²⁵
Dynamic contrast-enhanced MR imaging has been used to determine response to chemotherapy. In patients who respond to chemotherapy, MR images show a reduction in enhancement, whereas patients who do not respond show little or no reduction. Dynamic contrast-enhanced MR images should be acquired in short time intervals, since reactive changes may show contrast enhancement indistinguishable from that of tumor in the later phases of enhancement.²³^,²⁶^,²⁷ Subtraction MR imaging has been used to increase the detection of early arterial enhancement of residual tumor.²⁷
T2-weighted images are valuable in distinguishing muscle from tumor and can increase diagnostic specificity in evaluating marrow infiltration, seen as areas of low signal intensity on T1-weighted images.

MR imaging may not be as sensitive as CT to cortical disruption, fine periosteal reactions, and small calcifications; however, with proper imaging planes and resolution, early endosteal and cortical erosions as well as periosteal changes can be detected on MR images (Figs. 14-2, 14-3, and 14-4). Although each MR examination must be tailored to the patient

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and pathology in question, certain general guidelines can be followed:

FIGURE 14-2 ● Osteosarcoma of the distal tibia is seen on a sagittal proton density-weighted image. Cortical thinning and periosteal reaction are demonstrated (arrow).

FIGURE 14-3 ● Chondrosarcoma of the proximal femoral shaft is seen on a coronal T1-weighted image. Cortical thickening (arrow) demonstrates low signal intensity on the T1-weighted image.

The longitudinal extent of tumors can be seen on T1-weighted coronal or sagittal images.
If a lesion is difficult to differentiate from (i.e., is isointense with) adjacent tissues, a conventional T2, fat-suppressed T2-weighted fast spin-echo, fat-suppressed proton density-weighted fast spin-echo, or STIR sequence should be obtained.
T1- and T2-weighted images (including fat-suppressed T2-weighted fast spin-echo or STIR sequences) in the axial plane provide the most important images in delineating the relationship of a tumor to adjacent neurovascular structures and compartments—essential information in preoperative limb-salvage planning.
The proximal and distal extent of tumor involvement can be assessed by evaluating multiple axial sections.
The region of abnormality should be positioned as close to the center of the coil as possible.
The patient—s position (prone or supine) is determined by the area of abnormality.
Prior to imaging the region of interest, a large field-of-view localizer using an increased-diameter surface coil or body coil may be necessary to include skin lesions or to accurately determine the proximal and distal extension of a large mass.
MR spectroscopy using phosphorus-31 or protons has been used to improve lesion characterization and to evaluate response to therapy.²⁸^,²⁹ It is not, however, routinely used in most clinical settings. This technique is limited by the difficulty of obtaining representative spectra in all locations of the tumor and by contamination of tumor spectra with adjacent soft tissues.

FIGURE 14-4 ● Sagittal fat-suppressed T2-weighted fast spin-echo image demonstrates chondrosarcoma of the humeral shaft. Histologically confirmed hyperintense subcortical tumor infiltration (arrow) is identified.

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Staging

Pearls and Pitfalls

Staging

MR imaging is the method of choice in staging musculo-skeletal neoplasms.
MR images should be read in conjunction with radiographs.
MR imaging is accurate in determining the local extent of tumor and involvement of muscle compartments, joints, and neurovascular bundles.
Evaluation of intra- and extraosseous extent, joint invasion, neurovascular bundle involvement, skip metastases, and local adenopathy as well as distant metastases is important for accurate staging and subsequent therapy.
Consider FDG-PET for staging and evaluation of therapy response in equivocal cases.

Accurate preoperative staging is important to determine the prognosis and to aid clinicians in choosing the appropriate therapy. The purpose of staging is to define the local and distant extent of tumor, determine the surgical margins of resection, and facilitate interdisciplinary communication regarding treatment and results. The Enneking staging system for bone and soft-tissue tumors, the most widely used, is based on tumor grade, site, and the presence of metastases using histologic, radiologic, and clinical criteria (Table 14-1).³⁰^,³¹. In the Enneking system benign lesions are considered grade 0 (G0) and malignant lesions are either low grade (G1) or high grade (G2). For the assessment of the site and extent of tumors, T0 is reserved for benign tumors confined within a true capsule and the anatomic compartment of origin. An aggressive benign or malignant tumor confined within its anatomic compartment is T1. T2 indicates spread of the lesion beyond the anatomic compartment of origin. For the assessment of metastatic disease, M0 indicates no metastatic disease and M1 indicates the presence of regional or distant metastases. The Enneking staging system divides benign tumors into latent, active, or aggressive tumors.³⁰^,³¹

MR imaging has become the modality of choice for staging musculoskeletal neoplasms and assessing local spread of tumor. MRI can accurately detect tumor involvement of neurovascular structures, muscle compartments, and joints.

Recently, positron emission tomography (PET) has been found to be useful in following patients with bone and soft-tissue sarcomas after therapy.³² PET is an imaging technique that uses radiopharmaceuticals, typically a radionuclide-labeled analog of glucose, such as 2-F18-fluoro-2-deoxy-d-glucose (FDG), to detect abnormal metabolic activity. Because malignant tumors usually have increased cellular (and thus increased glucose) metabolism, PET is able to provide unique information, complementary to that provided by MR imaging, about the biologic activity of musculoskeletal tumors. FDG-PET has been shown to be able to distinguish recurrent tumor from post-therapeutic changes. In addition, FDG-PET has the ability to examine the entire body for both primary malignancies and metastatic disease during a single procedure. It can also be used to guide a biopsy to the most active tissue and to evaluate patients who have undergone limb-salvage procedures (Fig. 14-5).³²^,³³^,³⁴

FIGURE 14-5 ● FDG-PET in the evaluation of patients with sarcomas. Whole-body FDG-PET scan (A) and corresponding axial image (B) in a patient with neurofibromatosis type 1 demonstrates a region of intense FDG uptake in the right buttock area (arrow), which was found to represent a malignant peripheral nerve sheath tumor arising in a neurofibroma. (C and D) Images from a whole-body FDG PET scan in a patient with osteosarcoma show intense FDG uptake in the proximal tibia (D) in the area of the tumor (arrow). Images of the chest and abdomen demonstrate no evidence of metastatic disease. A marker was placed over the left chest wall (C) (arrowhead). (E) Coronal, axial, and sagittal images of a whole-body FDG-PET scan in a patient with osteosarcoma of the right femur. The patient underwent resection of the right femur and total knee arthroplasty. MR images and CT scans were deemed inadequate for tumor evaluation because of metallic artifact. FDG-PET scan shows a cold defect in the region of the right knee arthroplasty (arrows) and no increased FDG uptake in the area of the right femur and knee.

TABLE 14.1 ● The Enneking staging system for musculoskeletal neoplasms

Benign:
1	Latent	G0	T0		M0
2	Active	G0	T0		M0
3	Aggressive	G0	T0		M0-1
Malignant:
Ia	Low grade, intracompartmental			G1	T1	M0
Ib	Low grade, extracompartmental			G1	T2	M0
IIa	High grade, intracompartmental			G2	T1	M0
IIb	High grade, extracompartmental			G2	T2	M0
IIIa	Low or high grade, intracompartmental + metastases			G1-2	T1	M1
IIIb	Low or high grade, extracompartmental + metastases			G1-2	T2	M1
G1: low grade; G2: high grade; T1: intracompartmental; T2: extracompartmental; M: metastases

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General MR Appearance

Direct multiplanar imaging facilitates improved preoperative and pretherapy evaluation of the exact extent of a lesion. In addition, since there is minimal artifact from nonferromagnetic implants with MR imaging, it is often possible to identify postoperative and post-treatment fibrous tissues not seen on CT. The following MR findings are used in tumor evaluation:

A tumor bed with postoperative scarring demonstrates low signal intensity on both T1- and T2-weighted images (including fat-suppressed T2- or proton density-weighted fast spin-echo images).
If T1- and T2-weighted images show intermediate and high signal intensity, respectively, the possibility of recurrent tumor is more likely.
Frequently it is not possible to distinguish benign from malignant lesions of bone and soft tissue on the basis of MR signal characteristics alone. Both benign and malignant lesions may demonstrate areas of low signal intensity on T1-weighted images and increased signal intensity on T2-weighted images. Malignant lesions tend to be more extensive, involving marrow, cortical bone, and soft tissues, but these criteria do not always distinguish benign from malignant lesions. No correlations have been shown matching T1 and T2 relaxation times with corresponding histopathology.³⁵
If the neurovascular bundle is involved, the lesion is likely to be malignant.
A fluid–fluid level is a nonspecific finding in bone and soft-tissue lesions. Tumors with fluid–fluid levels include fibrous dysplasia, simple and aneurysmal bone cysts, malignant fibrous histiocytoma, osteosarcoma, chondroblastoma, and osteoblastoma.³⁶
Infectious processes (e.g., osteomyelitis) and benign neoplasms have been found to have a low-signal-intensity peripheral margin in 33% of patients.³⁷ This margin is rarely seen in malignant lesions because a well-defined sclerotic interface is not present.
It is important to perform MR studies prior to biopsy to avoid postsurgical inflammation and edema, which may prolong the T2 values of uninvolved tissues.
Muscle edema is nonspecific and may be associated with trauma, infection, and vascular insults.
On T1-weighted images, high signal intensity in the surrounding musculature can be seen in atrophy with fatty infiltration or neuromuscular disorders and should not be mistaken for tumor.
Venous varicosities may scallop subcutaneous tissue and demonstrate low signal intensity on T1-weighted images. This appearance is characteristic and can be distinguished from neoplastic soft-tissue extension or tumor vascularity.
Red-to-yellow marrow conversion in middle-aged women (marrow inhomogeneity is especially common in obese women with a history of smoking) is seen in metaphyseal or diaphyseal areas of low signal intensity (i.e., red marrow) without extension into the epiphysis (Fig. 14-6). These lesions become isointense with

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adjacent marrow on heavily T2-weighted images and demonstrate increased signal intensity on STIR images. Inhomogeneity of metaphyseal red and yellow marrow may also be observed in the immature skeleton.

FIGURE 14-6 ● Coronal T1-weighted image (A) and coronal STIR image (B) demonstrate red marrow heterogeneity (arrow) in the femoral diaphysis and metaphysis without extension across the physeal scar.

Bone Tumors

Benign Lesions

Benign Osteoblastic Lesions

Osteoid Osteoma

Osteoid osteomas are small, benign, osteoblastic lesions characteristically seen in younger patients (between 7 and 25 years of age).³⁸ They are composed of osteoid and trabeculae of newly formed osseous tissue embedded in a substriated, highly vascularized osteogenic connective tissue with a surrounding peripheral zone of dense sclerotic bone. They rarely exceed 1 cm in diameter and are usually located in either the spongiosa or the cortex of the bone. Most commonly, they affect long tubular bones; the femur is the most common site of involvement (Fig. 14-7). The clinical hallmark of this lesion is pain that is most prominent at night and can be relieved with salicylates such as aspirin.³⁸^,³⁹^,⁴⁰

FIGURE 14-7 ● Osteoid osteoma of the proximal femur. Coronal graphic illustration shows tumor nidus in red with surrounding sclerosis.

FIGURE 14-8 ● (A) Anteroposterior radiograph shows focal sclerosis of the medial tibial metaphysis (arrow). T1-weighted coronal (B) and sagittal (C) images reveal the osteoid osteoma nidus (straight arrows) and thickened cortex (curved arrow). (D) Scanning lens photomicrograph shows the nidus surrounded by sclerotic reactive bone. (E) High-power photomicrograph of the nidus demonstrates young neoplastic osteoid formation in an angiofibrotic background speckled with osteoblasts and giant cells.

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The characteristic MR appearance of osteoid osteoma includes the following findings:

On T1-weighted images, the nidus of the lesion demonstrates low to intermediate signal intensity (Fig. 14-8).
On T2-weighted images, the central nidus demonstrates moderate to increased signal intensity.⁴¹^,⁴²
Intracapsular osteoid osteomas of the hip usually cause a synovial inflammatory response and are associated with joint effusions and variable reactive marrow edema.⁴³^,⁴⁴ Reactive sclerosis demonstrates low signal intensity on T1- and T2-weighted images. There may be extensive marrow edema associated with osteoid osteomas, and STIR or fat-suppressed T2-weighted fast spin-echo sequences can be used to screen for this reactive edema (Fig. 14-9).
Occasionally, increased vascularity directed toward the nidus is seen on MR images.
Thin-section CT may be necessary to identify the nidus in intracortical lesions.⁴⁵
Enhancement with gadolinium in the presence of a reactive soft-tissue mass may be mistaken for a malignant tumor or osteomyelitis.⁸

FIGURE 14-9 ● Sagittal (A) and axial (B) T1-weighted images show subcortical osteoid osteoma with a low-signal-intensity nidus (arrow) of the talar neck. (C) Corresponding sagittal fat-suppressed T2-weighted fast spin-echo image demonstrates surrounding hyperintense bone marrow edema (arrowheads). A joint effusion and synovitis is present (arrows). (D) Sagittal CT image demonstrates the osteoid osteoma nidus in the talar neck (arrow).

FIGURE 14-10 ● Axial graphic illustration shows radiofrequency ablation of an osteoid osteoma. Nidus is shown in red with surrounding cortical thickening. The radiofrequency probe is positioned within the nidus.

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There are three fundamentally different approaches to the treatment of osteoid osteomas: surgical (resection or curettage), conservative (medical), or percutaneous (percutaneous excision, radiofrequency ablation, laser photocoagulation).⁴⁶^,⁴⁷^,⁴⁸^,⁴⁹ CT-guided percutaneous radiofrequency ablation has been found to be a safe and effective treatment for osteoid osteoma.⁵⁰^,⁵¹ It is performed under general or spinal anesthesia as an outpatient procedure. The patient is placed on the CT scanner and the lesion is identified using CT guidance. In general, the shortest distance through the bone is selected for access and a drill is used to enter the cortex and the nidus. A biopsy is usually obtained at the time of treatment. After needle biopsy, a radiofrequency electrode is introduced into the nidus and the lesion is heated to 90°C for 6 minutes (Figs. 14-10 and 14-11). If the lesion is large, multiple treatments can be performed during the same session.

Osteoblastoma

Initially known as giant osteoid osteoma, osteoblastoma commonly presents in patients between 10 and 20 years of age.⁸^,⁴⁰^,⁵² Local pain is inconsistently relieved with salicylates. Osteoblastoma most frequently involves the flat bones, posterior osseous elements of the vertebrae (Fig. 14-12), and long bones (Fig. 14-13). Histologically, osteoblastomas present a more variable picture than osteoid osteomas.

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There is well-vascularized connective tissue containing osteoid and primitive woven bone rimmed by osteoblasts. In contrast to osteoid osteoma, trabecular maturation, the degree of calcification, and overall architectural patterns vary greatly in osteoblastoma.⁴⁰

FIGURE 14-11 ● (A) CT images from a radiofrequency ablation show osteoid osteoma nidus in the anterior tibia (arrowhead) with surrounding cortical thickening. (B) Radiofrequency probe is positioned within the nidus for treatment.

FIGURE 14-12 ● Osteoblastoma of the spine. Axial graphic illustration shows expansile tumor in red arising from the posterior elements.

FIGURE 14-13 ● Sagittal graphic illustration shows cortical osteoblastoma of the distal femur in red.

Characteristic MR findings include the following:

Signal intensity may be heterogeneous or homogeneous on T2-weighted images (Fig. 14-14).
Fluid–fluid levels can be seen in osteoblastomas (Fig. 14-15).⁵³
A diffuse inflammatory response involving bone and adjacent soft tissue has been reported in a case of vertebral osteoblastoma.⁵⁴ CT may be more accurate for the identification of small foci of matrix calcification (Fig. 14-16).

Local excision involves marginal or intralesional resection with curettage.⁸

FIGURE 14-14 ● Cortical osteoblastoma of the femoral diaphysis. Coronal (A) and axial (B) T1-weighted images show osteoblastoma isointense to muscle with adjacent cortical thickening (arrows). Reactive bone marrow edema is hyperintense on sagittal (C) and axial (D) fat-suppressed T2-weighted fast spin-echo images (arrowheads).

FIGURE 14-15 ● Osteoblastoma arising in the posterior elements of the L2 vertebra is demonstrated on sagittal (A) and axial (B) fat-suppressed T2-weighted fast spin-echo images. Fluid–fluid levels are present (arrows). The tumor is isointense to muscle (arrow) on the axial T1-weighted image (C) and demonstrates enhancement (arrow) following intravenous injection of gadolinium (D). Note the mass effect upon the spinal canal from the tumor.

FIGURE 14-16 ● Internal calcifications (arrow) are identified on an axial CT image in this osteoblastoma arising in the left posterior elements of the L5 vertebra.

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Benign Chondroblastic Lesions

Pearls and Pitfalls

Benign Chondroblastic Lesions

On T2-weighted and STIR images, chondroid matrix is markedly hyperintense.
Enchondromas can be confused with bone infarcts. Infarcts show central fat signal and serpiginous borders.
Syndromes associated with multiple enchondromas have an increased risk of malignant transformation.
Osteochondromas show continuity with the marrow cavity of host bone.
Osteochondromas have their own growth plate and stop growing with skeletal maturity.
MR imaging is useful in determining the thickness of the cartilage cap.
Chondroblastomas involve the epiphyses/epiphysis equivalents.

Enchondroma

Enchondromas are benign intramedullary lesions composed of circumscribed lobules of hyaline cartilage that occasionally exhibit flecks of calcification (Fig. 14-17). Tumor cellularity may be increased in children and adolescents, but this does not imply chondrosarcoma. Radiologic findings are critical for accurate diagnosis.⁵⁵ Approximately 40% to 65% of solitary enchondromas occur in the hands, and approximately 25% occur in the long tubular bones.⁴⁰^,⁵⁵ Multiple enchondromatosis (Ollier—s disease) was originally described in 1899. This rare, nonhereditary disorder is characterized by multiple asymmetric chondromatous lesions (Fig. 14-18). In childhood, these lesions predispose the bone to fracture; in adulthood, there is an increased risk of malignant transformation.⁵⁶ Multiple enchondromas with associated soft-tissue hemangiomas are seen in Maffucci—s syndrome, a nonhereditary disorder with a high predisposition for malignant transformation (Fig. 14-19).⁵⁷ Lesions that continue to grow after the cessation of normal body growth should raise the suspicion of malignant transformation.

MR imaging is useful in identifying enchondromas, which are well defined and demonstrate low signal intensity on T1-weighted images. The following findings are characteristic:

Satellite cartilaginous foci are often seen in association with enchondromas.
Chondroid elements demonstrate marked hyperintensity on T2-weighted or STIR images (Figs. 14-20 and 14-21).
In the multiple enchondromas of Ollier—s disease, the foci of cartilaginous tissue or matrix are seen as areas of low to intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images. This appearance is secondary to the long T1 and T2 relaxation times of chondroid tissue. Enchondromas in Ollier—s disease may show aggressive features such as cortical irregularities, and there is an increased incidence of malignant degeneration (Fig. 14-22).
In Maffucci—s syndrome soft-tissue hemangiomas can be seen on T2-weighted images as high-signal-intensity tubular structures, consistent with slow flow in vascular channels.

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Enchondromas are common in the tubular bones of the hands and feet (Fig. 14-23).
Enchondromas can be differentiated from bone infarcts by the following features:⁵⁸^,⁵⁹
- Lack of central fat signal intensity on T1-weighted images
- Peripheral borders that are not as serpiginous as those seen in infarcted tissue
- Central areas of increased signal intensity on T2-weighted images
In juxtacortical or periosteal chondroma, soft-tissue calcification may mimic myositis ossificans (Figs. 14-24 and 14-25). A localized increase in signal intensity on T2-weighted images may occur without underlying involvement of the adjacent bone.⁶⁰

FIGURE 14-17 ● Coronal graphic illustration of the humerus (A) and femur (B) shows a well-defined lobulated metaphyseal enchondroma in purple.

FIGURE 14-18 ● Ollier—s disease of the lower extremity. Coronal graphic illustration of the femur (A) and lower leg (B) shows deformity with multiple enchondromas.

FIGURE 14-19 ● Maffucci—s syndrome of the hand. Coronal graphic illustration shows marked deformity of the hand with multiple enchondromas in purple and hemangiomas in red.

FIGURE 14-20 ● Benign enchondroma of the distal femur (arrows) demonstrates low signal intensity on a coronal T1-weighted image (A) and high signal intensity on sagittal (B) and axial (C) fat-suppressed T2-weighted fast spin-echo images. (D) Serpiginous enhancement is demonstrated on a contrast-enhanced axial fat-suppressed T1-weighted image.

FIGURE 14-21 ● (A) Coronal T1-weighted image shows a low-signal-intensity proximal humeral enchondroma (arrow). (B) On an axial fat-suppressed T2-weighted fast spin-echo image, the chondroid matrix is hyperintense (arrow). (C) Sagittal fat-suppressed T1-weighted image after administration of intravenous gadolinium demonstrates enhancement of the lesion (arrow).

FIGURE 14-22 ● Ollier—s disease with sarcomatous degeneration. (A) An anteroposterior radiograph demonstrates deformity of the humerus with large enchondroma (white arrow). Periosteal reaction (white arrowhead) is present. Multiple enchondromas are identified within the ribs (black arrows). (B) Coronal T1-weighted image shows a large enchondroma of the proximal humerus (arrow). (C) Axial proton density-weighted images show a large enchondroma of the proximal humerus (arrow) with cortical destruction and associated soft-tissue mass (arrowheads). (D) Axial fat-suppressed T2-weighted fast spin-echo image shows hyperintense cartilage matrix. There is disruption of the cortex with associated soft-tissue mass (arrows). (E) Coronal and (F) axial fat-suppressed T1-weighted images after administration of intravenous gadolinium demonstrate heterogeneous enhancement of the lesion and cortical destruction with soft-tissue mass (arrows). Nonenhancing areas of necrosis are present within the lesion (arrowheads).

It is difficult to differentiate enchondroma from low-grade chondrosarcoma on the basis of signal intensity alone. The presence of a positive bone scan in the absence of pain cannot be used as a criterion for malignant transformation of enchondromas. Nonaggressive features include the absence of a soft-tissue mass and absence of interval growth on serial MR studies. Treatment of atypical or symptomatic enchondromas includes marginal or intralesional resection.⁸ Autologous or allograft bone grafting is often required to repair defects in large and weight-bearing bones.

FIGURE 14-23 ● Expansile enchondromas (arrows) of the third proximal phalanx and metacarpal are seen on coronal T1-weighted (A) and sagittal (B) and axial (C) fat-suppressed T2-weighted fast spin-echo images.

FIGURE 14-24 ● Periosteal (i.e., juxtacortical) chondroma located in the proximal humerus is seen on coronal proton density-weighted (A), fat-suppressed T2-weighted fast spin-echo (B), and contrast-enhanced (C) images. Central osseous tissue is low in signal intensity on the proton density and fat-suppressed T2-weighted sequences (arrows).

FIGURE 14-25 ● (A) Radiograph of the knee of a young adult male with a heavily calcified juxta-articular chondroma in the infrapatellar fat pad. (B) T1-weighted sagittal image demonstrates the low-signal character of this heavily calcified chondroid lesion just beneath the patellar ligament. (C) Gross appearance of the resected specimen, with the upper faceted surface representing its prior articulation with the femoral condyle. (D) Low-power photomicrograph of benign-appearing chondrocytes in a heavily calcified chondroid matrix.

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Osteochondroma

Osteocartilaginous exostosis (osteochondroma) is one of the most common bone tumors in children. Solitary osteochondroma is a frequent lesion, and opinions differ as to whether it represents a true neoplasm or a developmental physeal growth defect.⁵⁵ The vast majority of these lesions present in patients younger than 20 years of age. With the exception of a proliferative cartilage cap, the spongiosa and cortex in exostosis are continuous with the adjacent shaft (Fig. 14-26). Osteochondromas have their own growth plate and stop growing with skeletal maturity. The thickness of the cartilaginous cap correlates with the age of the patient.⁵⁵ In children or adolescents, the cap may be as thick as 3 cm. In adults, however, the cap is thinner, presumably secondary to wear and tear. Findings of a cartilage cap thicker than 1 cm, an irregular cartilaginous cap, renewed growth with pain in an adult, or a combination of these raises the possibility of chondrosarcomatous transformation.⁴⁰^,⁵⁵^,⁶¹^,⁶² Such malignant transformation is rare (<1%) in solitary osteochondroma but has been reported with increased frequency in cases of multiple hereditary exostoses.

Osteochondromas are metaphyseal-based tumors that usually involve the long bones (Fig. 14-27). When flat bones are involved, the lesion may present with a more cauliflower or expansile configuration. On MR imaging, the following findings are characteristic:

Benign osteochondromas are isointense to normal marrow (Figs. 14-28 and 14-29).
The intact cartilage cap demonstrates intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images.
With malignant degeneration, the signal intensity of the exostosis decreases on T1-weighted images and

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increases on T2-weighted images. Disruption of the high-signal-intensity cartilaginous cap may also be observed with aggressive tumor invasion.
Fat-suppressed T2-weighted fast spin-echo sequences are useful in defining the thickness of the cartilaginous cap (as mentioned earlier, it may be as thick as 3 cm in adolescents and <1 cm in adults). Dispersed calcifications in the cartilaginous cap, as well as increased thickness of the cap, are features of malignant transformation that can be identified on MR scans.
MR imaging is superior to CT in demonstrating inflammatory changes that accompany enlargement of the bursa exostotica covering the cartilaginous cap (Fig. 14-30). This bursa is hypointense on T1-weighted images and hyperintense on T2, fat-suppressed T2-weighted fast spin-echo, or STIR sequences. Gadolinium administration causes enhancement of the periphery of the bursa exostotica only.
Growth deformity is associated with multiple hereditary exostoses (Fig. 14-31).⁶³

FIGURE 14-26 ● Coronal graphic illustration demonstrates an osteochondroma arising from the distal femur. Note the continuity of the lesion with the marrow cavity of the host bone. Calcifications within the osteochondroma are shown in red and the cartilage cap is shown in blue.

FIGURE 14-27 ● A metaphyseal osteochondroma arising from the proximal humerus with continuity of marrow and cortex is demonstrated on axial (A) and coronal (B) CT images.

FIGURE 14-28 ● A small osteochondroma arising from the proximal fibula (arrows) is demonstrated on an anteroposterior radiograph (A), a sagittal proton density-weighted image (B), and axial T1-weighted (C) and fat-suppressed T2-weighted fast spin-echo (D) images of the knee.

FIGURE 14-29 ● A sessile osteochondroma of the posterior distal femur (arrows) can be seen on coronal T1-weighted (A), sagittal proton density-weighted (B), and sagittal (C) and axial (D) fat-suppressed T2-weighted images.

FIGURE 14-30 ● Bursa exostotica associated with a tibial osteochondroma. (A) Anteroposterior radiograph shows an osteochondroma arising from the distal tibia (arrow). (B) The osteochondroma (arrow) demonstrates continuity with the medulla cavity at the tibia. The bursa exostotica (arrowhead) is hyperintense on a proton density-weighted axial image. (C) On a fat-suppressed T2-weighted fast spin-echo image, the bursa exostotica is hyperintense (arrows). (D) Peripheral enhancement (arrows) is seen on a fat-suppressed T1-weighted axial image after administration of intravenous gadolinium.

FIGURE 14-31 ● Undertubulation of the hips and knees in a patient with multiple hereditary exostoses. Exostoses (arrows) are seen on coronal (A) and axial (B) T1-weighted images of the hip. Sagittal proton density (C) and sagittal (D) and axial (E) fat-suppressed T2-weighted fast spin-echo images of the knee also demonstrate exostoses (arrows).

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Geirnaerdt et al. have described a pattern of septal enhancement with gadolinium-enhanced MR imaging that is helpful in identifying transformation of osteochondromas to low-grade chondrosarcomas.⁷ The enhancing curvilinear septa are thought to correlate histologically with fibrovascular septa around and between lobules of paucicellular cartilage and cystic mucoid tissue. Osteochondromas without signs of malignant transformation show a peripheral enhancement of fibrovascular tissue covering the nonenhanced hyaline cartilage cap without evidence of fibrovascular septa. Despite the presence of septa in low-grade chondrosarcoma, these patterns of enhancement are not observed in high-grade chondrosarcomas. Crim and Seeger point out that these findings have not been validated in a sufficient number of patients to conclude that septal enhancement is a specific sign for low-grade chondrosarcoma.⁶⁴ In fact, fibrovascular septa are uncommon in grade 1 chondrosarcoma. Additional studies need to be performed to more precisely determine the benefit of gadolinium enhancement in the diagnosis of grade 1 chondrosarcomas.

FIGURE 14-32 ● Chondroblastoma of the proximal femur. A well-defined lobulated lesion involving the femoral epiphysis is shown in red.

Symptomatic osteochondromas can be treated with simple resection of the base and cartilaginous tissue, including the perichondrium surrounding the cap.

Chondroblastoma

Chondroblastoma is a benign epiphyseal cartilage tumor that generally arises in a long tubular bone and occurs most frequently in the second decade of life (Fig. 14-32).⁶⁵ Joint effusion may accompany periarticular bone involvement. Histologic findings include a cellular proliferation of chondroblasts with irregularly dispersed multinucleated osteoclast giant cells.⁶⁵ In 50% of cases, benign peripheral sclerosis, intralesional punctate lace-like calcifications, and chondroid are present (Fig. 14-33). Older lesions may exhibit necrosis, resorption, or reparative fibrosis with metaplastic osseous areas. These features all contribute to the variability seen in this lesion.

On MR images, benign chondroblastomas demonstrate a well-defined area of low to intermediate signal intensity on T1-weighted images and heterogeneity with increased signal

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intensity on T2-weighted images (Figs. 14-34 and 14-35). Typically, the long-bone epiphyses of the humerus, tibia, or femur are affected. MR imaging can also be used to identify eccentric locations and the associated sclerotic border.⁶⁶ Other MR findings include the following:

FIGURE 14-33 ● A chondroblastoma of the calcaneus is shown in red on a sagittal graphic illustration. Areas of calcification are shown in white and a hemorrhagic cyst is shown in red.

FIGURE 14-34 ● Extensive marrow edema (arrowheads) in reaction to an epiphyseal-based chondroblastoma of the tibia (arrows) demonstrates low signal intensity on a coronal T1-weighted image (A) and hyperintensity on a coronal STIR image (B). Axial STIR image (C) demonstrates fluid–fluid levels within the lesions (arrow). (D) A coronal CT image shows a well-defined lytic defect in the tibial plateau (arrow). (E) An axial CT image obtained during radiofrequency ablation of the tumor shows the radiofrequency probe (arrow) within the chondroblastoma.

Epiphyseal and metaphyseal marrow edema associated with periosteal reaction is a frequent finding, seen in up to 57% of cases of long-bone involvement.⁶⁷ MR imaging is sensitive to this extensive reactive marrow edema, which demonstrates decreased signal intensity on T1-weighted images and increased signal intensity on T2-weighted or STIR images.
Fluid–fluid levels can be seen in chondroblastomas.⁵³
T2-weighted images are useful in demonstrating the lobular chondroid internal architecture and fine lobular margins of the lesion.⁶⁸

Treatment involves biopsy followed by curettage. Intralesional curettage may be combined with freezing with liquid nitrogen or application of phenol. Bony defects require autogenous or allogenic local graft and, depending on the degree of subchondral erosions, reconstruction of joint surfaces.⁸ Chondroblastomas have been successfully treated with CT-guided percutaneous radiofrequency ablation (Fig. 14-34E).⁶⁹

FIGURE 14-35 ● (A) On an axial T1-weighted image, a chondroblastoma of the femoral epiphysis (arrow) demonstrates intermediate signal intensity. (B) On an axial T2-weighted image, the chondroblastoma demonstrates increased signal intensity (arrow). (C) Contrast-enhanced axial fat-suppressed T1-weighted image shows mild peripheral enhancement (arrow). (D) Corresponding axial CT image shows a well-defined lytic lesion in the femoral epiphysis (arrow).

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Benign Fibrous and Related Lesions

Fibroxanthoma

Fibroxanthoma, fibrous cortical defect, and nonossifying fibroma are terms used to describe histologically similar lesions composed of spindle-shaped fibroblasts, giant cells, and foam (xanthomatous) cells.

Fibrous Cortical Defect

Fibrous cortical defects, histologically identical to nonossifying fibromas, demonstrate low to intermediate signal intensity on T1-weighted images and low to intermediate signal intensity on fat-suppressed T2-weighted fast spin-echo images (Fig. 14-36). The increased signal intensity observed on T2-weighted images is secondary to T2 prolongation in the varied cellular constituents (e.g., fibrous stroma, multinucleated giant cells, foam cells, cholesterol crystals, stromal red blood cells in hemorrhage).⁴⁰ Fibrous cortical defects have a predilection for the diaphyses of long bones.⁸

Nonossifying Fibroma

Nonossifying fibromas are frequently found in the long bones of children and are thought to represent one end of the spectrum of benign cortical defects

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with encroachment on the medullary cavity (Fig. 14-37). Histologically, both nonossifying fibromas and fibrous cortical defects are composed of spindle-shaped fibroblasts arranged in an interlacing pattern. Unlike the fibrous cortical defect, nonossifying fibromas are more commonly found in the metaphyseal area.⁸ On T1-weighted images, these lesions are usually low in signal intensity and have a lobulated contour with an eccentric epicenter. T2, fat-suppressed T2-weighted fast spin-echo, or STIR images may demonstrate areas of hyperintensity (Fig. 14-38). Nonossifying fibromas may also display a central area of fat signal intensity isointense to adjacent marrow, especially during healing or involution. The Jaffe-Campanacci syndrome is characterized by multiple nonossifying fibromas with extraskeletal congenital anomalies such as café-au-lait spots, mental retardation, hypogonadism or cryptorchidism, and cardiovascular malformations (Fig. 14-39).

FIGURE 14-36 ● Anteroposterior (A) and lateral (B) radiographs show a fibrous cortical defect with a sclerotic border (arrows) in the posterior femoral cortex. A bipartite patella is incidentally noted on the anteroposterior radiograph (arrowhead). The defect demonstrates low signal on coronal T1-weighted (C) and coronal (D) and axial (E) fat-suppressed T2-weighted images (arrows).

Gadolinium has been used to enhance the peripheral border in fibrous metaphyseal defects.⁷⁰ Benign fibrous histiocytomas, also histologically similar to nonossifying fibromas, are present in an older population and may be associated with symptoms of bone pain and local recurrence after treatment.⁶⁶ Asymptomatic fibrous cortical defects and nonossifying fibromas do not require treatment.⁸ A pathologic fracture may occur through a nonossifying fibroma.

Ossifying Fibroma

Ossifying fibromas are benign fibro-osseous lesions that most commonly arise in the facial bones

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of young females.⁴⁰ These lesions are well demarcated and therefore amenable to surgical curettage and enucleation. The histologic features of this lesion consist of random trabecular woven bone or dystrophic calcification set in a fibrous stroma.

FIGURE 14-37 ● Coronal graphic illustration demonstrates a well-defined nonossifying fibroma of the proximal tibia in reddish-brown. Note the sclerotic scalloped borders.

FIGURE 14-38 ● Anteroposterior (A) and lateral (B) radiographs show nonossifying fibromas in the proximal tibia and fibula (arrows). Coronal T1-weighted images (C and D) demonstrate the low-signal-intensity sclerotic border (arrows). Corresponding coronal (E) and sagittal (F) fat-suppressed T2-weighted fast spin-echo images show areas of hyperintensity (arrows). A contrast-enhanced fat-suppressed T1-weighted image (G) shows mild peripheral enhancement (arrow).

Periosteal Desmoid

The periosteal desmoid is a fibrous proliferation of the periosteum involving the posteromedial cortex of the medial femoral condyle. The sclerotic saucer-shaped lesion is usually hypointense on T1- and T2-weighted images. A distal femoral cortical irregularity, called linea aspera, is seen in a similar age group (10–20 years of age) and may be related to traction of the adductor magnus aponeurosis. This lesion may in fact represent a variant of the periosteal desmoid (Fig. 14-40). Biopsy should not be performed on periosteal desmoids, and they require no treatment.

Hyman et al. used MR imaging to identify an excavation of the distal femoral metaphysis corresponding to the origin of the medial head of the gastrocnemius muscle.⁷¹ Stress-induced remodeling was inferred by the presence of inflammation along the floor of the excavation. The high-signal-intensity edema associated with the inflammation could be mistaken for a malignant bone tumor on T2-weighted images.

FIGURE 14-39 ● Coronal graphic illustration of the knees shows multiple bilateral nonossifying fibromas in a patient with Jaffe-Campanacci syndrome. Patients with this syndrome also have extraskeletal abnormalities, such as café-au-lait spots, mental retardation, and hypogonadism.

FIGURE 14-40 ● Femoral cortical irregularity distal to the linea aspera (arrows). This area of cortical roughening is usually located medially, along the posterior femoral cortex, as shown on a sagittal proton density-weighted image (A) and a fat-suppressed T2-weighted image (B). Both the periosteal desmoid and distal femoral cortical irregularity may resemble a fibrous cortical defect.

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Fibrous and Osteofibrous Dysplasia

Both monostotic fibrous dysplasia and osteofibrous dysplasia demonstrate low signal intensity on T1-weighted images and intermediate to increased signal intensity on T2-weighted images (Fig. 14-41). The lesions are bordered by a thick, low-signal-intensity sclerotic border, and the homogeneous increased signal intensity seen on T2-weighted images is less than that of fluid (Fig. 14-42). The femoral neck is a common location for monostotic fibrous dysplasia. In 10% to 20% of patients the skull and facial bones are involved (Fig. 14-43). The lesions in polyostotic fibrous dysplasia are similar, but it is a more aggressive condition. The shepherd—s crook deformity is

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caused by a pathologic fracture at the femoral neck (Fig. 14-44). Treatment of fibrous dysplasia involves cortical grafting or implant fixation to stabilize the long bone.⁸

FIGURE 14-41 ● Monostotic fibrous dysplasia of the humerus. Anteroposterior radiograph (A) demonstrates an expansile lesion involving the humeral diaphysis (arrow). Fibrous tissue expansion of the medullary cavity is intermediate in signal intensity (arrows) on coronal T1-weighted image (B). Fibrous tissue expansion of the medullary cavity is intermediate in signal intensity on axial (C) T1-weighted image (arrow) and high in signal intensity (arrow) on a sagittal fat-suppressed T2-weighted fast spin-echo image (D). Fat-suppressed coronal (E) and axial (F) T1-weighted images following the intravenous administration of gadolinium demonstrate peripheral enhancement (arrows).

FIGURE 14-42 ● Monostotic fibrous dysplasia of the left femoral neck, a common location, demonstrates low signal intensity on a coronal T1-weighted image (A) and increased signal intensity on a fat-suppressed T2-weighted fast spin-echo image (B) (arrows). Note the areas of lower signal intensity on the fat-suppressed T2-weighted image (arrowhead). Coronal (C) and axial (D) fat-suppressed T1-weighted images following the intravenous administration of gadolinium demonstrate peripheral enhancement (arrows).

The intracortical cystic lesions of osteofibrous dysplasia (Kempson-Campanacci lesion) may mimic adamantinoma and demonstrate heterogeneity on T2-weighted images. Osteofibrous dysplasia is a benign lesion with a predilection for the tibia in children, affecting the proximal to mid-segment of the anterior tibial cortex. Anterior bowing, cortical disruption, and medullary cavity involvement may occur (Fig. 14-45). Anterior tibial cortical disruption may be associated with reactive muscle edema and present with anterior compartment-like symptoms (Fig. 14-46). Fat-suppressed T2-weighted fast spin-echo or STIR sequences are sensitive to these soft-tissue changes.

FIGURE 14-43 ● Fibrous dysplasia of the facial bones. Coronal graphic illustration shows an expansile, well-defined lesion involving the angle of the mandible.

FIGURE 14-44 ● Coronal graphic illustration shows bowing deformity of the proximal femur (shepherd—s crook deformity). Note multiple lucent lesions of polyostotic fibrous dysplasia involving the femur.

FIGURE 14-45 ● Osteofibrous dysplasia of the tibia is shown in blue on a sagittal graphic illustration. Surrounding sclerosis is shown in light brown. Note the anterior bowing of the tibia, often seen in osteofibrous dysplasia.

FIGURE 14-46 ● Osteofibrous dysplasia with cortical destruction and reactive edema within the tibialis anterior muscle group. This lesion presented with symptoms of anterior compartment syndrome. (A) T1-weighted and (B) fat-suppressed T2-weighted fast spin-echo coronal images. (C) T1-weighted, (D) fat-suppressed T2-weighted fast spin-echo (arrow), and (E) STIR axial images (arrow).

FIGURE 14-47 ● Unicameral bone cyst of the calcaneus. Sagittal graphic illustration demonstrates a well-defined lytic lesion with intact adjacent cortex.

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Benign Tumors and Non-Neoplastic Lesions

Pearls and Pitfalls

Benign Tumors and Non-Neoplastic Lesions

Unicameral bone cysts demonstrate fluid signal on MR imaging without evidence of enhancement.
Unicameral and aneurysmal bone cysts can be treated with CT-guided percutaneous injections.
Fluid–fluid levels are characteristic of aneurysmal bone cysts.
Secondary aneurysmal bone cysts can be seen in a variety of benign and malignant tumors.
Intraosseous hemangiomas are characterized by fat signal intensity.
Intraosseous lipomas follow the signal intensity of fat on all pulse sequences.
Langerhans cell histiocytosis is a “great imitator” of various benign and malignant lesions.

Unicameral Bone Cyst

In patients younger than 16 years of age, unicameral bone cysts are most likely to occur in the proximal humerus or femur, in proximity to the cartilaginous growth plate, and they may recur following surgical removal. Involvement of the calcaneus is more common in older patients (Fig. 14-47). Malignancy in a solitary unicameral bone cyst has been reported, but it is extremely rare. Treatment includes steroid injection (Fig. 14-48) or curettage and bone grafting.⁸

The unicameral bone cyst has a characteristic MR appearance:

On T1-weighted images, it is well defined and demonstrates low signal intensity secondary to simple fluid within the cyst.
On T2-weighted images, the fluid contents demonstrate uniformly increased signal intensity (Fig. 14-49).
Internal hemorrhage may alter T1 and T2 signal characteristics.

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A low-signal-intensity peripheral border representing reactive sclerosis often demarcates this lesion.⁶⁶
Although fracture may occur as a complication, there should be no associated soft-tissue mass.

FIGURE 14-48 ● (A) Axial CT images from a percutaneous CT-guided steroid injection demonstrate a well-defined lytic lesion in the humeral metaphysic (arrow). (B) Corticosteroid mixed with iodinated contrast is seen within the lesion following therapy (arrow).

Aneurysmal Bone Cyst

The aneurysmal bone cyst is an expansile, blood-filled lesion that is osteolytic on conventional radiographs or CT (Fig. 14-50). In the hip or pelvis, aggressive cortical expansion and soft-tissue extension may simulate a malignant process or pseudotumor. Aneurysmal bone cysts, which occur in children and young adults (approximately 80% occur in patients younger than 20 years of age), are most commonly found in the posterior osseous elements of the vertebrae (Fig. 14-51) and the shafts of the long bones.⁷² They are reported to involve the femur in 13% of cases and the innominate bone in 9% of cases. Aneurysmal bone cysts

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contain varying amounts of blood, fluid, and fibrous tissue. In many cases, they arise from preexisting bone lesions such as a giant cell tumor, osteoblastoma, chondroblastoma, fibrous dysplasia, or chondromyxoid fibroma.⁷² Histologic features include blood-filled channels supported by a fibrous septum, which contains multinucleated giant cells and osteoid. Careful microscopic assessment is required to rule out the elusive telangiectatic variant of osteosarcoma, which may mimic an aneurysmal bone cyst both clinically and radiographically.⁷²^,⁷³^,⁷⁴

FIGURE 14-49 ● Unicameral bone cyst (arrow) of the calcaneus, a common location. (A) Lateral radiograph demonstrates a well-defined lytic lesion (arrow). The cystic contents demonstrate low to intermediate signal on a sagittal T1-weighted image (B) and high signal on a sagittal fat-suppressed T2-weighted fast spin-echo image (C). (D) No enhancement is visualized on an axial fat-suppressed T1-weighted image following the intravenous administration of gadolinium.

FIGURE 14-50 ● Aneurysmal bone cyst of the distal femur. (A) Sagittal and (B) axial graphic illustrations show an expansile blood-filled lesion with fluid–fluid levels in red.

FIGURE 14-51 ● Aneurysmal bone cyst of the spine. An axial CT image of the lower thoracic spine demonstrates a lytic expansile lesion arising from the posterior elements of a thoracic vertebral body (arrows).

The favored theory regarding the pathogenesis of aneurysmal bone cysts suggests an altered vascular flow resulting in local circulatory failure.⁷² According to the Armed Forces Institute of Pathology (AFIP), aneurysmal bone cysts occur secondary to hemorrhagic “blowout” in a preexisting lesion.⁷⁵ In the AFIP classification, juxtacortical lesions are grouped with cystic subperiosteal giant cell tumors or subperiosteal myositis ossificans. Central lesions, attributed to secondary changes in preexisting lesions, are identified by the predominant lesion or tumor histology. Intramedullary lesions, without any associated preexisting lesion, are thought to represent giant cell tumors of bone.

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On MR imaging, aneurysmal bone cysts tend to be circumscribed but heterogeneous, with areas of both low and high signal intensity.⁷⁶ The following findings are typical:

The expansile lesion may have internal septations, fluid–fluid levels, and areas of bright signal intensity on both T1- and T2-weighted images, depending on the chronicity of the associated hemorrhage (Figs. 14-52 and 14-53).⁷⁷
The fluid–fluid level probably represents layering of uncoagulated blood within the lesion.⁷⁸ On T1-weighted images, increased signal intensity in the gravity-dependent layer represents methemoglobin. It may be difficult to exclude the possibility of malignancy when severe inhomogeneity of signal intensity is observed.
Cortical bowing and septation (trabeculation) may be seen in a low-signal-intensity contour of cortical bone.

Treatment consists of curettage and bone grafting.⁸ Local adjuvants include freezing with liquid nitrogen and application of phenol. Aneurysmal bone cysts may also be treated with percutaneous calcitonin injection.⁷⁹^,⁸⁰^,⁸¹

FIGURE 14-52 ● Aneurysmal bone cyst of the talus is eccentric and expansile (arrows), as seen on sagittal T1-weighted image (A). Fluid–fluid levels (arrows) are seen on sagittal (B) and axial (C) fat-suppressed T2-weighted fast spin-echo images and an axial proton density-weighted image (D). Septal enhancement (arrows) is noted on this fat-suppressed T1-weighted axial image (E) following administration of intravenous gadolinium.

FIGURE 14-53 ● Aneurysmal bone cyst of the sacrum. (A) Coronal T1-weighted image defines the extent (arrows) of this well-marginated mass. (B) Axial fat-suppressed T2-weighted fast spin-echo image shows hyperintense signal intensity of the lesion (arrows). (C) Peripheral enhancement (arrows) is seen on an axial fat-suppressed T1-weighted image following administration of intravenous gadolinium.

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Hemangioma

Hemangiomas are benign bone lesions classified as capillary, cavernous, or venous vascular proliferations. Hemangiomas that are highly cellular may be confused with a malignant vascular neoplasm.⁸² Hemangiomas may occur in patients from 20 to 60 years of age, but the incidence increases after middle age. The spine and flat bones of the skull and mandible are most frequently involved (Fig. 14-54). When there is vertebral involvement, conventional radiographs or CT demonstrate coarse vertical trabeculations with a corduroy-cloth appearance.

Hemangiomas are variable in MR appearance and may demonstrate low, intermediate, or high signal intensity on T1-weighted images. There is usually some increase in signal intensity on T2-weighted or STIR sequences. Reinforced trabeculae are more difficult to appreciate on MR than CT images (Figs. 14-55 and 14-56).

Intraosseous Lipoma

Intraosseous lipomas were thought to represent rare tumors, constituting only approximately 0.1% of bone tumors.³⁷ However, with the advent of MR imaging and CT, it is likely that intraosseous lipomas will be identified more frequently.⁸³ Approximately 10% of these lesions are discovered within the calcaneus (Fig. 14-57).³⁷ Both simple bone cysts and lipomas exhibit a pyramidal shape

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and are situated in the center of the trigonum calcis, which raises the question of some form of relationship between the two entities. However, the precise explanation remains uncertain.³⁷

FIGURE 14-54 ● Hemangioma of the spine is shown in red on a sagittal graphic illustration.

FIGURE 14-55 ● (A) T1-weighted sagittal image showing thoracic vertebral body involvement with a bright-signal-intensity hemangioma with extension into the posterior elements (arrows). Smaller hemangiomas are present in the lower thoracic vertebral bodies (arrowheads). (B) Corresponding fat-suppressed T1-weighted image after intravenous administration of gadolinium shows marked enhancement of the hemangioma with involvement of the posterior elements (arrows). The smaller hemangiomas demonstrate only minimal enhancement (arrowheads). (C) Hemangioma in a different patient (arrows) has a corduroy appearance on an axial CT image.

The staging classification for intraosseous lipomas has three categories based on the degree of histologic involvement.³⁷^,⁸⁴ In stage 1 tumors, viable fat cells can be demonstrated; in stage 2 tumors, there are both viable fat cells and mixed areas of fat necrosis and calcification; and in stage 3 tumors, necrotic fat, calcification, cyst formation, and reactive woven bone are present.

On MR examination, intraosseous lipomas demonstrate the following findings:

Diffuse fat signal intensity or a thick rind of adipose tissue differentiates intraosseous lipomas from simple cysts in the calcaneus. This tissue demonstrates high signal intensity on T1-weighted images and circumscribes a central hemorrhagic or fluid component, which demonstrates low signal intensity on T1-weighted images and bright signal intensity on T2-weighted images.
Central calcifications are of low signal intensity on T1- and T2-weighted images.
In addition to their appearance in the calcaneus, intra-osseous lipomas are known to affect the long tubular bones, including the fibula, tibia, and femur.

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Intraosseous lipomas demonstrate the same signal intensity as that of fat (i.e., bright on T1-weighted images and intermediate on T2-weighted images) and therefore appear black on STIR and fat-suppressed T2-weighted images (Fig. 14-58).

FIGURE 14-56 ● A hemangioma of the right iliac bone (arrows) is seen on (A) an axial proton density-weighted image, (B) axial and (C) sagittal fat-suppressed T2-weighted fast spin-echo images, and (D) coronal contrast-enhanced fat-suppressed T1-weighted image. A corresponding axial CT image (E) demonstrates characteristic coarsened trabeculae (arrows).

FIGURE 14-57 ● Sagittal graphic illustration shows an intraosseous lipoma of the calcaneus in yellow. Central calcifications are shown in tan.

FIGURE 14-58 ● (A) Lateral radiograph shows an intraosseous lipoma as a lytic area in the calcaneus (arrow). (B) On a sagittal T1-weighted image, the high-signal-intensity fat (arrow) surrounds small low-signal-intensity hemorrhagic fluid contents (arrowhead). (C) On a sagittal fat-suppressed T2-weighted fast spin-echo image, the fat demonstrates decreased signal intensity (arrow), whereas small central fluid contents (arrowhead) generate increased signal intensity. (D) Axial proton density-weighted image demonstrates high-signal-intensity fat (arrow).

FIGURE 14-59 ● Langerhans cell histiocytosis of the skull. Osteolytic lesion in the frontal bone with sharp margins, giving it a punched-out appearance. Uneven involvement of the inner and outer tables results in beveled appearance.

Langerhans Cell Histiocytosis

Langerhans cell histiocytosis presents with solitary or diffuse lesions that involve the flat bones (Fig. 14-59), the long bones, and the spine. The lesions of Langerhans cell histiocytosis may be confused with osteomyelitis or Ewing sarcoma on MR images. Subperiosteal new bone and high-signal-intensity peritumoral edema have been identified on both T1- and T2-weighted images (Fig. 14-60).⁸⁵^,⁸⁶ The site of histiocytic proliferation demonstrates increased signal intensity on T2-weighted images. Treatment involves biopsy and curettage with or without grafting.⁸ CT-guided percutaneous steroid injection has been performed successfully (Fig. 14-61).⁸⁷^,⁸⁸

FIGURE 14-60 ● Solitary Langerhans cell histiocytosis of the intertrochanteric femur. (A) Coronal T1-weighted image shows areas of low-signal-intensity histiocytic infiltration (arrow). (B) Bright-signal-intensity edema and marrow involvement (arrow) are seen on a fat-suppressed T2-weighted fast spin-echo image.

FIGURE 14-61 ● Axial CT images from a percutaneous CT-guided steroid injection for treatment of Langerhans cell histiocytosis. (A) A lytic lesion with pathologic fracture (arrow) of the iliac bone. (B) Injection of corticosteroid mixed with iodinated contrast (arrow).

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Intermediate Lesion (Giant Cell Tumor)

Giant cell tumors exhibit unpredictable biologic behavior. They can be aggressive and are usually treated by surgical removal. Forty percent recur locally, and approximately 10% to 20% have potential for malignant transformation.⁸⁹ Others exhibit local soft-tissue extension or systemic tumor implantation, and secondary aneurysmal bone cyst formation is sometimes seen (Fig. 14-62). The majority of these tumors are seen in patients 20 to 40 years of age.⁸⁹^,⁹⁰ The long tubular bones are most commonly affected, and giant cell tumors often pre-sent around the knee (e.g., distal femur, proximal tibia) and abut the subchondral bone (Fig. 14-63).⁹¹ When spinal bones are involved, there is a predilection for anterior elements (i.e., vertebral bodies). Giant cell tumors of bone originate in the distal metaphysis of the long bones and abut the physeal plate.⁹² They do not occur in or cross the physis prior to fusion of the epiphysis and metaphysis, at which time the tumor occupies a subchondral position.

Histologically, giant cell tumors exhibit a uniform distribution of giant cells against a cellular background of ovoid spindle stromal cells that exhibit mitotic activity. Osteoid foci can be seen within these lesions and are usually associated with areas of hemorrhage or are found within a fracture callus.⁸⁹ A histologic grading system was instituted in an attempt to predict the biologic aggressiveness of these lesions, but it has fallen out of favor in light of variable behavior in the presence of histologically benign features. In patients with histologically benign lesions, tumor implantation at distant sites, typically the lungs, occurs in approximately 1% to 3% of cases.⁸⁹ These implants are regarded as passive vascular transports related to surgical curettage. If these pulmonary lesions are identical histologically to the primary giant cell tumor, they generally do not lead to the demise of the patient.

Malignant transformation, which can occur in a previously benign-appearing giant cell tumor, usually follows irradiation of the original tumor.²⁰^,²³^,³⁵^,³⁶

On MR examination, giant cell tumors show the following:

Low to intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images (Fig. 14-64)⁹²

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Heterogeneity in T2-weighted images may represent central areas of liquefaction, hemorrhage, or necrosis (Fig. 14-65).
Aoki et al. reported MR detection of hemosiderin in 63% of giant cell tumors.⁹³ The low-signal-intensity hemosiderin is attributed to extravasated erythrocytes in the tumor and the phagocytic function of the tumor cells. Areas of hemosiderin were most evident on gradient-echo sequences.
T2, fat-suppressed T2 or proton density fast-spin echo, or STIR images may help to identify areas of tumor recurrence after excision and packing with methylmethacrylate. Recurrent tumors have been identified on MR scans with low to intermediate signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences.
Fluid–fluid levels are sometimes seen on MR images.⁹⁴
Areas of necrosis (i.e., signal inhomogeneity), cortical erosion, and associated effusions may also be identified on MR images.
Intratumoral hemorrhage may produce bright signal intensity on T1- and T2-weighted images, although the tumor generally demonstrates low signal intensity on T1-weighted images and increased signal intensity on T2-weighted images.
Soft-tissue involvement and rare joint invasion can be evaluated with MR imaging.⁹⁵
A thin rim of sclerotic bone may be identified at the tumor interface with uninvolved fatty marrow in less aggressive lesions.
Contrast inhomogeneity and a multilobular configuration was seen in one case of giant cell tumor involving the flat pubic bone of the pelvis.

FIGURE 14-62 ● Secondary phenomena observed in giant cell tumors.

FIGURE 14-63 ● Giant cell tumor of the distal femur. The tumor extends to the epiphysis.

FIGURE 14-64 ● Giant cell tumor. (A) Anteroposterior and (B) lateral radiographs show an osteolytic lesion involving the subchondral bone of the tibial plateau. Cortical thinning and expansion can be seen. (C) Coronal T1-weighted image shows subchondral extension of the tumor (arrow). Tumor contents demonstrate low signal intensity on the coronal (C) and axial (D) T1-weighted images and are hyperintense on the coronal (E) and axial (F) T2-weighted images. Peritumoral edema (arrow) is noted on the axial fat-suppressed T2-weighted fast spin-echo image (F).

Because of the high rate of recurrence with curettage or intralesional resection, a more extensive approach, with an en bloc resection, has been attempted.⁸ This technique is not without its own complications, however, including infection, resorption, collapse, and fracture. More extensive local excision (with wide decortication of bone overlying the tumor), combined with the use of polymethylmethacrylate cement, has been used in an attempt to remove adequate tissue without destabilizing the surgical area.

FIGURE 14-65 ● Expansile giant cell tumor of the proximal fibula. Anteroposterior radiograph (A) shows a lytic lesion with periosteal reaction (arrow). Coronal T1-weighted image (B) demonstrates low-signal-intensity tumor (arrow). Cystic areas within the tumor (arrows) are shown on axial T2-weighted (C) and fat-suppressed T2-weighted fast spin-echo (D) images. T1-weighted fat-suppressed coronal (E) and axial (F) images after administration of intravenous gadolinium demonstrate enhancement of the lesions. Nonenhancing areas corresponding to cystic/necrotic changes are seen on the axial image (arrowhead). (G) Axial CT image demonstrates low-density cystic/necrotic areas within the tumor (arrows).

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Malignant Lesions

Pearls and Pitfalls

Malignant Lesions

MR imaging is nonspecific in differentiating benign from malignant processes.
Most benign and malignant tumors show increased T1 and T2 signal.
Cortical destruction, soft-tissue masses, and involvement of neurovascular bundles are more common in malignant tumors.
MR imaging findings suggestive of tumor recurrence include new edema, convex margins of the treatment site, and nodular enhancement.
With MR imaging, intramedullary marrow involvement, soft-tissue extension, areas of necrosis, skip-lesions, metastases, and postoperative recurrence can be accurately assessed.
Osteosarcoma can arise de novo (primary osteosarcoma) or in association with preexisting lesions of bone such as Paget—s disease, prior radiation, or bone infarcts.
Telangiectatic osteosarcoma can simulate an aneurysmal bone cyst.
Frank cortical disruption, soft-tissue extension, and periosteal reaction in a preexisting enchondroma or osteochondroma are suspicious for malignant transformation to chondrosarcoma.
Ewing sarcoma predominately involves the flat bones and diaphyses of long bones.
The tibia is involved in 90% of cases of adamantinoma.
MR imaging should be performed if there is suspicion of metastatic disease and radiographs are normal.
Distinguishing primary lymphoma of bone from secondary involvement is important because the latter has a worse prognosis and is treated differently.

Since the primary component of most malignant bone tumors has prolonged T1 and T2 tissue relaxation times, these tumors demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.¹ Heterogeneous signal intensity is most evident on T2-weighted images in areas of necrosis or hemorrhage. Neurovascular bundle encasement, peritumoral edema, and irregular margins are secondary evidence of the malignant nature of these lesions.

Except in cases of postradiation marrow edema, extensive marrow involvement by edema or tumor is highly suggestive of either a malignancy or an infection. When there is marrow involvement in neoplastic or inflammatory disease, the MR signal characteristics depend on whether the marrow in the affected limb is yellow or red. On T1-weighted images, neoplastic or inflammatory involvement of yellow marrow demonstrates low signal intensity within or adjacent to bright marrow fat and increased signal intensity with progressive T2 weighting. When the affected limb is primarily red marrow, however, lesions often appear isointense with normal hematopoietic marrow on T1-weighted images and bright on T2-weighted images.

In the postoperative evaluation of malignant neoplasms, initial edema and inflammatory changes demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. This bright signal intensity on T2-weighted images is feathery and infiltrative and conforms to the contour of the muscle. With increased time, the surgical field is replaced with fibrous tissue that demonstrates low to intermediate signal intensity on T1- and T2-weighted scans. Postoperative hematomas and seromas are well marginated and confined to the region of surgery and demonstrate uniform increased signal intensity with long repetition time (TR) and echo time (TE) settings. Chronic hemorrhage with hemosiderin deposits remains dark on T1- and T2-weighted images because of the paramagnetic effect of iron.

Certain MR changes, including the following, strongly suggest tumor recurrence:

The reappearance of edema, characterized by low signal intensity on T1-weighted images and high signal on T2-weighted images
A new area of increased signal intensity on T2-weighted images with a corresponding area of intermediate signal intensity on T1-weighted images
A change in the contour of a muscle or postoperative surgical field, such that the margins are convex instead of concave
New areas of nodular enhancement in the postoperative surgical field

Osteosarcoma

Osteosarcoma is the most common primary malignant bone tumor in childhood. Except for multiple myeloma, osteosarcoma is the most common primary malignancy of bone and is considered one of the most aggressive and histologically varied neoplasms. The identifying feature of all osteosarcomas is the presence of malignant stromal cells that produce osteoid. From a clinical viewpoint, there are two main categories of osteosarcoma, primary (de novo) and secondary.⁵⁶^,⁹⁶

Primary Osteosarcoma

Primary (de novo) conventional osteosarcomas most often affect individuals younger than 20 years. There is a slight predominance in male patients. In 96% of cases, the tumor develops in the long bones and limbs. The lesion is usually metaphyseal, destroys trabecular and cortical bone, invades the soft tissues, and may extend into the epiphysis (Fig. 14-66).⁹⁶

The role of genetic factors in the pathogenesis of osteosarcoma is of particular interest in children. Patients with genetic retinoblastoma who have a point mutation at chromosome 13q14 band demonstrate a 514-fold increased risk of developing osteosarcoma.⁶¹ With the use of cytogenetic and molecular techniques, the Rb (retinoblastoma) and p53 suppressor genes have been discovered to play a role in the pathogenesis of osteosarcoma.⁶¹

Secondary Osteosarcoma

Osteosarcoma in older individuals (secondary osteosarcoma) arises in a setting of preexisting bone disease (e.g., Paget—s disease, bone infarcts) or

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after exposure to a mutagenic event such as irradiation. In osteosarcoma associated with Paget—s disease, bones other than the long bones (e.g., the pelvis, skull, facial bones, and scapula) are involved. Osteosarcomas arising in Paget—s disease have a poorer prognosis than conventional de novo osteosarcomas.⁶¹

FIGURE 14-66 ● Primary osteosarcoma of bone. Coronal graphic illustrations of the knee and shoulder show osteosarcoma with sclerotic and lytic areas and aggressive periosteal reaction involving the proximal tibia (A), distal femur (B), and proximal humerus (C).

Classification

Several pathologic patterns of osteosarcomas have been identified:

Telangiectatic osteosarcomas are especially difficult to assess histologically because the viable cells and anaplastic cells are obscured by hemorrhage and necrosis or camouflaged within benign reactive cells of the walls, which simulates an aneurysmal bone cyst (Fig. 14-67).⁷²^,⁷⁴ It was thought to have a worse prognosis than conventional osteosarcoma, but now the prognosis is considered similar. On radiographs, the telangiectatic variant appears lytic because of the lack of demonstrable bone production. MR images show an aggressive, destructive lesion, usually accompanied by an associated soft-tissue mass. The inhomogeneity of signal intensity, with both low- and high-signal-intensity areas on T1- and T2-weighted images and fluid–fluid levels, reflects the high degree of vascularity and the presence of large hemorrhagic cystic spaces (Fig. 14-68).
Approximately 10% of primary osteosarcomas are tremendously rich in bone production and are classified as

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sclerosing or osteoblastic osteosarcoma.⁹⁶ At times, the bone and osteoid are deposited in massive solid amounts. Frequently, the mineralization extends into the soft tissues.⁹⁷ Blastic components remain low in signal intensity on T1- and T2-weighted images, but associated peritumoral edema or nonsclerotic areas generate increased signal intensity on T2-weighted or STIR images (Fig. 14-69).
Chondroblastic osteosarcoma is seen in approximately 5% of cases.⁹⁶ The generation of chondroid elements, whether benign or malignant, contributes to increased signal intensity on T2-weighted images.
Osteosarcomas that produce large amounts of spindly fibroblasts are classified as fibroblastic osteosarcoma, seen in approximately 4% of cases.⁹⁶ There is no convincing evidence that chondroblastic or fibroblastic subtypes of osteosarcoma have different prognoses.⁹⁶
Central, low-grade osteosarcoma, a rare type of osteosarcoma arising in medullary bone, is so well differentiated that it is frequently mistaken for a benign condition both radiologically and histopathologically. Patients are usually young adults and may have had symptoms for years at the time of presentation. Much of the lesion appears circumscribed, simulating fibrous dysplasia. A search for small foci of cortical destruction may be essential for accurate diagnosis. The prognosis in low-grade osteosarcoma is excellent. Metastases occur late, if ever, but local recurrences may be a serious problem if surgery is inadequate.⁹⁶ On MR imaging, the low-grade central osteosarcoma may be difficult to distinguish from a conventional osteosarcoma, which has an identical radiographic appearance.

FIGURE 14-67 ● Coronal graphic illustration shows telangiectatic osteosarcoma of the distal femur. Hemorrhagic areas are shown in red.

FIGURE 14-68 ● Telangiectatic osteosarcoma. (A) Coronal T1-weighted and (B) axial T2-weighted images show intraosseous and extraosseous tumor extension (white arrows) with heterogeneous cystic hemorrhagic components (white arrowhead) and fluid–fluid levels (black arrows).

There are three additional subtypes of osteosarcomas, referred to as juxtacortical osteosarcomas. They account for 7% of all osteosarcomas and arise on the surface of the bone rather than within the medulla. These juxtacortical subtypes are periosteal osteosarcoma, parosteal osteosarcoma, and high-grade surface osteosarcoma:

Periosteal osteosarcoma, which accounts for 0.3% of all osteosarcomas, favors the diaphyseal surface of long tubular bones and forms an irregular thickened cortex. Histologically, hyaline chondrosarcomatous elements are a prominent feature.⁹⁸
Parosteal osteosarcoma occurs most frequently on the metaphyseal surface of long tubular bones within the parosteal soft tissue and forms a lobulated mass. Histologically it is characterized by a fibrous proliferation surrounding parallel lamellar bony spicules. The fibrous component can appear externally bland, mimicking a fibrous dysplasia. Unlike periosteal osteosarcomas, chondrosarcomatous elements are minimal or absent in parosteal osteosarcomas.⁹⁹^,¹⁰⁰
High-grade surface osteosarcoma is the least common of the surface osteosarcomas. It is typically seen in the second decade of life and is histologically identical to the conventional intramedullary form. These lesions can be distinguished from dedifferentiated parosteal osteosarcoma by a lack of adjacent low-grade areas. The prognosis for high-grade surface osteosarcoma is

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the worst of surface osteosarcomas and is similar to that of the conventional type.

FIGURE 14-69 ● Osteosarcoma of the proximal tibia. Coronal (A) and sagittal (B) T1-weighted images show marrow replacement with cortical break and a soft-tissue mass of intermediate signal (arrows). Areas of sclerosis are hypointense (arrowheads). Osteosarcoma and soft-tissue mass are hyperintense on the sagittal fat-suppressed T2-weighted fast spin-echo image (C). Areas of sclerosis are hypointense (arrowhead). Non-enhancing necrotic areas are demonstrated on axial (D) and sagittal (E) fat-suppressed T1-weighted images after intravenous administration of gadolinium (arrowheads).

MR imaging performed in the early stages of developing myositis ossificans may display changes that can be mistaken for a more aggressive process, such as parosteal or periosteal osteosarcoma.¹⁰¹ Rim enhancement with intravenous contrast injection and hyperintensity on T2-weighted images mimic the appearance of an abscess or necrotic tumor.

Treatment of Osteosarcoma

The management of patients with conventional osteogenic sarcoma has changed dramatically since the early 1970s for two reasons. First is the introduction of adjuvant multidrug chemotherapy, which has changed the prognosis for survival from 25% at 5 years to the present figure of 75%. The effectiveness of preoperative chemotherapy is assessed by evaluating the clinical picture of reduced pain and tumor size. Imaging studies are also helpful in monitoring the effectiveness of chemotherapy by quantifying the amount of tumor necrosis taking place after the first or second cycle of chemotherapy. The degree of chemotherapy-induced tumor necrosis is further quantified by the pathologist, who gives a percentile determination to the surgical specimen taken at the time of amputation or limb-salvage resection. If this percentage is greater than 90%, the 5-year survival prognosis climbs to 85% to 90%, compared with only 50% to 60% for the nonresponding group.¹⁰²

The second major event that took place in the 1970s was the advent of total joint replacement surgery, which was rapidly applied to the local management of osteosarcoma by means of limb-salvage reconstruction instead of ablative amputation. At present, limb-salvage reconstructive procedures are used in 90% of cases, with a local recurrence rate of 6% and a prosthetic survival rate of 83% at 5 years and 67% at 10 years.¹⁰³^,¹⁰⁴ A modular titanium system with a rotating hinge device for the knee joint has been used in the United States since 1982 with a high degree of success

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(Fig. 14-70). The major cause for failure is stress shielding secondary to stem loosening. This is avoided in newer systems with more compliant, stress-sharing fixation devices.

FIGURE 14-70 ● (A) A T2-weighted sagittal image shows osteosarcoma involving the femoral metaphysis (small arrows). Bright contrast portions of the high-signal-intensity tumor represent necrosis with hemorrhage (large arrow). (B) The corresponding gross specimen shows central tumor necrosis (arrow). (C) Macrosection of amputated specimen cut in the coronal plane displaying a large extracortical tumor mass with various tissue types, including young tumor osteoid at the outer edge (short arrow) contrasting with malignant chondroid tissue (curved arrow) that would demonstrate higher signal on T2-weighted images. The long arrow indicates Codman—s reactive triangle. (Reprinted with permission from

Skinner HB, ed. Current diagnosis and treatment in orthopaedics. Norwalk, CT: Appleton & Lange, 1995.

) (D) Low-power photomicrograph of malignant tumor osteoid (arrow). (E) High-power photomicrograph of the malignant chondroid portion of heterogeneous tumor tissue. (F) Photograph of the titanium modular rotating hinge prosthesis used in reconstructive limb-salvage surgery following wide surgical resection of the distal femur. (G) Photograph of the implant at the time of surgical reconstruction prior to wound closure. (H) Radiographic appearance in lateral projection 7 years following surgery. (F and H reprinted with permission from

Meyer JL, Vaeth JM, eds. Frontiers in radiation therapy and oncology: organ conservation in curative cancer treatment, vol. 27. Basel: S. Karger AG, 1993.

)

MR Appearance

MR imaging affords superior marrow and soft-tissue discrimination in the evaluation of osteosarcoma. Osteosarcoma, chondrosarcoma, giant cell tumors, and Ewing sarcoma are statistically the primary bone neoplasms that are likely to involve the lower extremities. The distal femur, proximal tibia, and humerus are common sites of involvement in conventional osteosarcoma (Fig. 14-71). With MR imaging, intramedullary marrow involvement, soft-tissue extension, areas of necrosis, skip-lesion metastases, and postoperative recurrence can be accurately assessed (Fig. 14-72). Some advantages of MR imaging include the following:

Marrow infiltration can be identified on MR images, even when standard radiographic results are negative or when conventional radiographic findings are misleading.¹⁰⁵
MR imaging can also be used to differentiate exuberant bone, which is isointense with yellow marrow, from osteosarcoma, which causes both T1 and T2 prolongation.
MR scans of aggressive, dedifferentiated osteosarcomas may demonstrate a necrotic, hemorrhagic fluid–fluid level within the more distal marrow cavity.
In many cases, there is MR evidence of tumor crossing the physis, although this extension is not evident on conventional radiographs.

In general, osteosarcomas demonstrate low signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences. Specific cellular constituents (e.g., fibrous, chondroid, blastic, or telangiectatic components), however, can modify signal characteristics.⁹⁷ There is excellent correlation between the MR appearance of the extent of marrow, cortical bone, and soft-tissue involvement and the gross pathologic specimen:¹⁰⁶

Lesions that are primarily blastic, and therefore sclerotic on radiographs, demonstrate low signal intensity on both T1- and T2-weighted sequences. However, even in lesions with extensive sclerosis, areas of increased signal intensity may be identified on T2-weighted images.
Hemorrhagic components within telangiectatic osteosarcoma demonstrate focal areas of high signal intensity on T1- and T2-weighted images.
Skip lesions and multiple sites of involvement are common in multicentric osteosarcoma, which affects a younger age group.¹⁰⁷

FIGURE 14-71 ● (A) Anteroposterior radiograph showing blastic osteosarcoma as a sclerotic metaphyseal focus with aggressive periosteal reaction (arrows). (B) On a T1-weighted axial image, the lesion is seen as a low-signal-intensity area of marrow replacement with associated soft-tissue extension (arrows). On coronal (C) and axial (D) fat-suppressed T2-weighted fast spin-echo images, hyperintense tumor and soft-tissue component are demonstrated. Soft-tissue component is hyperintense on sagittal fat-suppressed T2-weighted fast spin-echo image (E) (arrows).

FIGURE 14-72 ● Osteosarcoma of the distal femur. (A) Coronal T1-weighted image demonstrates marrow infiltration (arrowhead) with cortical destruction and extension into the adjacent soft tissues (arrows). (B) On a sagittal proton density-weighted image, aggressive periosteal reaction and soft-tissue extension are demonstrated (arrows). (C) On a coronal fat-suppressed T2-weighted fast spin-echo image, the tumor is hyperintense (black arrows) with hypointense areas corresponding to sclerosis (white arrow). Aggressive periosteal reaction is hyperintense (arrowhead). (D) Coronal contrast-enhanced fat-suppressed T1-weighted image shows central nonenhancing areas, consistent with necrosis (arrows).

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In cases of parosteal osteosarcoma, the precise involvement of cortex and marrow can be assessed on MR imaging (Fig. 14-73). Both edema and tumor extension may encase vessels, and multiple axial images are usually necessary to distinguish edema that conforms and tracks along specific muscle groups.

MR imaging is superior to CT in determining the exact extent of marrow involvement by tumor and edema, particularly important in limb-salvage procedures. MR imaging is also sensitive in identifying joint invasion by osteosarcoma; however, false-positive interpretations may result in overstaging of the tumor.¹⁰⁸ Contrast enhancement is helpful in identifying joint involvement. The effectiveness of preoperative chemotherapy is determined by evaluating the sensitivity of the tumor to chemotherapy. Although this can be approximated on a clinical and radiologic basis, histopathologic assessment of the amount of tumor necrosis in the postresection specimen remains the best way to evaluate response.¹⁰² Direct coronal or sagittal MR images of the tumor are also helpful in planning preoperative or postoperative therapeutic regimens. Prior to surgery, interval responses of both tumor and edema to chemotherapy can be monitored by serial MR scans, including contrast-enhanced MR images.¹⁰⁹^,¹¹⁰^,¹¹¹^,¹¹²^,¹¹³ Fast dynamic contrast-enhanced sequences are useful for identifying residual tumor prior to surgery.¹¹³ Viable tumor is correlated with a short (<6 seconds) time interval between arterial enhancement and tumoral enhancement.

Pan et al. reported on four MR patterns found in post-chemotherapy osteosarcoma:¹¹²

A dark pattern of low- to intermediate-signal-intensity areas on T1- and T2-weighted images, corresponding

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to calcified osteoid or cartilage matrix, dense granulation tissue, and hemosiderin
A mottled or speckled pattern of intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images, best appreciated on T2-weighted images, which corresponds to tumor matrix, edematous granulation tissue, and hemosiderin deposits
A distinct multicystic or bubbly appearance in the cystic pattern, of intermediate to high signal intensity on T1-weighted images and high signal intensity on T2-weighted images, thought to correspond to either blood-filled cysts with viable tumor cells or tumor matrix mixed with edematous granulation tissue. However, foci of residual viable tumor cannot be specifically diagnosed.
Skip metastases, a reduction in peritumoral edema, the development of a dark rim of collagenous capsule continuous with the periosteum, bone infarcts, and intramedullary vascular channels can all be identified.

FIGURE 14-73 ● Parosteal osteosarcoma. Anterior coronal (A) and axial (B) T1-weighted images show a large lobulated mass of low signal intensity arising from the metadiaphyseal surface of the tibia (arrows). Tumor is hyperintense on sagittal (C) and axial (D) fat-suppressed T2-weighted fast spin-echo images. Tumor enhancement is noted on axial fat-suppressed T1-weighted image (E) following administration of intravenous gadolinium.

Holscher et al. have reported a correlation between a positive response to chemotherapy and signal intensity changes on T2-weighted images in the extraosseous tumor component and changes in overall tumor volume.¹¹⁰ No correlation was observed between chemotherapy and intraosseous tumor signal intensity or volume and histology. Sanchez et al. have studied patients receiving intra-arterial chemotherapy and reported similar signal intensities in viable tumor, necrosis, edema, and hemorrhage, which demonstrated the limitations of nonenhanced spin-echo images in the accurate assessment of the percentage of tumor necrosis.¹¹⁴

Chondrosarcoma

Chondrosarcomas occur about half as often as osteogenic sarcomas. They are malignant cartilage-producing neoplasms, which arise primarily in adulthood and old age. The peak incidence is in the fourth, fifth, and sixth decades of life. Chondrosarcomas most commonly arise in the central skeleton (the pelvis and ribs) and within the metadiaphysis of the femur and humerus (Fig. 14-74). More than in other primary bone malignancies, their clinical and biologic behavior depends on their histologic grade, which is proportional to the level of anaplasia:

Grade 1 (i.e., well-differentiated) chondrosarcomas consist of pure hyaline cartilage exhibiting mild anaplasia and mild increased cellularity.
Cellularity and anaplasia are greatest in poorly differentiated or grade 3 variants.
The 5-year survival rates of patients with grades 1, 2, and 3 disease are approximately 90%, 81%, and 43%, respectively.⁶¹
Grades 1 and 2 neoplasms (i.e., the majority of chondrosarcomas) do not usually metastasize, whereas grade 3 neoplasms tend to form hematogenous metastases.

The majority of chondrosarcomas (approximately 75%) arise de novo and are considered primary chondrosarcomas. The remainder, secondary chondrosarcomas, arise by malignant transformation of preexisting cartilaginous lesions such as multiple enchondromatosis (e.g., Ollier—s disease or Maffucci—s syndrome) and exostosis, especially multiple hereditary osteochondromatosis.⁶¹ These neoplasms, whether situated peripherally or

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centrally, are composed of characteristic lobules of gray-white translucent tissue with spotty calcification.⁶¹ The adjacent cortex may be thickened or eroded, with extension of the neoplasm into the surrounding soft tissues.

FIGURE 14-74 ● Chondrosarcoma of the proximal femur (A) and calcaneus (B). Chondroid tissue is shown in blue. Note cortical destruction and soft-tissue extension.

Treatment of Chondrosarcoma

Unlike osteosarcoma and Ewing sarcoma, low-grade chondrosarcomas are not responsive to adjuvant chemotherapy or radiation therapy. Instead, these tumors are considered surgical problems and must be removed with clean margins, if at all possible, in which case the 5-year survival rate is approximately 81%.¹¹⁵ Figure 14-75 shows a typical primary central chondrosarcoma of the proximal humerus that has been treated by a wide resection of the proximal 15 cm (6 inches) of the humerus and reconstructed

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with a combination of a metallic Neer prosthesis and a cadaver allograft. Tendinous tags have been left intact for reattachment to the original structures, allowing for excellent limb function for many years to come. Preoperative MR imaging is extremely helpful in planning these reconstructive procedures by providing accurate evaluation of the extent of tumor growth down the medullary canal and out into the surrounding soft tissue. These same preoperative imaging studies assist the prosthesis manufacturers in the design of the prosthetic implant.

FIGURE 14-75 ● (A) A coronal T1-weighted image shows aggressive destruction of the proximal humerus (curved arrows) with a large soft-tissue component (straight arrow). This is characteristic of chondrosarcoma. Axial T1-weighted (B) and T2*-weighted (C) images show the degree of anterior, posterior, and lateral tumor extension (curved arrows), as well as the degree of marrow expansion (open arrows). (D) Macrosection of tumor breaking out of the humoral head beneath the deltoid muscle with benign enchondral ossification (white arrow), which is seen as signal void on T2-weighted images. (Reprinted with permission from Henderson C, Wilson C, Phillips T, et al., eds. Current cancer diagnosis and treatment. Norwalk, CT: Appleton & Lange, in press.) (E) High-power photomicrograph demonstrating numerous dividing chondroblasts (arrow) typical of chondrosarcoma. (F) Surgical photograph of allograft used in limb-salvage reconstruction following wide resection with rotator cuff tendon (arrow). (G) Long-stem metallic Neer prosthesis with pectoralis tendon allograft (arrow). (H) Photograph of completed alloprosthetic reconstruction with reattachment of the rotator cuff (curved arrow) and pectoralis and latissimus dorsi tendons (straight arrow). (I) Postoperative anteroposterior radiograph with Neer stem cemented into the distal humerus. (G through I reprinted with permission from

Meyer JM, Vaeth JM, eds. Frontiers of radiation therapy and oncology: organ conservation in curative cancer treatment, vol. 27. Basel, S. Karger AG, 1993.

)

MR Appearance

The center of a primary, central chondrosarcoma can be located in the metaphysis or diaphysis of a long bone.¹¹⁶ The cortex is irregularly thinned, and a circumferential periosteal reaction can be seen. Dedifferentiated chondrosarcomas are more aggressive and show rapid growth,

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cortical destruction, and the frequent association of an extraosseous component. MR findings include the following:

FIGURE 14-76 ● (A) An anteroposterior radiograph of the femur shows characteristic annular and comma-shaped calcifications within central or medullary chondrosarcoma (arrows). (B) Coronal T1-weighted and (C) axial fat-suppressed T2-weighted fast spin-echo images accurately depict marrow involvement. Tumor is hyperintense on the fat-suppressed T2-weighted fast spin-echo image. Calcifications are seen as dark-signal-intensity foci (arrow). (D) Contrast-enhanced axial fat-suppressed T1-weighted image shows cortical thinning posteriorly (arrow).

FIGURE 14-77 ● Coronal T1-weighted (A) and fat-suppressed T2-weighted (B) fast spin-echo images demonstrate low-grade chondrosarcoma involving the sacrum and ilium. The chondroid matrix is hyperintense on the T2-weighted image. (C) Contrast-enhanced fat-suppressed T1-weighted image demonstrates peripheral enhancement (arrows). Cortical destruction and soft-tissue extension are seen.

MR imaging can be used to characterize changes of marrow involvement prior to medullary expansion (Fig. 14-76).
Fat-suppressed T2-weighted fast spin-echo images are sensitive to associated soft-tissue masses (Fig. 14-77).
On T2-weighted images, calcifications are identified as low-signal-intensity foci that contrast with the bright-signal-intensity tumor.
A low-signal-intensity fibrous septum may be seen, separating homogeneous high-signal-intensity lobules of hyaline cartilage.⁷⁸
Heterogeneity of signal intensity correlates with higher-grade, more cellular lesions.¹¹⁷
Gadolinium administration may produce enhancement in a ring and arc septal pattern (Fig. 14-78).⁷^,⁶⁴

Secondary chondrosarcoma develops in a preexisting enchondroma or osteochondroma. Although extremely rare in childhood, chondrosarcoma has been known to arise from malignant transformation of an enchondroma in Ollier—s disease or Maffucci—s syndrome. Secondary chondrosarcomas exhibit the following MR characteristics:

On T2-weighted images, areas of malignant transformation demonstrate greater signal intensity than adjacent enchondromas.

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Malignant degeneration or transformation can be inferred by the presence of frank cortical disruption, soft-tissue extension, periosteal reaction, and disproportionate size of lesion relative to satellite enchondromas.
Although these findings have been confirmed by pathologic evaluation, reliable MR criteria have not been developed to characterize these aggressive cartilage lesions. MR imaging is useful in the evaluation of aggressive extraskeletal mesenchymal chondrosarcoma, where soft tissue origin is common.¹¹⁸ In these extraskeletal lesions, MR imaging demonstrates enhancement, lobulation, and extent of disease, whereas CT is more accurate in characterizing calcifications.

FIGURE 14-78 ● (A and B) Axial CT images showing a large chondrosarcoma arising from the left superior pubic ramus. Punctate calcifications (arrows) are well visualized on the CT images. T2-weighted sagittal (C) and axial (D) images show a large heterogeneous cartilage tumor arising from the left pubic ramus. (E) Axial fat-suppressed T1-weighted contrast-enhanced image shows enhancement in a ring and arc septal pattern.

Ewing Sarcoma

Ewing sarcoma is seen primarily during the last part of the first decade and the first half of the second decade of life. It is the second most common malignant bone tumor in childhood, and although any bone in the body may be affected, the femur, ilium, humerus, and tibia are the most common sites (Fig. 14-79). In long bones the diaphysis is frequently involved. Ewing sarcoma is one of the malignant round cell tumors involving bone. Although its histogenesis remains uncertain, evidence suggests a neuroectodermal origin.¹¹⁹ Translocation of the long arms of chromosomes 11 and 22 has been found in Ewing sarcoma.¹²⁰

Treatment of Ewing Sarcoma

As is the case with osteosarcoma, the prognosis for Ewing sarcoma has changed dramatically since the early 1970s with the advent of multidrug adjuvant chemotherapy. The 10-year survival rate is now 55% to 70%.¹²¹ The effectiveness of chemotherapy can be monitored with MR imaging (Fig. 14-80), which can be useful to the surgeon in planning surgical resection of the tumor and limb-salvage reconstruction. If the tumor can be successfully removed with safe margins, there is no need for postoperative radiation therapy, and the prognosis for survival

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is increased to 74%.¹²² For nonresectable tumors, adjuvant radiation therapy rather than surgery is used in conjunction with chemotherapy. In these cases, the prognosis is less favorable.

FIGURE 14-79 ● (A) Sagittal and (B) axial graphic illustrations of the femur and (C) coronal graphic illustration of the ilium show destructive intramedullary Ewing sarcoma with lamellated periosteal reaction. Large soft-tissue component is seen on the axial image (arrows).

FIGURE 14-80 ● (A) Proton density axial image of a 12-year-old boy with an extensive intermediate-signal soft-tissue abnormality representing extensive round cell infiltrate and edema into the surrounding muscle compartments from a Ewing sarcoma of the femur. (B) Same proton density study after two cycles of chemotherapy, with dramatic shrinkage of the tumor margins (arrow). (C) Photograph of the amputated specimen, with arrow indicating the same tumor margin seen in B. (D) Gross macrosection of same specimen, with extracortical tumor margin indicated by arrow.

MR Appearance

Ewing sarcoma demonstrates low signal intensity on T1-weighted sequences and high signal intensity on T2, fat-suppressed T2-weighted fast spin-echo, and STIR sequences (Fig. 14-81).¹²³^,¹²⁴ Marrow involvement and peritumoral edema are clearly delineated on fat-suppressed T2-weighted fast spin-echo images (Figs. 14-82 and 14-83). Additional MR findings include the following:

There is usually a substantial soft-tissue component associated with Ewing sarcoma, which is well evaluated with MR imaging. Since MR imaging provides excellent delineation of soft tissues, the extent of muscular and neurovascular involvement in extraosseous Ewing sarcoma can be accurately assessed.
In addition, Ewing sarcoma originating in bone marrow can be identified in the early stages, before cortical erosion and periostitis have developed.
Multilamellar periosteal reactions and an atypical pattern of cortical thickening and saucerization in Ewing sarcoma have also been identified on MR scans.¹²⁵ The rare periosteal Ewing sarcoma can be distinguished from the more common medullary and soft-tissue Ewing sarcoma because of its location, predominance in males, and lack of presenting metastases.¹²⁶
MR imaging has been used after chemotherapy to more accurately define tumor margins and assess the interval decrease in adjacent peritumoral edema.

FIGURE 14-81 ● Ewing sarcoma of the femur. (A) Coronal T1-weighted MR image demonstrates low-signal-intensity tumor involving the femoral diaphysis with extension into the metaphysis. (B) Coronal and (C) axial fat-suppressed T2-weighted fast spin-echo images show peritumoral edema and soft-tissue components (arrows). Edema, marrow, and soft-tissue components demonstrate low signal intensity on the T1-weighted image and high signal intensity on the T2-weighted images. (D) Axial fat-suppressed T1-weighted image after the administration of intravenous gadolinium shows enhancement of the soft-tissue component (arrows).

FIGURE 14-82 ● Ewing sarcoma. High-signal-intensity tumor and soft-tissue mass (arrows) are seen on coronal (A) and axial (B) fat-suppressed T2-weighted fast spin-echo images.

FIGURE 14-83 ● (A) On a T1-weighted image, marrow involvement (arrows) of Ewing sarcoma is seen as foci of decreased signal intensity. (B) On a fat-suppressed T2-weighted fast spin-echo image, foci of involved marrow show increased signal (arrows). (C) Tumor enhancement is seen on contrast-enhanced axial fat-suppressed T1-weighted image.

FIGURE 14-84 ● A lytic lesion involving the distal tibia from plasmacytoma is shown on a sagittal graphic illustration. Areas of internal hemorrhage are shown in orange.

FIGURE 14-85 ● Plasmacytoma of the right ilium. (A) Coronal T1-weighted MR image reveals an intermediate-signal-intensity mass (arrows) with aggressive cortical transgression and soft-tissue extension. (B) Axial fat-suppressed T2-weighted fast spin-echo image shows hyperintense tumor and soft-tissue mass (arrows). (C) Axial fat-suppressed T1-weighted image following intravenous administration of contrast demonstrates non-enhancing central areas of necrosis (arrows).

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Plasmacytoma

Plasmacytoma is a solitary neoplasm of plasma cells that sometimes converts into multiple myeloma.¹²⁷ Solitary plasmacytomas most commonly affect those in middle age (50 years) and frequently target the spine and pelvis. A solitary lytic or expansile lesion is common (Fig. 14-84), although an ivory vertebral pattern has been reported in the spine. MR characteristics of plasmacytoma are nonspecific low to intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images (Fig. 14-85). Aggressive cortical disruption and infiltration into soft tissue and adjacent structures, with discontinuity of adjacent low-signal-intensity cortical bone, can be identified on MR examination.

Adamantinoma

Adamantinoma of bone is a rare neoplasm with an uncertain histogenesis. The tibia is the site of involvement in 90% of cases (Fig. 14-86). Immunohistochemical data indicate an epithelial nature. Histologically, there is a spectrum of patterns ranging from spindled to squamoid, with characteristic basaloid cells. The peak incidence is in the third to fourth decades of life. Although these tumors are slow-growing and appear histologically bland, approximately 20% metastasize.¹²⁸

The MR imaging characteristics of adamantinoma may be similar to lesions of osteofibrous dysplasia, with low-signal-intensity sclerosis identified on T1- and T2-weighted images.

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Peritumoral edema or small focal areas of hyperintensity may be identified within the tibial cortex. These areas of increased signal intensity may correspond to osteolytic defects adjacent to sclerotic bone (Fig. 14-87). Adamantinomas are characteristically located in the middle third of the tibia and have a cortical or eccentric center.

Metastatic Disease

MR imaging is very sensitive in evaluating bone metastases.¹²⁹^,¹³⁰^,¹³¹^,¹³² Skeletal metastases of known origin usually involve the breast or prostate, whereas metastases of unknown origin often originate in the lung or kidney.¹³³ In general, metastatic bone disease appears as focal lesions that involve both cortical bone and marrow and demonstrate low signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences (Fig. 14-88). Metastatic deposits can be identified on MR images in patients with negative findings on conventional radiographs. Vascularity may assist in determining the site of tumor origin (e.g., renal cell carcinoma with low-signal-intensity vessels). Metastatic lung cancer often affects the tubular bones in the hands and feet (Fig. 14-89). High bone detail in cortical or trabecular destruction may require correlation with corresponding CT scans (Fig. 14-90).

The MR appearance of metastatic lesions is nonspecific for the site of origin.²⁴ Peritumoral soft-tissue edema may be

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observed in the absence of a soft-tissue mass. Sequential postcontrast MR images can help to differentiate tumor from perineoplastic edema.⁹ Osteolytic metastatic lesions, commonly from carcinoma of the lung or breast, often demonstrate a more uniform signal intensity on T1- and T2-weighted images than do mixed sclerotic or osteoblastic deposits from carcinoma of the prostate or medulloblastoma. Lesions of enostosis or osteopoikilosis may mimic blastic metastatic deposits; however, no aggressive features are associated with enostosis or osteopoikilosis (Fig. 14-91).

FIGURE 14-86 ● Adamantinoma of the tibia. Sagittal graphic illustration shows an adamantinoma involving the anterolateral cortex of tibia with associated anterior bowing.

FIGURE 14-87 ● (A) Sagittal T1-weighted images show an eccentric sclerotic adamantinoma in the anterior tibial diaphysis (arrows). (B) The tumor is hypo-intense on the axial T1-weighted image. (C) Axial fat-suppressed T2-weighted fast spin-echo image shows areas of cortical hyperintensity (arrow). (D) Corresponding axial CT image demonstrates sclerotic eccentric tumor with central lucency (arrow).

FIGURE 14-88 ● Metastatic thyroid cancer. (A) An anteroposterior radiograph shows cortical destruction and a large lytic lesion of the iliac wing (arrows). Axial CT scans (B and C) show destruction of the left iliac wing with a large associated soft-tissue mass (arrows). On coronal T1-weighted (D) and axial proton density-weighted (E) images, the low- to intermediate signal-intensity metastatic tumor and soft-tissue mass are noted (arrows). On the corresponding axial (F) and sagittal (G) STIR images, the metastatic tumor (arrows) demonstrates intermediate signal intensity, with central areas of fluid signal, indicating necrosis. (H) The axial fat-suppressed T1-weighted image following administration of intravenous gadolinium shows central nonenhancing areas, indicating necrosis (arrows).

FIGURE 14-89 ● Metastatic lung cancer to the foot. Coronal graphic illustration shows a metastatic lesion with a pathologic fracture (arrow) involving the metatarsal. Metastases to the hands and feet are usually from lung cancer.

FIGURE 14-90 ● Metastatic renal cell carcinoma involving the L1 and L3 vertebral bodies is seen on sagittal (A) and axial (B) CT images. The CT scan reveals the lytic destruction (arrows) in precise detail. (C) This sagittal STIR image shows hyperintense tumor with involvement of the posterior elements and extension into the spinal canal, causing cord compression. Marrow infiltration of the L3 vertebral body (arrowhead) is noted.

Primary Lymphoma of Bone

Originally designated “reticulum cell sarcoma of bone,” this disorder is now defined as a malignant lymphoma that arises within the medullary cavity of a single bone and occurs without concurrent regional lymph node or visceral involvement within a 6-month period (Figs. 14-92 and 14-93).¹³⁴^,¹³⁵ This rare disorder represents approximately 2% to 3% of malignant bone tumors and 5% of all extranodal lymphomas. Although all pathohistologic types of non-Hodgkin lymphoma have been reported to occur in bone, large cell (i.e., reticulum cell) lymphoma is the most common. Primary lymphoma of bone carries a better prognosis than disseminated non-Hodgkin lymphoma with secondary involvement of bone.¹³⁶ Mulligan and Kransdorf have reported sequestration in 11% of cases of primary lymphoma.¹³⁷ Sequestra also occur in osteomyelitis, Langerhans cell histiocytosis, fibro-sarcoma, malignant fibrous histiocytoma, and desmoplastic fibroma.

FIGURE 14-91 ● Asymptomatic low-signal-intensity multiple bone islands are seen on a sagittal proton density-weighted image (A), an axial T1-weighted image (B), and an axial fat-suppressed T2-weighted image (C) of the knee. The morphology and distribution of these bone islands are characteristic of osteopoikilosis (arrows).

FIGURE 14-92 ● Primary lymphoma of bone is shown as a hemorrhagic intramedullary mass in red on coronal graphic illustrations of the knee (A) and distal forearm (B).

FIGURE 14-93 ● Primary lymphoma of bone involving the distal femur. (A) Coronal T1-weighted images show low-signal-intensity lymphomatous marrow replacement crossing the physeal scar and involving the subchondral bone. (B) An extraosseous soft-tissue component is present on the axial T1-weighted image (arrows). (C) This sagittal proton density-weighted image demonstrates lymphomatous involvement of intermediate signal with associated soft-tissue component (arrows). On (D) sagittal and (E) axial fat-suppressed T2-weighted fast spin-echo images, the tumor is of intermediate signal and hyperintense compared to adjacent bone marrow. Associated soft-tissue mass (arrows) and surrounding bone marrow edema (black arrowhead) are noted. (F) Coronal and (G) axial contrast-enhanced fat-suppressed T1-weighted images show enhancement of the tumor with nonenhancing areas (arrows) representing necrosis.

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Soft-Tissue Neoplasms

In the evaluation of soft-tissue tumors, MR imaging offers the following advantages over CT:¹¹^,¹⁴^,¹³⁸^,¹³⁹^,¹⁴⁰^,¹⁴¹^,¹⁴²

Ability to assess associated cortical and marrow involvement
Improved depiction of the tissue planes surrounding the lesion and muscle textural patterns
Ability to assess neurovascular involvement

Gadolinium enhancement may be used to further characterize the extent, degree of necrosis, and recurrence of soft-tissue tumors.⁷⁰^,¹⁴³^,¹⁴⁴

MR demonstrates changes of radiation therapy and chemotherapy in the treatment of soft-tissue sarcomas.¹⁴⁵ These changes include irradiation effects on the epiphyseal plates (which are widened) and metaphysis, and suppression of myeloid elements. Chemotherapy-related myeloid suppression may lead to anemia, neutropenia, and subsequent stimulation of hematopoietic marrow. Pretherapy MR imaging and spectroscopy have been used by Sostman et al. in predicting the prognosis of patients with soft-tissue sarcomas.¹⁴⁴ By monitoring the relationship between pretherapy pH and T2 with tumor necrosis, they determined that higher values were associated with increased tumor necrosis.

Benign Soft-Tissue Neoplasms

Benign soft-tissue tumors are usually homogeneous and clearly marginated and do not involve neurovascular structures. Malignant soft-tissue lesions tend to be heterogeneous, with irregular margins and surrounding muscle edema. With long TR/TE sequences (TR 2,500 msec, TE 80 msec), a tumor may demonstrate higher signal intensity than associated edema.

Pearls and Pitfalls

Benign Soft-Tissue Neoplasms

Benign soft-tissue tumors are usually homogeneous and well marginated.
Lipomas follow the signal intensity of fat on all pulse sequences. Low-signal-intensity septations may be present.
Hemangiomas are ill defined, with areas of high T1 signal due to fat or slow-flowing blood.
If MR imaging is nondiagnostic for hemangioma, it is important to obtain radiographs, since phleboliths can be missed on MR examinations.
Fibromatosis represents a group of benign disorders characterized by fibrous growth and a tendency to infiltrate adjacent tissues and to recur. They may appear to be very aggressive and may be mistaken for sarcoma.
Juvenile fibromatosis demonstrates low signal on T1- and T2-weighted images.
Benign peripheral nerve sheath tumors are best seen on long axis (coronal or sagittal) imaging planes and appear as fusiform masses entering and exiting a nerve.

Lipoma

Soft-tissue lipomas are homogeneous and clearly marginated, with or without internal fibrous separations. Lipomas are the most frequent soft-tissue tumor, and they usually appear in the fifth or sixth decade of life. Most commonly, these soft-tissue neoplasms are found in the subcutaneous regions of the back and shoulder, but they can be found in any subcutaneous location (Fig. 14-94). Less frequently, they occur in deeper locations such as within the thigh, anterior mediastinum, retroperitoneum, or gastrointestinal wall. Deep-seated intramuscular lipomas, especially in the paraspinal region, are rarely encapsulated and frequently recur.¹⁴⁶

FIGURE 14-94 ● Subcutaneous lipoma, shown in yellow, of the anterior thigh.

FIGURE 14-95 ● A soft-tissue lipoma (arrows) located in the posterior chest wall, superficial to the deltoid muscle, demonstrates subcutaneous fat signal intensity on an axial T1-weighted image (A), which suppresses on the fat-suppressed T2-weighted fast spin-echo image (B). (C) No enhancement is seen on the corresponding fat-suppressed T1-weighted image after intravenous gadolinium administration.

FIGURE 14-96 ● Intramuscular lipoma of the vastus intermedius (arrows) showing fat signal intensity on a T1-weighted axial image (A), which suppresses on the fat-suppressed T1-weighted coronal image (B) after intravenous gadolinium administration. No significant enhancement is seen.

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Lipomas demonstrate high signal intensity on T1-weighted images and do not increase in signal intensity on T2 or fat-suppressed T2-weighted fast spin-echo sequences (Fig. 14-95).¹¹ Low-signal-intensity septations may be seen within these lesions on T1- and T2-weighted images. Lipomas do not show contrast enhancement.

Deep lipomas occurring in the extremities commonly involve either the shoulder or thigh (Figs. 14-96 and 14-97). Although intramuscular lipomas are almost always well-defined lesions, on occasion they may have ill-defined borders and may demonstrate infiltration into adjacent muscle tissue (Figs. 14-98 and 14-99). The lesions of lipomatosis are ill defined, with infiltration into surrounding structures (Fig. 14-100). MR imaging is useful for presurgical evaluation of parosteal lipomas, clearly identifying associated muscle atrophy and nerve involvement.¹⁴⁷

FIGURE 14-97 ● Intramuscular lipoma of the shoulder (arrows) demonstrates bright fat signal intensity on coronal T1-weighted image (A). Fat signal suppresses on the corresponding coronal (B) and axial (C) fat-suppressed T2-weighted fast spin-echo images.

FIGURE 14-98 ● Axial graphic illustration demonstrates ill-defined intramuscular lipoma (in yellow) of the thigh.

FIGURE 14-99 ● Large infiltrative lipoma of the calf demonstrates high signal intensity on coronal (A) and axial (B) T1-weighted images and low signal on corresponding coronal (C) and axial (D) fat-suppressed T2-weighted fast spin-echo images. (E) No significant enhancement is seen on the contrast-enhanced axial T1-weighted fat-suppressed image.

FIGURE 14-100 ● Axial graphic illustrations showing lipomatosis of the chest and abdominal wall in yellow. (A) Diffuse infiltration of the paraspinal muscles and left chest wall and (B) infiltration of the paraspinal muscles and left abdominal wall.

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Hemangioma

Hemangiomas can occur in bone or soft tissues and may vary widely in appearance. Histopathologically, they range from the cavernous type, with very large vascular spaces separated by fibrous tissue, to the capillary type—very cellular and lacking fibrous septa. Radiographs may reveal phleboliths within a soft-tissue mass and underlying osseous changes, such as periosteal or cortical thickening (Figs. 14-101 and 14-102).¹⁴⁸ Intramuscular hemangiomas (Fig. 14-103) are ill-defined tumors with a predilection for the thigh muscles of young adults.¹⁴⁹ Pain is frequently the presenting symptom. The intramuscular type of hemangioma is associated with variable amounts of fat, smooth muscle, myxoid stroma, and hemosiderin. Although clearly benign histologically, these intramuscular hemangiomas may recur.¹⁴⁶

On MR examination, hemangiomas demonstrate intermediate signal intensity with areas of high signal intensity (representing fat or slow-flowing blood) on T1-weighted images and high signal intensity on T2-weighted images (Fig. 14-104).¹⁵⁰ Because of paramagnetic effects, central hemorrhage with hemosiderin deposits or peripheral hemosiderin-laden macrophages demonstrate low signal intensity on T1- and T2-weighted images. In selected cases, feeding vessels can be identified. There may be secondary involvement of

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cortical bone. Hemangiomas can be treated by CT-guided percutaneous ethanol injection (Fig. 14-105).¹⁵¹^,¹⁵²^,¹⁵³^,¹⁵⁴^,¹⁵⁵^,¹⁵⁶

FIGURE 14-101 ● Cavernous hemangioma of the forearm is shown in red. Central phleboliths are shown in gray.

FIGURE 14-102 ● Hemangioma of the hand with calcified phleboliths. Extensive soft-tissue swelling involving the thumb, index finger, and thenar region is noted.

FIGURE 14-103 ● Hemangioma within the vastus medialis (arrows) demonstrates high signal intensity on a coronal T1-weighted image (A) and bright signal intensity on coronal (B) fat-suppressed T2-weighted fast spin-echo images. (C) Hemangioma within the vastus medialis (arrow) demonstrates bright signal intensity on axial fat-suppressed T2-weighted fast spin-echo images. Fat-suppressed T1-weighted image following the intravenous administration of gadolinium (D) shows enhancement of the lesion.

FIGURE 14-104 ● Intramuscular hemangioma of the brachialis muscle (arrows). Sagittal (A) and axial (B) T1-weighted and corresponding fat-suppressed T2-weighted sagittal (C) and axial (D) images show an ill-defined serpiginous soft-tissue mass. Fat content within the hemangioma is bright on the T1-weighted images (A and B) and suppresses on the fat-suppressed T2-weighted fast spin-echo images (C and D).

FIGURE 14-105 ● Axial CT images from alcohol ablation of intramuscular hemangioma of the biceps. (A) A soft-tissue mass is noted on this pretreatment image. Fat within the lesion is hypodense (arrow). (B through D) Injection of iodinated contrast and ethanol is demonstrated on axial CT images.

Arteriovenous Malformation

Arteriovenous malformations of the soft tissues are seen as an irregular tangle of vessels that demonstrate low signal intensity on T1- and T2-weighted images in areas of rapidly flowing blood. In Klippel-Weber-Trenaunay syndrome (unilateral cutaneous capillary hemangiomas, varicose veins, and local soft-tissue and osseous hypertrophy), arteriovenous malformations are associated with bone lesions and are hyperintense on T2, fat-suppressed T2-weighted fast spin-echo, and STIR sequences. The high-signal-intensity lesion has a serpiginous morphology or pattern of tangled vessels. Low-signal-intensity foci of hemosiderin or vessels can also be seen.

Hemangiopericytoma

Hemangiopericytomas are hypervascular soft-tissue tumors thought to originate from the capillary cell wall.⁴⁰ Pericytes (cells present in the walls of capillaries and venules) can be demonstrated with electron microscopy and identified as the exact cell of origin. Most of these neoplasms are small, but on rare occasions they may achieve diameters of 8 cm or more. Despite a benign histopathologic pattern, hemangiopericytomas can recur, and as many as 50% metastasize to lungs, bone, and liver. Their biologic behavior, therefore, cannot always be predicted from histology. Hemangiopericytomas usually present as a deep soft-tissue mass of the thigh.¹⁴⁶

The peripheral arterial branching seen in angiograms is depicted on T2-weighted MR images as an intricately packed network of vessels demonstrating signal void. It is in contrast to the adjacent tumor, which is bright (Fig. 14-106).

Fibromatosis

Fibromatosis refers to a group of benign lesions characterized by proliferation of benign fibrous tissue and the tendency to infiltrate adjacent tissues and to recur. The MR imaging appearance of soft-tissue fibromatosis is variable, with hyperintense, isointense, hypointense, or mixed signal intensity relative

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to adjacent skeletal muscle.¹⁵⁷ Increased signal intensity on T1-weighted images can be attributed to fat, protein, or both. Lesions of fibromatosis may also show varying degrees of increased signal intensity on T2, T2*, and STIR sequences (Fig. 14-107). Axial and sagittal MR images are useful in assessing the fibrous lesions of plantar fibromatosis. These fibrous nodules, which involve the plantar aponeurosis, demonstrate low signal intensity on T1- and T2-weighted sequences (Figs. 14-108 and 14-109).

FIGURE 14-106 ● (A) Hemangiopericytoma of the vastus medialis and intermedius muscles shows hypervascularity (large arrow) and erosion of the adjacent cortex (small arrows) on an anteroposterior radiograph. T2-weighted coronal (B) and axial (C) images show low-signal-intensity hypervascularity (small black arrows), cortical erosion (small white arrows), and bright signal intensity (large arrow).

Desmoid Tumor

Desmoid tumors (i.e., aggressive fibromatosis) exhibit biologic aggressiveness that lies somewhere between a reactive fibrous proliferation and a low-grade fibrosarcoma.¹⁴⁶ These lesions appear most frequently in the second to fourth decades, although they may occur at any age. They are usually large, infiltrative, poorly demarcated fibrous masses that tend to recur if incompletely surgically excised. They do not, however, exhibit the ability to metastasize.¹⁴⁶^,¹⁵⁸ On MR examination desmoid tumors are characterized by low-signal-intensity fibrous bands traversing the tumor, which demonstrates low to intermediate signal intensity on T1-weighted images and T2-weighted images (Fig. 14-110). An association between multicentric fibromatosis and metaphyseal dysplasia (widening and flaring of the metaphysis and cortical thinning) has been reported.¹⁵⁹

Juvenile Fibromatosis

Juvenile fibromatosis is a locally invasive tumor that demonstrates low signal intensity on T1- and T2-weighted images (Fig. 14-111). It resembles fibrous desmoid tumors and may recur after initial excision.

Cystic Hygroma

Cystic hygromas form as the result of budding lymphatics and may arise anywhere in the body. They are most frequently encountered in the neck and axilla and often encompass

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neurovascular structures. They exhibit the same signal intensity as fluid on T1- and T2-weighted images (Fig. 14-112).¹⁶⁰

FIGURE 14-107 ● Fibromatosis of the calf. (A) Sagittal T1-weighted and (B) fat-suppressed T2-weighted fast spin-echo images show a soft-tissue mass in the upper calf. The tumor is of intermediate signal intensity on the T1-weighted image and demonstrates high signal intensity with low-signal-intensity fibrous stroma on the fat-suppressed T2-weighted images (arrows).

FIGURE 14-108 ● Plantar fibromatosis is seen as a heterogeneous mass in gray infiltrating the adjacent muscles.

FIGURE 14-109 ● Plantar fibromatosis (arrows). A soft-tissue mass is shown at the plantar surface of the foot on (A) a sagittal T1-weighted image, sagittal (B) and axial (C) STIR images, and (D) an axial proton density-weighted image. The mass is of intermediate signal on the T1- and proton density-weighted images and displays intermediate to low signal on the STIR image. (E) There is enhancement on this post-contrast fat-suppressed T1-weighted sagittal image.

FIGURE 14-110 ● Desmoid tumor of the calf (arrows). Coronal (A) and axial (B) T1-weighted images show a large heterogeneous soft-tissue mass in the posterior calf. (C) On an axial fat-suppressed T2-weighted fast spin-echo image, the mass is of intermediate signal with low-signal fibrous strands. (D) A contrast-enhanced axial fat-suppressed T1-weighted image shows heterogeneous enhancement. (E) A corresponding axial CT image demonstrates a hetero-geneous mass with areas of low density.

FIGURE 14-111 ● Juvenile fibromatosis of the forearm. Fibrous tissue demonstrates low signal intensity (arrows) on coronal (A) and axial (B) T1-weighted images and sagittal (C) and axial (D) fat-suppressed T2-weighted fast spin-echo images. Enhancement is noted on a coronal T1-weighted fat-suppressed image (E) after intravenous administration of gadolinium.

Benign Peripheral Nerve Sheath Tumor

Benign peripheral nerve sheath tumors (BPNST) are divided into schwannomas (neurilemmomas) and neurofibromas. They represent a group of benign lesions of neural origin that occur in peripheral nerves, soft tissue, skin, or bone. They contain cells that are closely related to Schwann cells. Neurofibromas represent an unencapsulated nerve sheath tumor and occur in three forms:

Solitary localized nodules
Diffuse thickening of skin and subcutaneous tissues
“Plexiform” tumors representing worm-like multinodular growths that expand to contiguous major or minor nerves

Neurofibromatosis type 1 (von Recklinghausen disease) is a hereditary, autosomal dominant, hamartomatous disorder that

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involves the neuroectoderm, mesoderm, and endoderm and is associated with multiple neurofibromas. The plexiform pattern is characteristic of neurofibromatosis (Fig. 14-113).¹⁴⁶

FIGURE 14-112 ● Cystic hygroma of the parapharyngeal space. (A) Axial T1-weighted image shows areas of low signal intensity (arrows) deep to the right mandible. (B) On the corresponding axial fat-suppressed T2-weighted fast spin-echo image, the fluid-filled cystic structure demonstrates high signal intensity. (C) No enhancement of the mass is seen on this fat-suppressed axial T1-weighted image after administration of gadolinium (arrows).

Benign peripheral nerve sheath tumors are best seen on long axis MR images as circumscribed fusiform masses entering and exiting a nerve (Fig. 14-114). The following characteristics are seen:

They demonstrate low to intermediate signal intensity on T1-weighted images and uniform bright signal intensity on T2-weighted images and on gadolinium-enhanced T1-weighted images.
The target sign with a center of low signal due to collagen and condensed Schwann cells is frequently seen (Fig. 14-115).¹⁶¹
In patients with neurofibromatosis, MR imaging is used not only to evaluate the soft-tissue extent of disease, but also to assess spinal canal, adjacent cortical bone, and marrow involvement.¹⁶²
Plexiform neurofibromas may be distinguished from non-plexiform types by the presence of longitudinal tracking along neural fascicles in a lobulated fashion (Figs. 14-116 and 14-117).

FIGURE 14-113 ● Multiple spinal plexiform neurofibromas in a patient with neurofibromatosis type 1. Hemorrhage within a lesion is shown in red.

FIGURE 14-114 ● Large neurofibroma (arrows) arising from the sciatic nerve in a patient with neurofibromatosis type 1. The tumor is hyperintense on the coronal fat-suppressed T2-weighted fast spin-echo image. There is enlargement of the sciatic nerve.

FIGURE 14-115 ● Schwannoma arising from the lumbar spine. (A) A sagittal T1-weighted image showing a circumscribed hypointense mass (arrow) causing expansion of the neural foramen and scalloping of the L2 vertebral body. The schwannoma is hyperintense on sagittal fat-suppressed T2-weighted fast spin-echo images (B) (arrow). (C) The schwannoma is hyperintense on axial fat-suppressed T2-weighted fast spin-echo images. An axial contrast-enhanced T1-weighted image (D) and a sagittal contrast-enhanced T1-weighted image with fat suppression (E) demonstrate the target sign with a central low-signal-intensity area and surrounding hyperintensity (arrows).

FIGURE 14-116 ● Plexiform neurofibroma (arrows) in a child with neurofibromatosis type 1. An extensive lobular soft-tissue mass is seen extending along the anterior tibia. It displays low signal on a sagittal T1-weighted image (A) and high signal on the corresponding fat-suppressed T2-weighted fast spin-echo image (B). (C) Contrast enhancement of the mass is seen on a sagittal fat-suppressed T1-weighted image following administration of intravenous gadolinium. Tibial dysplasia with bowing is noted.

FIGURE 14-117 ● Sagittal (A) and axial (B) fat-suppressed T2-weighted fast spin-echo images display multiple T2 hyperintense plexiform neurofibromas (arrows) arising from the lumbar spine in a patient with neurofibromatosis type 1. No compression of the spinal canal is seen.

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Malignant Soft-Tissue Neoplasms

Pearls and Pitfalls

Malignant Soft-Tissue Neoplasms

Liposarcomas often do not contain visible fat on MR images.
Myxoid liposarcomas may appear as cystic lesions on T2-weighted images, so gadolinium contrast is necessary to diagnose solid lesions.
Consider malignant transformation when a preexisting neurofibroma enlarges.
FDG-PET is useful in detecting malignant change in plexiform neurofibromas.
Synovial sarcomas do not arise from synovium or joints.
Synovial sarcomas can be misdiagnosed as benign lesions, such as ganglions, if no intravenous contrast is administered.
Malignant fibrous histiocytoma often presents as a hemorrhagic mass, and extensive areas of hemorrhage can obscure the underlying neoplasm. Therefore, intravenous contrast is required for evaluation of suspected hematomas.

Liposarcoma

Arising from primitive mesenchymal cells rather than adult fat cells, liposarcomas may appear anywhere in the body without regard to adipose tissue. They are frequently found in the extremities, and the myxoid type commonly involves the thigh and popliteal region.¹⁶³ Peak incidence is in the fifth to seventh decades. These neoplasms arise in deep structures, and although they appear circumscribed, they are commonly multilobular, with projections that creep between tissue planes. Areas of hemorrhage, necrosis, and cystic change may be present (Fig. 14-118). Four histopathologic variants have been characterized:

Well differentiated
Round cell
Myxoid
Pleomorphic

Myxoid liposarcomas are the most commonly observed variant. Biologic aggressiveness and a tendency for distant metastases are expressed primarily in the round cell and pleomorphic variants.

On MR images liposarcomas appear more heterogeneous than lipomas (Fig. 14-119)¹⁴⁶^,¹⁶⁴ and display the following additional characteristics:

On T1-weighted images, lesions with a greater cellular component may demonstrate lower signal intensity relative to fatty elements.
Focal areas of malignant change within a lipomatous matrix demonstrate low to intermediate signal intensity on T1-weighted sequences and high signal intensity on T2-weighted and contrast-enhanced sequences (Fig. 14-120).¹⁶⁵
Well-differentiated liposarcomas demonstrate fat composition with thick septa, which are hyperintense on T2-weighted images.¹⁵⁹
Although well-differentiated subtypes contain fat, other histopathologic types may not demonstrate fat signal intensity.
Myxoid tumors are mildly heterogeneous, and the more aggressive round cell and pleomorphic subtypes are

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more heterogeneous. Heterogeneity is associated with high-grade liposarcomas.
Myxoid liposarcomas are hyperintense on fat-suppressed T2-weighted fast spin-echo images (Fig. 14-121). Gadolinium enhancement may help to demonstrate central myxoid degeneration, not always seen on T2-weighted images.

FIGURE 14-118 ● Axial graphic illustration showing liposarcoma in yellow. Central areas of necrosis are shown in brown.

FIGURE 14-119 ● (A) T1-weighted axial image through the lower leg displays a high-signal-intensity lipomatous mass in the soleus muscle (black arrows). Central areas of low signal intensity are present (white arrow). Corresponding axial fat-suppressed T2-weighted fast spin-echo image (B) and axial fat-suppressed T1-weighted contrast-enhanced image (C) show sarcomatous change as high signal intensity and enhancement (arrows).

FIGURE 14-120 ● Liposarcoma of the medial thigh. (A) Coronal T1-weighted image and (B) coronal contrast-enhanced fat-suppressed T1-weighted image demonstrate a mixture of high- and low-signal abnormalities. Sarcomatous change is low signal on the T1-weighted image and demonstrates enhancement on the fat-suppressed T1-weighted image following administration of intravenous gadolinium (arrows).

FIGURE 14-121 ● Myxoid liposarcoma of the anterior thigh (arrows). (A) Axial T1-weighted image demonstrates a low-signal soft-tissue mass, isointense to muscle, located between the sartorius, vastus intermedius, and rectus femoris muscles. The mass appears cystic and hyperintense on axial (B) and sagittal (C) fat-suppressed T2-weighted fast spin-echo images. Axial (D) and coronal (E) fat-suppressed T1-weighted images after the administration of intravenous gadolinium show heterogeneous enhancement, indicating a solid mass.

Leiomyosarcoma

Leiomyosarcomas are uncommon malignant neoplasms of soft tissues arising from smooth muscle cells. They may arise in the retroperitoneum, peripheral soft tissues, or blood vessels (Fig. 14-122). Leiomyosarcomas demonstrate isointensity with muscle tissue on T1-weighted images and a uniform increase in signal intensity on T2-weighted images.

FIGURE 14-122 ● Leiomyosarcoma of the thigh on coronal (A) and axial (B) graphic illustrations. Bone metastasis is shown in brown. Axial graphic illustration (C) shows leiomyosarcoma of the foot.

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Malignant Peripheral Nerve Sheath Tumor

The development of malignant peripheral nerve sheath tumors (MPNST) (neurofibrosarcoma and malignant schwannoma) may be characterized by a large infiltrative, often hemorrhagic mass related to the neurovascular bundle (Figs. 14-123 and 14-124). Irregular areas of necrosis and heterogeneity on T1- and T2-weighted images are frequently seen (Fig. 14-125).¹⁶⁶^,¹⁶⁷^,¹⁶⁸

Synovial Sarcoma

The term synovial sarcoma is considered a misnomer, because studies do not support the synovial cell as the cell of origin for these tumors. Instead, a multipotential mesenchymal cell has been identified as the likely source.¹⁴⁶^,¹⁴⁷^,¹⁴⁸^,¹⁴⁹^,¹⁵⁰^,¹⁵¹^,¹⁵²^,¹⁵³^,¹⁵⁴^,¹⁵⁵^,¹⁵⁶^,¹⁵⁷^,¹⁵⁸^,¹⁵⁹^,¹⁶⁰^,¹⁶¹^,¹⁶²^,¹⁶³^,¹⁶⁴^,¹⁶⁵^,¹⁶⁶^,¹⁶⁷^,¹⁶⁸^,¹⁶⁹ Synovial sarcomas tend to occur in younger patients, 15 to 35 years of age, in proximity to joints. They commonly involve the lower extremities in an extra-articular location (Fig. 14-126). Less than 10% of these tumors arise within the joint cavity. Spotty calcification is present in 15% of cases, and the histopathologic pattern is biphasic, with epithelial and spindled areas.¹⁴⁶ Synovial sarcomas have a propensity to metastasize to lymph nodes.

MR imaging is useful for staging the intra-articular and extra-articular involvement of synovial sarcoma. The lesions demonstrate the following characteristics:

Synovial sarcomas display low to intermediate signal intensity on T1-weighted images and are homogeneously bright on T2-weighted images. It is important to note that the increased signal intensity on T2 or fat-suppressed T2-weighted fast spin-echo sequences is less than that of fluid, and it should not be mistaken for a cystic fluid collection or ganglion if intermediate-signal-intensity areas are present.
Small areas of central necrosis can be seen as regions of higher signal intensity (Fig. 14-127).
Focal calcifications, although detected on MR images, are better delineated on CT scans or radiographs (Fig. 14-128).
A heterogeneous, multilocular mass with internal septation may be the characteristic feature on MR images. Extensive loculations with multiple fluid–fluid levels secondary to hemorrhage have also been observed.¹⁷⁰

FIGURE 14-123 ● Malignant peripheral nerve sheath tumor associated with neurofibromatosis. Coronal graphic illustration shows a large fusiform mass in the thigh. The sciatic nerve is seen entering and exiting the mass. Numerous superficial neurofibromas are present.

FIGURE 14-124 ● (A) Coronal T1-weighted image of a malignant peripheral nerve sheath tumor of the adductor compartment shows high-signal hemorrhage (arrow) within the tumor. (B) Axial fat-suppressed T2-weighted fast spin-echo image shows a hyperintense cystic area (arrow) with a fluid–fluid level, corresponding to the hemorrhage. (C) Axial fat-suppressed T1-weighted image after the intravenous administration of gadolinium shows large nonenhancing hemorrhagic area (arrow) within the tumor.

FIGURE 14-125 ● Malignant peripheral nerve sheath tumor arising from a plexiform neuro-fibroma in a patient with neurofibromatosis type 1. (A) Coronal T1-weighted image shows two large masses arising from the right sciatic nerve (arrows). Enlargement of the left sciatic nerve is also noted. (B) Corresponding coronal fat-suppressed T2-weighted fast spin-echo image demonstrates typical T2 hyperintensity of the more inferior mass (black arrow), likely representing a benign neurofibroma. The superior mass is of intermediate signal intensity (white arrow). (C) Axial fat-suppressed T2-weighted fast spin-echo image demonstrates heterogeneity and intermediate signal of the more superior mass arising from the sciatic nerve. (D) The corresponding axial contrast-enhanced fat-suppressed T1-weighted image shows heterogeneous enhancement, with nonenhancing areas (arrows) representing necrosis. (E) An axial image from a whole-body PET scan shows markedly increased FDG uptake within the mass (arrow), indicating malignant transformation.

FIGURE 14-126 ● Coronal graphic illustration shows a synovial sarcoma of the foot in gray. Internal calcifications are present. The mass erodes the proximal phalanx of the second toe.

FIGURE 14-127 ● Large synovial sarcoma of the thenar eminence. (A) Coronal T1-weighted image demonstrates a large heterogeneous mass within the thenar eminence. Areas of high signal are consistent with hemorrhage/necrosis (arrows). (B) Sagittal fat-suppressed T2-weighted fast spin-echo image shows hyperintensity of the mass. Sagittal (C) and axial (D) contrast-enhanced fat-suppressed T1-weighted images show heterogeneous enhancement, with nonenhancing areas (arrows) corresponding to necrosis/hemorrhage.

FIGURE 14-128 ● Synovial sarcoma of the popliteal fossa. (A) Lateral radiograph shows subtle calcifications in the popliteal fossa (arrow). (B) Sagittal T1-weighted image demonstrates a large lobulated soft-tissue mass of low to intermediate signal intensity (arrows). (C) The mass is hyperintense on an axial fat-suppressed T2-weighted fast spin-echo image (arrow). After administration of intravenous gadolinium, the corresponding sagittal (D) and axial (E) fat-suppressed T1-weighted images show a lobulated heterogeneously enhancing soft-tissue mass (arrows).

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Malignant Fibrous Histiocytoma

Malignant fibrous histiocytoma (MFH) of both bone and soft tissue has been characterized on MR images (Figs. 14-129 and 14-130).¹¹ MFH of soft tissues is a sarcoma that occurs most frequently in patients 50 to 70 years of age. Areas of hemorrhage and necrosis are commonly seen within this often large, multinodular, hypervascular tumor. MR features of MFH are nonspecific. Signal inhomogeneity with low to intermediate intensity on T1-weighted images and high intensity on T2-weighted images reflects the distribution of hemorrhage, necrosis, and calcification. Low-signal-intensity vessels correspond to hypervascular regions of the tumor. This lesion may be mistaken for a traumatic fluid collection such as hemorrhage when the signal intensity is homogeneous and no aggressive features are seen. Intravenous gadolinium enhancement is useful in this situation, since fluid will not enhance (Figs. 14-131 and 14-132).

FIGURE 14-129 ● Pleomorphic malignant fibrous histiocytoma. Coronal graphic illustration shows a large soft-tissue mass in the lateral thigh.

FIGURE 14-130 ● Malignant fibrous histiocytoma of bone. Coronal graphic illustration shows an intramedullary tumor of the distal femur and proximal tibia with hemorrhagic and necrotic changes and cortical disruption.

FIGURE 14-131 ● Malignant fibrous histiocytoma of the proximal tibia is shown on (A) a coronal T1-weighted image, (B) an axial fat-suppressed T2-weighted fast spin-echo image, and fat-suppressed contrast-enhanced T1-weighted coronal (C) and axial (D) images. On the contrast-enhanced images (C and D), tumor necrosis is hypointense (white arrows), whereas viable tumor enhances (black arrows). (E) Axial CT image demonstrates cortical destruction and associated soft-tissue mass (arrows).

FIGURE 14-132 ● Malignant fibrous histiocytoma is seen as a large soft-tissue mass involving the posterior calf. (A) The axial T1-weighted image demonstrates areas of high signal intensity, consistent with hemorrhage (arrows). The mass is of heterogeneous signal intensity on the fat-suppressed T2-weighted fast spin-echo image (B) and demonstrates rim enhancement (arrows) on the fat-suppressed T1-weighted axial image following administration of intravenous gadolinium (C).

FIGURE 14-133 ● Fibrosarcoma of soft tissues. Axial graphic illustration shows a heterogeneous mass in the gluteus maximus with central necrosis.

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Fibrosarcoma

Fibrosarcoma is a malignant tumor with a fibrous, proliferative matrix devoid of any cartilage, osteoid, or bone (Figs. 14-133 and 14-134). Fibrosarcoma of bone has a poorer prognosis than primary soft-tissue fibrosarcoma.¹¹ MR characteristics of fibrosarcoma include low signal intensity on T1-weighted images, reflecting histologic differentiation; hyperintensity on T2-weighted images; and areas of enhancement with intravenous gadolinium¹⁷¹ (Fig. 14-135). Soft-tissue fibrosarcoma may be identified with or without evidence of secondary cortical erosion or destruction. Fibrosarcomas that occur in children are different from those seen in adults. In children, the tumor usually occurs within the first 2 years of life, and the prognosis is more favorable than in adults (Fig. 14-136).

FIGURE 14-134 ● Fibrosarcoma of bone. Coronal graphic illustration shows a gray-white to tan mass in the distal femur with cortical destruction and soft-tissue extension. Internal hemorrhage is shown in red.

FIGURE 14-135 ● Soft-tissue fibrosarcoma presenting as a heterogeneous mass at the lateral aspect of the knee (arrows). (A) Coronal T1-weighted, (B) axial fat-suppressed T2-weighted, and (C) coronal fat-suppressed T1-weighted contrast-enhanced images show subcutaneous tumor mass.

FIGURE 14-136 ● Soft-tissue fibrosarcoma of the thigh in a newborn. The tumor demonstrates low signal intensity on a T1-weighted sagittal image (A) (arrows) and high signal intensity on a fat-suppressed T2-weighted axial fast spin-echo image (B). Septal enhancement is noted on the axial fat-suppressed T1-weighted image following administration of intravenous gadolinium (C).

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Tumorlike Lesions

Myositis Ossificans

Myositis ossificans is a circumscribed mass of granulation tissue that may calcify or ossify with time (Fig. 14-137). A history of blunt trauma to muscle is often present. If the trauma is not recalled and no calcification is seen on radiographs or CT, patients are frequently evaluated for a soft-tissue mass. The MR imaging appearance is variable and depends on the stage of the lesion. Acute lesions are isointense to muscle on T1-weighted images and demonstrate heterogeneous signal on T2-weighted images. There may be a large area of surrounding edema. Mature lesions (older than 3 months) may have central signal intensity on T1- and T2-weighted images that is isointense to fat, representing bone marrow. Edema is generally not present after the first few weeks following the injury. Radiographs and CT scans that demonstrate peripheral calcification and a cleft between the mass and cortex of underlying bone (Fig. 14-138) are often more helpful in making the specific diagnosis of myositis ossificans.¹⁷²^,¹⁷³^,¹⁷⁴^,¹⁷⁵

Bone Infarct

Bone infarcts, usually metaphyseal-based, are circumscribed by a serpiginous low-signal-intensity border that corresponds to the rim of reactive sclerosis that is often detected on radiographs (Fig. 14-139).¹⁷⁶ In the acute and subacute stages, signal from the central island of the infarct is isointense with fatty marrow signal. With T2 weighting there may be linear bright signal parallel with the outline of the lesions (Fig. 14-140).

FIGURE 14-137 ● Myositis ossificans shows peripheral mineralization with surrounding edema.

FIGURE 14-138 ● Myositis ossificans of the thigh. Axial T1-weighted image (A) demonstrates a heterogeneous soft-tissue mass with high T1 signal (arrow) in the anterior thigh. On the sagittal (B) and axial (C) fat-suppressed T2-weighted fast spin-echo images, central cystic areas (black arrows) and marked surrounding edema (white arrows) are noted. Sagittal (D) and axial (E) fat-suppressed T1-weighted contrast-enhanced images show central nonenhancing cystic areas (arrows) and surrounding enhancement. On MR images, the lesion has an aggressive appearance and might be mistaken for a soft-tissue sarcoma. Anteroposterior radiograph (F) demonstrates characteristic peripheral calcifications in the area of the soft-tissue lesion (arrows), confirming the diagnosis of myositis ossificans. Axial CT images (G and H) demonstrate characteristic peripheral calcifications in the area of the soft-tissue lesion (arrows), confirming the diagnosis of myositis ossificans.

FIGURE 14-139 ● Bone infarcts of the shoulder and knee with characteristic serpentine lines.

FIGURE 14-140 ● (A) Low-signal-intensity circumscribed bone infarcts are present in the distal tibia and talus (arrows), as seen on a sagittal T1-weighted image. (B) The infarcts are hyperintense on this coronal fat-suppressed T2-weighted fast spin-echo image (arrows). Central fatty marrow signal intensity is characteristic of bone infarction.

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Hematoma

Hematomas are confined collections of blood that are well defined with a masslike character (Fig. 14-141). The appearance of hematomas on MR imaging is variable and is age-dependent. Acute hemorrhage demonstrates intermediate signal on T1- and T2-weighted images, whereas subacute hemorrhage shows hyperintensity on T1-weighted images similar to that of fat. Chronic hematomas often have peripheral low signal intensity on T1- and T2-weighted images due to hemosiderin deposits and fibrosis (Fig. 14-142).¹⁷⁷^,¹⁷⁸^,¹⁷⁹

FIGURE 14-141 ● Axial graphic illustration shows organized hematoma of the chest wall in red.

FIGURE 14-142 ● Intramuscular hematoma of the vastus intermedius occurring along the myotendinous junction. A soft-tissue mass with areas of high signal intensity is seen on coronal (A) and axial (B) T1-weighted images (arrows). (C) Marked surrounding edema is noted on this axial fat-suppressed T2-weighted fast spin-echo image (arrows). Following the administration of intravenous gadolinium, there is only mild surrounding enhancement on the coronal (D) and axial (E) fat-suppressed T1-weighted images (arrows).

FIGURE 14-143 ● A synovial wrist ganglion (arrows) is seen on both axial T1-weighted (A) and fat-suppressed T2-weighted (B) fast spin-echo images. Mucinous synovial contents demonstrate intermediate signal intensity on the T1-weighted images and high signal intensity on the fat-suppressed T2-weighted images.

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Ganglion Cyst

Ganglion cysts, thin-walled cysts filled with clear mucinous fluid, occur in soft tissues near joints. They are most commonly found around the hands and feet, especially the wrist, and are believed to represent either synovial herniations or mucinous degenerations of dense fibrous connective tissue, possibly related to trauma. These lesions may erode adjacent bone and become completely intraosseous (intraosseous ganglia).¹⁸⁰^,¹⁸¹ The most common site of intraosseous ganglion cysts is the medial malleolus of the tibia. Synovial ganglion cysts project into the soft tissues, are well defined, and demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images (Fig. 14-143). Septations, when present, are best seen on T2-weighted images, where they are outlined by the bright signal intensity of fluid.

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