Benign Soft Tissue Tumors

By admin Last updated Mar 31, 2023

Ovid: Oncology and Basic Science

Editors: Tornetta, Paul; Einhorn, Thomas A.; Damron, Timothy A.

Title: Oncology and Basic Science, 7th Edition

> Table of Contents > Section III – Specific Soft Tissue Neoplasms and Simulators > 11 – Benign Soft Tissue Tumors

Benign Soft Tissue Tumors

Timothy A. Damron

Gustavo De La Roza

Benign soft tissue tumors outnumber soft tissue sarcomas
by approximately 100:1. In adults, lipomas are the most common soft
tissue tumor; in children, hemangiomas are the most common. Recognition
of these relatively more common benign tumors is important not only to
avoid overtreatment but also to allow distinction from their malignant
counterparts. Soft tissue tumors, both benign and malignant, are
classified according to the purported cell of origin or resemblance,
and the organization of this chapter will follow the latest World
Health Organization (WHO) scheme in that regard.

Fatty Benign Soft Tissue Tumors

Lipoma

Lipoma is the most common soft tissue tumor in adults.
However, because it is, soft tissue sarcomas are sometimes assumed to
be lipomas, unnecessarily delaying the diagnosis. Hence, recognition of
the distinction between benign lipomas and soft tissue sarcomas is very
important.

Pathogenesis and Pathophysiology

Etiology is unknown.
More common in obese patients
Peak age: 40 to 60 (rare in children)
Multiple lipomas in 5%

Histopathology

Lobules of mature adipocytes
Occasionally other areas of tissue formation
- Bone = osteolipoma
- Cartilage = chondrolipoma
- Myxoid change = myxolipoma

Genetics

Chromosomal aberrations in up to 75%
Three patterns
- 12q13-15 aberrations
- 6p21-23 aberrations
- Loss of material from 13q

Classification

Histologic variant subtypes have no prognostic significance.
Infiltrative

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Intermuscular Versus Intramuscular

Intermuscular: easily shelled out between muscles
Intramuscular: two types within muscle
- Well demarcated
- Infiltrative: infiltrates and encases atrophic muscle fibers

Lipoma Arborescens

Subsynovial located lipoma
One type of intra-articular lipoma

Diagnosis

Clinical Features

Most common: painless mass with characteristic doughy feel
Most superficial lipomas are small (<5 cm).
Most deep lipomas are large (>5 cm).
Lipoma arborescens presents as articular swelling.

Radiologic Features

Superficial lipomas: difficult to see on radiographs or magnetic resonance imaging (MRI)
Deep lipomas (Fig. 11-1)
- Fatty intramuscular shadow
- Homogeneous fatty signal on all MRI sequences
- No gadolinium enhancement
- Occasional entrapped muscle fibers or fibrous strands
Lipoma arborescens
- Fatty infiltration throughout affected synovium with fat-distended villi

Figure 11-1 Inramuscular lipoma of the proximal forearm. (A) The fatty soft tissue shadow is shown within the muscle of the proximal forearm by plain radiograph. Axial T1-weighted (B) and T2-weighted (C) MR imaging studies show homogenous fatty signal characteristics identical to that of the subcutaneous fat.

Diagnostic Work-up

Most superficial lipomas are
distinguished by their doughy characteristics on physical examination
and do not warrant radiographic evaluation.
Larger or deep masses warrant plain radiographic and MRI evaluation, which is usually diagnostic.
Biopsy is rarely indicated.
Also see Chapter 2, Evaluation of Soft Tissue Tumors.

Treatment

Surgical and Nonoperative Options

Superficial lipomas
- Observation favored
- Excision only if symptomatic
Deep lipomas
- Observation if radiology identifies clearly as benign lesion (not atypical lipoma or liposarcoma)
- Excision for most

Surgical Goals and Approaches

Marginal excision (complete excision through pseudocapsule)
Avoid transaction of nerves running within deep lipomas (piecemeal excision preferred).

Results and Outcome

Rarely recur following marginal excision

Lipomatosis

Lipomatosis is a diffuse systemic or regional overgrowth of mature adipose tissue that differs from normal fat only in

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its distribution. Lipomatosis should be distinguished from lipomas, as
the former conditions may be correctable by addressing the underlying
condition and tend to recur after attempted excision.

Pathogenesis and Pathophysiology

Etiology is poorly understood.
Possibly due to point mutations in mitochondrial genes
See Table 11-1 for epidemiology.
Histopathology: mature fat in poorly circumscribed lobules or sheets infiltrating surrounding tissues

Classification

Classified by distribution for idiopathic types (diffuse, pelvic, symmetric) or by etiology (steroid, HIV lipodystrophy)

Diagnosis

Notable accumulations of fat in affected areas may resemble neoplasms (see Table 11-1).
Radiologic studies document extent of fatty deposition.
Neither radiological evaluation nor biopsy is usually necessary to diagnosis the lipomatoses.
See Chapter 2.

Treatment and Outcome

Palliative surgery is rarely indicated
unless life-threatening fat accumulation (such as that causing
laryngeal compression in Madelung’s disease).
Correction of steroid lipomatosis follows lowering of steroid levels.

Table 11-1 Epidemiology and Clinical Features of Lipomatosis Subtypes

Subtype	Epidemiology	Distribution	Potential Complications or Associated Symptoms
Diffuse	Typically <2 yr	Trunk, large part of extremity, head and neck, pelvis, or intestinal Macrodactyly or digital gigantism
Pelvic	Black males all ages, 9 yr and up	Perivesicular, perirectal	Bowel or bladder symptoms (frequency, constipation), perineal/abdominal/low back pain
Symmetric (Madelung’s disease)	Middle-aged men of Mediterranean descent	Upper body, especially neck	Cranial or peripheral neuropathy Airway obstruction Superior vena cava syndrome Associated with alcoholism, hyperuricemia, hyperlipidemia
Steroid	Steroid treatment Primary adrenal cortical axis overstimulation	Facial, sternal, or buffalo hump	Secondary manifestations of hypercortisolism
HIV lipodystrophy	HIV patients on treatment	Visceral, breast, cervical fat Fat wasting in face and limbs	Associated with hyperlipidemia, diabetes

Lipomatosis of Nerve (Lipofibromatous Hamartoma)

Lipofibromatosis, also referred to variously as
lipofibromatous hamartoma, fibrolipomatosis, and intraneural lipoma,
among other names, is a fatty and fibrous infiltrative process
affecting the epineurium and leading to enlargement of the affected
nerve. The median and ulnar nerves are most commonly affected.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: 10 to 40
Frequently evident at birth or early childhood
Female > male if macrodactyly present; male > female if none
Gross histopathology: yellow fibrofatty infiltration of nerve (Fig. 11-2)
Microscopic histopathology: epineurial and perineural fibrofatty infiltration isolating individual nerve bundles

Diagnosis

Clinical Features

Gradually enlarging mass with variable neurologic deficits
Median nerve or branches > ulnar nerve
Foot, brachial plexus less common sites
Macrodactyly in one third

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Figure 11-2 A
Fibrous hamartoma of infancy. Organoid pattern composed of intersecting
fascicles of spindle cells separated by collagen, mature fat, and
islands of immature mesenchymal cells with myxoid matrix. B Close-up view of characteristic island of immature mesenchyma in fibrous hamartoma of infancy.

Radiologic Features

MRI findings pathognomonic
- Fusiform neural enlargement following branching pattern of nerve
- Hypointense serpentine nerve bundles on both T1- and T2-weighted images
- Variable intramuscular fatty deposition

Diagnostic Work-up

Because MRI is diagnostic, biopsy is usually not needed.

Treatment

Goal of surgical intervention is to decompress nerve if symptomatic.
Avoid attempted excision, which may damage nerve.

Lipoblastoma

Lipoblastoma is a tumor resembling fetal adipose tissue
that occurs predominately in infants and may be localized
(lipoblastoma) or diffuse (lipoblastomatosis). Since lipomas generally
do not occur in this age group, lipoblastoma should be considered
highly in the differential diagnosis of any tumor with fatty imaging
characteristics in a child.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: birth to age 3; less common in older children
Males > females

Histopathology

Variable mixture of mature adipocytes and immature fat cells (lipoblasts)
Variable myxoid background with plexiform vascular pattern suggestive of myxoid liposarcoma
Pronounced lobular pattern and lack of atypia distinguishes lipoblastoma from liposarcoma

Genetics

8q11-13 rearrangement in most cases
Fusion gene products: HAS2/PLAG1, COL1A2/PLAG1

Classification

Solitary (lipoblastoma) versus diffuse (lipoblastomatosis)

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Sites: extremities predominate
Typically small (2 to 5 cm), superficial mass
Lipoblastomatosis often involves muscle as well.

Radiologic Features

Fatty signal characteristics
- Bright on T1-weighted MRI, dark on fat-suppressed sequences, identical to surrounding fat
Indistinguishable from lipoma, well-differentiated liposarcoma (atypical lipoma) by radiology

Treatment and Outcome

Lipoblastoma: marginal en bloc excision
Lipoblastomatosis: wide surgical excision
Recurrence rare in lipoblastoma; up to 22% in lipoblastomatosis

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Table 11-2 Histological Lipoma Variants

Tumor	Peak Ages/ Gender	Distribution	Clinical Features	Histology
Angiolipoma	Late teens to early 20s Rare >50 yr	Forearm > trunk > upper arm Superficial	Small Frequently multiple Painful mass	Mature adipocytes Branching capillary-sized vessels (see Fig. 11-3)
Chondroid lipoma	Adults Female:male 4:1	Proximal extremities or limb girdles > trunk Deep or involving fascia	Painless mass Half with enlargement	Nests and cords of lipoblasts embedded in myxoid or hyaline cartilage matrix
Spindle cell lipoma	Adults 45 to 65 yr Male:female 9:1	Posterior neck and shoulder > upper arm Superficial	Small Rarely multiple	Adipocytes, spindle cells, and collagen bundles Variable inflammatory cells (especially mast cells)
Pleomorphic lipoma	Adults >45 yr Male:female 9:1	Posterior neck and shoulder > upper arm Superficial	Small Rarely multiple	Spindle cells and giant cells with nuclei arranged in “floret-like” pattern
Hibernoma	Young adults 20 to 40 yr (60%)	Thigh > trunk > upper extremity > head and neck Subcutaneous > deep (9:1)	Painless mass Enhances with MRI contrast	Polygonal brown fat cells Lipoblast-like multivacuolated cells (see Fig. 11-4)

Histologic Lipoma Variants

The histologic variants of lipoma are summarized in Table 11-2, and examples are shown in Figures 11-3 and 11-4.

Fibroblastic Benign Soft Tissue Tumors

Nodular Fasciitis

Nodular fasciitis is a tumor predominately of the upper
extremity and neck region that is distinguished by its rapid growth and
its tissue culture–like histology pattern.

Pathogenesis and Pathophysiology

Etiology: unknown, but history of trauma common

Epidemiology

Peak age: young adults, but may involve any age

Histopathology (Fig. 11-5)

Plump, regular myofibroblasts
Frequent mitoses but not atypical mitoses
Tissue culture–like, “torn” or “feathery” appearance at low power
Prominent small vessels may resemble granulation tissue.
Nodular infiltrative pattern of organization
Extravasated red blood cells, chronic inflammatory cells, and even giant cells may be seen.
SMA and MSA positive
Desmin, cytokeratin, and S100 negative

Figure 11-3
Angiolipoma composed of mature adipose tissue intermixed with numerous
vascular channels. The vascularity is often more prominent in
subcapsular areas.

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Figure 11-4
Hibernoma: rare fatty tumor composed of mature adipose tissue and a
large number of multivacuolated brown fat cells with abundant granular
cytoplasm and a centrally located nucleus.

Genetics

Some clonality suggesting neoplastic nature demonstrated, but may represent artifact of culture conditions

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Subcutaneous >> intramuscular
Upper extremity, trunk, head and neck most common
Rapid growth is characteristic.
Clinical history usually <1 to 2 months
Pain and local tenderness common
Small size, usually <2 cm

Figure 11-5
Nodular fasciitis: cellular proliferation of stellate cells with vague
storiform pattern and extravasated red blood cells. Inflammatory cells
and mitoses are also commonly seen, but there is no cytologic atypia.

Radiologic Features

Nonspecific soft tissue mass (Fig. 11-6)

Treatment and Outcome

Marginal excision
Rare recurrence should warrant reconsideration of diagnosis.

Proliferative Fasciitis and Myositis

These two processes are similar to nodular fasciitis in
their rapid growth, predilection for the upper extremity, and plump
myofibroblastic cells, but they are distinguished by the large
ganglion-like cells that are also present. Proliferative fasciitis (PF)
involves the fascia, while proliferative myositis (PM) involves the
muscle.

Pathogenesis and Pathophysiology

Etiology is unknown, although a history of trauma may be elicited.
Peak age: middle-aged or older adults, older than nodular fasciitis
Much less common than nodular fasciitis

Histopathology

Two cellular types: plump myofibroblasts and ganglion-like cells
Plump myofibroblasts resemble those of nodular fasciitis.
Ganglion-like cells: large with one to three rounded nuclei, prominent nucleoli, and abundant cytoplasm
Numerous mitoses but not atypical mitoses
Infiltrative growth pattern (along fascial planes for PF and between muscle groups in PM)
Checkerboard pattern of infiltration between muscle fibers for PM
SMA and MSA positive
Desmin, cytokeratin, and S100 negative

Genetics

Similar to nodular fasciitis

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

PF: upper extremity (especially forearm) > lower extremity > trunk
PM: trunk > shoulder girdle > upper arm > thigh

Rapid growth is characteristic.

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Figure 11-6 Nodular fasciitis of the shoulder. (A)
The patient presented with a 3-week history of a rapidly enlarging,
painful soft tissue mass over the anterior deltoid. Sagittal MR studies
show nonspecific findings of a heterogeneous intramuscular mass
hypointense to muscle on T1-weighted sequences (B), hyperintense on T2-weighted images (C), and enhancing on post-gadolinium sequences (D).

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Clinical history usually <1 or 2 months
Pain and local tenderness more common with PF
Small size, usually <5 cm

Radiologic Features

Nonspecific soft tissue mass

Treatment and Outcome

Marginal excision
Recurrence is rare.

Myositis Ossificans

Myositis ossificans is a benign reparative process
characterized by bone formation within the soft tissues in response to
trauma. On the one hand, it may simulate malignancy, but conversely,
sarcomas that give a similar appearance may be mistaken for this
benign, self-limited condition.

Pathogenesis and Pathophysiology

Trauma history elicited in up to 75% of patients; repetitive trauma in others
Peak age: young, physically active adults (rare in infants or elderly)
Males > females

Histopathology

Zonal proliferation with central fibroblasts and peripheral osteoblast-rimmed bone trabeculae
Progression from initially fibrous tissue to peripheral woven bone and then eventually lamellar bone
Peripheral ossification usually evident by 3 weeks
Mitoses frequent but no atypical mitoses

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Initial 1 to 2 weeks after injury: swollen, tender
From 2 to 6 weeks after injury: tenderness and pain resolve, and painless, very firm mass forms
Chronically, the mass becomes less prominent over time.
Any location in body susceptible to trauma
Most common locations: thigh, shoulder, buttock, elbow

Radiologic Features

Initial 1 to 2 weeks after injury: x-ray,
computed tomography (CT), MRI shows soft tissue shadow, heterogeneous
MRI signal with edema
From 2 to 6 weeks after injury: x-ray, CT best show peripheral rim of calcifications (Fig. 11-7)
Chronically, MRI eventually shows low-signal rim with fatty marrow signal centrally.

Treatment and Outcome

Observation is best, but marginal excision is appropriate if sufficiently symptomatic.
May recur if excised incompletely early in course

Elastofibroma

Elastofibroma (elastofibroma dorsi) is a unique process
that occurs almost exclusively in a subscapular location in adults and
is characterized by histologic evidence of elastic fibers.

Pathogenesis and Pathophysiology

Etiology is unknown, although repetitive trauma has been implicated.
Peak age in seventh and eighth decades (nearly always after age 50)
Females > males

Histopathology

Low-cellularity collagenous tissue with intermixed elastic fibers (Fig. 11-8)
Immunohistochemistry positive for elastin

Genetics

Familial occurrence reported in Okinawa

Diagnosis

Clinical Features

Slowly growing mass, typically painless and nontender
May cause popping scapula or local discomfort
Classic location
- Nearly exclusively subscapular, applied to chest wall at the lower portion of the scapula
- Deep to latissimus dorsi and rhomboid major
- Often attached to periosteum of ribs
- Rare musculoskeletal sites: upper arm, hip
- May be bilateral (especially subclinical) in up to one third

Radiologic Features

Fibrous and fatty elements create layered picture.
MRI may be highly suggestive (Fig. 11-9).
- Fatty areas: bright areas on T1-weighted images, intermediate on T2-weighted images
- Fibrous areas: similar to muscle

Diagnostic Work-up

Biopsy is generally recommended to
confirm diagnosis, although the combination of classic location and
radiologic appearance is highly suggestive.
Also see Chapter 2.

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Figure 11-7 (A)
Initial plain radiographs within the first 2 to 3 weeks may not show
the characteristic peripheral rim of calcification associated with
myositis ossificans. (B) Subsequently, the peripheral rim is best demonstrated on computed tomography. (C) MR studies show a heterogeneous mass that is sometimes confused with a soft tissue sarcoma.

Treatment and Outcome

Marginal excision indicated for symptomatic patients but may be observed if asymptomatic and histologically proven
Rarely recurs after complete excision

Superficial Fibromatoses

The superficial fibromatoses include palmar fibromatosis
(Dupuytren’s disease or contracture) and plantar fibromatosis
(Ledderhose disease), both fibroblastic proliferations with
infiltrative growth that are usually easily recognizable clinically and
have a high rate of local recurrence if excised.

Pathogenesis and Pathophysiology

Etiology is multifactorial.
- Familial component
- Trauma
- Associated diseases: epilepsy, diabetes, alcohol- induced liver disease
Peak age: adults with increasing frequency in advanced ages (rare before age 30)
Males >> females (3 to 4:1)
Ethnicity: highest incidence in northern Europe and in

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those of northern European descent; rare in non-Caucasians
Pathophysiology is shown in Figure 11-10.

Figure 11-8 (A) Elastofibroma composed of intertwining, swollen collagen and elastic fibers with fibroblasts. (B) Elastic fibers highlighted by EVG stain in elastofibroma.

Diagnosis

Clinical Features

Palmar fibromatosis

Volar surface, 50% bilateral

Typical progression

Begins with firm painless nodule

Progresses to multiple nodules connected by tight cords (distinguished from pre-existing normal fascial bands)

Figure 11-9 (A)
The characteristic location for an elastofibroma is at the inferior
medial border of the scapula in a subscapular position. MR images show
a heterogeneous subscapular soft tissue mass on T1-weighted (B) and proton density (C) axial images.

Puckering of overlying skin
End-stage flexion contractures favoring ring and little fingers

Plantar fibromatosis
- Plantar aponeurosis
- Typical course
  - Begins with firm nodule adherent to skin
  - Often painful with shoe wear
  - Progressive enlargement may result.
  - Rarely leads to contracture

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Figure 11-10 (A) Plantar fibromatosis: dense fibrous proliferation infiltrating adipose tissue. (B)
Plantar fibromatosis: early lesions consist of cellular proliferations
of bland-looking spindle cells and collagen deposition similar to
desmoid tumors. Older lesions tend to be less cellular and contain more
collagen.

Radiologic Features

Palmar fibromatosis
- Cords and hypocellular nodules dark on T1- and T2-weighted images
- Hypercellular nodules intermediate on T1- and T2-weighted images
Plantar fibromatosis
- Isointense to slightly hyperintense always on T1-weighted images and nearly always on T2-weighted images
- Usually hyperintense on short tau inversion recovery (STIR) sequence
- Enhancement common

Diagnostic Work-up

Dupuytren’s contracture and many cases of
plantar fibromatosis are easily recognized clinically and do not
require either further imaging or biopsy.
If plantar masses are not characteristic, MRI imaging may establish the diagnosis, but biopsy is sometimes needed.

Treatment and Outcome

Palmar fibromatosis
- Flexion contracture is most common indication for operative treatment.
- Details of operative treatment are beyond the scope of this text.
Plantar fibromatosis
- Usually managed nonoperatively unless recalcitrant to adaptive shoe wear and inserts
- If operative treatment is elected, complete fasciectomy achieves lowest recurrence rates.
- High recurrence rate for plantar fibromatosis ranging from 50% with complete fasciectomy to 100% with local excision

Desmoid-Type Fibromatoses

Extra-abdominal desmoid tumors are the subset of
interest in orthopaedics. They consist of a neoplastic fibroblastic
proliferation that may result in pain, infiltrative growth resulting in
recurrence, and absence of metastatic potential.

Pathogenesis and Pathophysiology

Etiology

Familial component
- Component of Gardner syndrome (autosomal dominant transmission, variant of familial adenomatous polyposis; Box 11-1)
Endocrine factors
- Frequent exacerbation/appearance postpartum
- Estrogen receptors in desmoid tumors
- Response to endocrine agents
Trauma may be contributory factor.

Epidemiology

Pediatric patients: extra-abdominal > abdominal
Puberty to 40: abdominal > extra-abdominal, female > male
Older than 40: abdominal = extra-abdominal

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Figure 11-11 (A) Extra-abdominal fibromatosis showing a proliferation of bland-looking spindle cells separated by collagen. (B) Infiltration of surrounding soft tissues in fibromatosis. Note the presence of entrapped skeletal muscle cells.

Pathophysiology

Histopathology (Fig. 11-11)

Low-cellularity spindle cells in collagenous stroma
Extremely infiltrative process arranged in sweeping bundles

Genetics

Two mutations with common endpoint of increased beta-catenin expression
- Inactivation of APC tumor suppressor gene on 5q may be initiating event, especially those with familial occurrence.
- Activating beta-catenin mutations render beta- catenin resistant to inhibitory effect of APC.
Trisomies for chromosomes 8 or 20 in 30% of cells

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Deep (as distinguished from superficial fibromatoses)
Painful or painless, poorly circumscribed, very hard mass
Occasionally multifocal
Occasional joint stiffness or neurological symptoms

Radiologic Features

MRI nonspecific (Fig. 11-12)
- Isointense to muscle on T1-weighted images
- Intense contrast enhancement
- Hyperintense on STIR and T2-weighted images

Treatment

Surgical indication
- Wide resection preferred if adequate margin attainable with acceptable function
Surgical contraindications
- Vital structures involved or unresectable disease
- Consider alternative treatments in this situation:
  - Radiotherapy
  - Pharmacologic treatment
    - Hormonal agents
    - Nonsteroidal anti-inflammatory agents
    - Interferons
    - Cytotoxic chemotherapy
Radiotherapy consultation is recommended for positive surgical margins, although efficacy not well established.

Results and Outcome

Surgical resection
- Wide margins: 85% to 90% local control
- Marginal margins: 50%
- Intralesional margin: <50%
Radiotherapy
- Typically recommended for positive margins, but proof of efficacy not clearly established
Pharmacologic treatment
- 40% to 50% response rate

Solitary Fibrous Tumor and Hemangiopericytoma

According to the latest WHO classification, extrapleural
solitary fibrous tumors (SFT) and hemangiopericytoma (HP) share enough
clinical, histological, and immunohistochemical features to be
classified together and may actually represent a spectrum of the same
entity. They typically behave in a benign fashion, but up to 30% are
malignant.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age 20 to 70 years (median 50)

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Figure 11-12 (A)
Desmoid tumor typically presents as a slow-growing, sometimes painful
soft tissue mass. MR imaging shows nonspecific hypointense signal on
T1-weighted sequences (B) and hyperintense signal on T2-weighted sequences (C). (D) Gadolinium enhancement is intense.

Histopathology

SFT
- Intermixed hypercellular and hypocellular areas separated by bands of hyalinized collagen
- Round to spindle cells with indistinct cytoplasm
- Immunohistochemistry
  - Vimentin, CD34, and CD99 positive
  - Variable focal positivity for others
HP
- Similar to SFT with addition of prominent variably branching thin-walled vessels with staghorn configuration (Fig. 11-13)
- Immunohistochemistry
  - Vimentin, CD34, and CD99 positive
  - Cytokeratin, actin, and desmin negative

Figure 11-13
Hemangiopericytoma: cellular neoplasm composed of spindle and round
cells with large vascular spaces with a “staghorn” configuration.

Genetics

Inconsistent chromosomal aberrations

Diagnosis

See Chapter 2 for diagnostic work-up.

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Clinical Features

Superficial or deep, slowly growing, small or large, painless mass in any location
Rare paraneoplastic syndrome due to IGF-1
production results in hypoglycemia or acromegaly; oncogenic
osteomalacia is very rare with these tumors.

Radiologic Features

Nonspecific radiologic features
MRI nonspecific
- Isointense to slightly hyperintense on T1-weighted images
- Hyperintense on T2-weighted images
- Heterogeneous, enhancing

Treatment and Outcome

Wide surgical resection
Postoperative management with adjuvant radiotherapy and/or chemotherapy may play a role in some malignant cases.
85% 5-year survival rate including malignant forms

Fibrohistiocytic Benign Soft Tissue Tumors

Giant Cell Tumor of Tendon Sheath

Also referred to as localized tenosynovial giant cell
tumor or nodular tenosynovitis, these common tumors represent the most
common neoplasms of the hand. In some classifications, such as that of
the current WHO, this terminology also includes what in the past has
been referred to as the localized form of pigmented villonodular
synovitis (PVNS).

Figure 11-14 (A)
Pigmented villonodular synovitis showing characteristic intra-articular
villous pattern. Abundant hemosiderin pigment seen within macrophages
in fibrovascular core. (B) Pigmented
villonodular synovitis: solid areas composed of oval to spindle-shaped,
histiocytoid, mononuclear cells and multinucleated giant cells.

Pathogenesis and Pathophysiology

Current evidence favors a neoplastic etiology over the traditional belief that this was a reactive process.
Peak age: 30 to 50 years (but any age may be involved)
Female > male

Histopathology (Fig. 11-14B)

Grossly well circumscribed with yellow and brown areas
Composed of mixture of background
mononuclear cells, variable numbers of giant cells, lipid-laden
xanthoma cells, and hemosiderin-laden macrophages
Immunohistochemistry
- Mononuclear cells: CD68 positive
- Giant cells: positive for CD68, CD45, and tartrate resistant acid phosphatase (TRAP)

Genetics

Cytogenetic aberrations of chromosome 1p in particular, frequently with translocation, are common.
No trisomies of chromosomes 5 or 7, as seen with PVNS

Diagnosis

Clinical Features

Predominately in the hand (85%) near tendon sheath or joint > wrist, foot/ankle, knee
Small painless nodules

Radiologic Features

Approximately 20% erode adjacent bone.

Nonspecific MRI features (Fig. 11-15)

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Figure 11-15 Giant cell tumor of tendon sheath shows a soft tissue shadow on plain lateral radiograph (A), hypointense signal on axial T1-weighted MRI (B), hyperintense signal on axial T2-weighted MRI (C), and gadolinium enhancement on post-contrast fat-suppressed T1-weighed sagittal MRI (D).

T1-weighted images: hypointense to slightly hyperintense to muscle
T2-weighted images: hypointense to hyperintense to muscle
Heterogeneous due to hypointense areas
Diffuse contrast enhancement

Diagnostic Work-up

Usually plain radiographs and MRI are sufficient for these small masses.
Also see Chapter 2.

Treatment and Outcome

Marginal excision
Up to 30% recurrence rate, easily controlled by re-excision

Diffuse-Type Giant Cell Tumor (Pigmented Villonodular Synovitis)

Pigmented villonodular synovitis (PVNS) is a destructive
proliferative synovial process predominately seen affecting the knee.
It was formerly subdivided into localized and diffuse forms; the former
is now classified as localized giant cell tumor.

Pathogenesis and Pathophysiology

Current evidence favors a neoplastic etiology over the traditional belief that this was a reactive process.
Peak age: <40 years (but any age may be involved)
Female > male

Histopathology

Intra-articular forms: villous pattern with yellow and brown areas (Fig. 11-14A)
Extra-articular forms: grossly well circumscribed with yellow and brown areas (Fig. 11-14B)
Expansile sheets of infiltrative cells
with intermixed cellular regions and discohesive areas creating
blood-filled pseudoalveolar spaces
Composed of mixture of background
mononuclear cells, variable numbers of giant cells, lipid-laden
xanthoma cells, and hemosiderin-laden macrophages
- Giant cells less common than in giant cell tumor of tendon sheath, sparse or absent in up to 20%
- Bimodal mononuclear cells: small histiocyte-like cells and larger dendritic cells
Immunohistochemistry
- Mononuclear cells: CD68 positive
- Larger dendritic cells: desmin positive
- Giant cells: CD68, CD45, and TRAP positive

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Genetics

Chromosome 1p translocations common and similar to giant cell tumor of tendon sheath
Trisomies of chromosomes 5 and 7 seen only in PVNS, not in giant cell tumor of tendon sheath

Classification

Intra-articular and extra-articular forms

Diagnosis

Clinical Features

Usually large and associated with pain, local tenderness, and decreased joint motion
Recurrent hemarthroses common

Radiologic Features (Fig. 11-16)

Juxta-articular erosions evident in intra-articular form in advanced cases
MRI
- Characteristic hemosiderin attenuation of signal resulting in at least foci of hypointense signal within all sequences
- Fatty areas due to lipid accumulation
- Joint effusion

Figure 11-16
Pigmented villonodular synovitis shows characteristic signal
attenuation from the hemosiderin deposition within the synovium on all
MRI sequences. In this case, low-signal areas can be seen within the
suprapatellar pouch and popliteal region on these sagittal T1-weighted (A), axial proton density (B), and axial T2-weighted (C) MR images.

Diagnostic Work-up

Usually plain radiographs and MRI are sufficient for these processes.
Also see Chapter 2.

Treatment

Intra-articular form: arthroscopic or
open complete synovectomy; radiosynovectomy or external-beam
radiotherapy used in recalcitrant cases
Extra-articular form: marginal excision

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Up to 50% recurrence rate in both forms
Rare cases of lung involvement (n = 2)

Neural Benign Tumors

Neurofibroma and Neurofibromatosis

Neurofibromas are benign peripheral nerve sheath tumors
that may occur as solitary lesions or as a component of
neurofibromatosis. Neurofibromas are more characteristically
intertwined with the peripheral nerve and difficult to separate.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: young adults 20 to 30 years

Figure 11-17 (A) Cross-section of a plexiform neurofibroma showing its distinctive arrangement in large fascicles. (B)
Characteristic neurofibroma with wavy spindle cells in a loose stroma.
Toward the center, cells are more tightly apposed, resembling
peripheral nerve fibers. (C) Diffuse and strong immunostaining of neurofibroma with S-100 protein stain.

Histopathology (Fig. 11-17)

Mixture of Schwann cells, perineurial-like cells, and fibroblasts
Hypocellular, widely spaced cells with thin, elongated nuclei
S100 positive

Genetics

NF-1 gene mutation in neurofibromatosis (NF)
Solitary neurofibromas not as clearly established

Classification (Table 11-3)

Localized cutaneous neurofibroma (Fig. 11-18)
- Most common type
- 10% occur in NF-1
Diffuse cutaneous neurofibroma
- Plaque-like thickenings of dermis and subcutaneous tissue

Localized intraneural neurofibroma (Fig. 11-19)

Second most common type
Fusiform segmental enlargement of nerve (Fig. 11-20)

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Table 11-3 Subtypes of Neurofibroma

Subtype	Depth	Site	Size	Pain?	% in NF-1	Malignant Transformation?
Diffuse cutaneous neurofibroma	Skin and subcutaneous	Any	1 to 2 cm	No	10%	No
Localized cutaneous neurofibroma	Skin and subcutaneous	Head and neck	Several centimeters	No	10%	Rare
Localized intraneural neurofibroma	Superficial or deep	In NF-1, predilection for cervical spine	1 cm to many centimeters	Neural pain if deep	Minority	Infrequent
Plexiform neurofibroma	Usually deep	Predilection for larger nerves	Large	Neural pain	Almost exclusively	Highest
Massive soft tissue neurofibroma	Skin and deeper tissues	Shoulder, pelvis, lower extremity	Large	Variable	100%	Rare

Plexiform neurofibroma (Fig. 11-21)
- Uncommon tumor usually affecting large nerve
- “Bag of worms” appearance in plexus regions (Fig. 11-22)
- Firm, ropy cylinder in nonbranching nerves
- Highly suggestive of diagnosis of NF-1
Massive soft tissue neurofibroma (Fig. 11-24)
- Least common form always seen in NF-1
- Formerly called “elephantiasis neuromatosa”
- Localized gigantism or massive regional tissue enlargement

Diagnosis

See Chapter 2 for diagnostic work-up.

Figure 11-18
Localized cutaneous neurofibromas, as shown in this patient with type I
neurofibromatosis, are the most common type of neurofibroma.

Clinical Features

Solitary painless or painful masses or in the setting of NF-1 (von Recklinghausen’s disease)
Intraneural neurofibromas may cause pain or paresthesias along distribution of affected nerve.
- Tinel’s sign may be positive.
- More medial–lateral movement than proximal– distal on palpation

Radiologic Features

MRI features may be highly suggestive of benign peripheral nerve sheath tumors.
- Target sign more frequent with neurofibromas than schwannomas (Fig. 11-25)
- Dark on T1-weighted images, bright on T2-weighted images
Dumbbell tumors (partially intraspinal and partially extraspinal) may cause foraminal enlargement.

Treatment

Localized cutaneous neurofibroma and diffuse cutaneous neurofibroma
- Excision usually curative
- No neurological deficit expected
Localized intraneural neurofibroma and plexiform neurofibroma
- Avoid resection due to need to resect functional nerve fibers.
- Indications for excision: suspicion for malignant degeneration
  - Rapid increase in size
  - Pain
Massive soft tissue neurofibroma
- May be amenable only to debulking

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Figure 11-19
Localized intraneural neurofibroma is usually recognizable as a benign
peripheral nerve sheath tumor, but it may be difficult to distinguish
from a schwannoma. This case shows a discrete enhancing mass in
continuity with deep nerve fibers on coronal T1-weighted (A), post-gadolinium T1-weighted (B), and axial T2-weighted (C)
images. Although these features could be seen with either neurofibroma
or schwannoma, this tumor was found to be a neurofibroma histologically.

Figure 11-20
Schematic shows appearance of an intraneural neurofibroma, where normal
nerve fibers entering and exiting the tumor are intimately intertwined
with those of the neurofibroma.

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Figure 11-21
Plexiform neurofibroma, which is highly suggestive of the diagnosis of
type I neurofibromatosis, is shown here as the classic “bag of worms”
appearance on plain radiograph (A), MR images (B,C), and gross resected specimen (D).

Figure 11-22 Schematic shows typical “bag of worms” appearance of a plexiform neurofibroma.

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Figure 11-23 Schematic shows typical eccentric position of a neurilemoma, where normal nerve fibers are displaced peripherally by the mass.

Figure 11-24 Massive soft tissue neurofibroma is manifest in this leg as a bag-like collection.

Figure 11-25 (A)
The target sign of peripheral nerve sheath tumors on an axial MR image
for a neurofibroma, which more commonly shows this sign than does
schwannoma. Coronal T1-weighted (B) and T2-weighted (C) images.

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Schwannoma (Neurilemoma)

Schwannoma is, along with neurofibroma, one of the two
most common benign peripheral nerve sheath tumors. While there are four
histological variants, this section will concentrate on the
conventional schwannoma.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak ages: third to sixth decades
A schwannoma is a neoplasm composed of Schwann cells, and its immunohistochemical and ultrastructural profile reflects that.

Histopathology (Fig. 11-26)

Biphasic Antoni A and B areas
- Antoni A pattern
  - Collagenous background
  - Compact, elongated spindle cells aligned in bundles
  - Verocay bodies: palisaded arrangement of peripheral aligned nuclei surround central eosinophilic cell processes
- Antoni B pattern
  - Mucopolysaccharide myxoid background
  - Less organized cells with plump nuclei and delicate cobweb-like interconnecting processes
Cystic degeneration and hemorrhage frequent (formerly termed “ancient schwannoma”)
Immunohistochemistry: S100 positive

Genetics

Not associated with NF-1 but may be associated with NF-2

Figure 11-26 (A)
Schwannoma with characteristic areas of high cellularity (Antoni A) and
low cellularity (Antoni B). Note the palisading of spindle cells known
as Verocay bodies. (B) Strong immunoreactivity of schwannoma cells with S-100 protein stain.

Classification

Conventional: most common form; characterized by Antoni A and Antoni B areas
Cellular: higher cellularity, mostly Antoni A areas, absent Verocay bodies
Plexiform: rare variant with intraneural pattern but not usually associated with NF-1
Melanotic: unusual variant with melanin-producing cells

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Head and neck, flexor surfaces of extremities (Fig. 11-27)
Slowly growing masses <10 cm
May elicit neural symptoms of pain or paresthesias
Physical examination may show positive Tinel sign, limited proximal–distal mobility.
Spinal schwannomas usually affect sensory nerves.

Radiologic Features (Fig. 11-28)

Well-demarcated soft tissue masses on plain x-ray
MRI heterogeneity related to cystic degeneration
- Hypointense on T1-weighted images, hyperintense on T2-weighted images
- Enhancing
Target sign on MRI less common than neurofibroma
Foraminal enlargement with dumbbell schwannomas and giant sacral schwannomas

Treatment and Outcome

Marginal complete excision usually
possible without resecting nerve due to eccentric growth (unlike
intraneural and plexiform neurofibromas; Fig. 11-23)
Recurrence unusual except in sacral schwannomas

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Figure 11-27 Schwannomas of the extremity typically occur on the flexor surface near a joint, such as the knee in this patient.

Figure 11-28 Schwannomas are hypointense to muscle on T1-weighted MR image (A) and hyperintense on T2-weighted image (B). Note how the nerve fibers splay around the round to oval mass.

Figure 11-29 The schwannoma is usually easily separable from the associated nerve, unlike the intraneural neurofibroma.

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Smooth Muscle Benign Soft Tissue Tumors

Leiomyoma of Deep Soft Tissue

Leiomyoma of the soft tissue is a tumor comprising
normal-appearing smooth muscle cells, but it is extremely rare in the
extra-abdominal soft tissues.

Pathogenesis and Pathophysiology

Etiology is unknown.
Extremely rare
Cells resemble smooth muscle cells with
eosinophilic cytoplasm and blunt-ended, cigar-shaped nuclei arranged in
intersecting bundles (Fig. 11-30).
Immunohistochemistry
- Positive for actin, desmin, and h-caldesmon
- S100 negative

Diagnosis

Clinical features: superficial or deep soft tissue masses
Radiologic features: frequently calcified, but otherwise non specific
See Chapter 2 for diagnostic work-up.

Treatment and Outcome

Marginal excision
Local recurrence rate ~3%

Figure 11-30 (A)
Leiomyoma. Fusiform cells with eosinophilic cytoplasm arranged showing
a vague fascicular pattern closely resembling normal smooth muscle. (B) Strong and diffuse reactivity for smooth muscle actin stain in leiomyoma.

Skeletal Muscle Benign Tumors

Rhabdomyoma

Analogous to leiomyomas, rhabdomyomas are extremely rare tumors of the soft tissue.

Pathogenesis and Pathophysiology

Etiology is unknown.
Very rare
Peak ages:
- Adult form: median 60 years (33 to 80 years)
- Fetal form: median 4 years

Histopathology (Fig. 11-31)

Lobules of closely packed cells
Spider cells (large, polygonal cells with abundant eosinophilic cytoplasm)
Cytoplasmic inclusion bodies, cross-striations
Immunohistochemistry: MSA, desmin, and myoglobulin positive

Classification

This section refers only to extracardiac rhabdomyomas at the exclusion of cardiac rhabdomyomas.
The extracardiac rhabdomyomas are subdivided into adult and fetal types.

Diagnosis

Clinical features: head and neck (90%), soft tissue mass
Radiologic features are nonspecific.
See Chapter 2 for diagnostic work-up.

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Figure 11-31
Rhabdomyoma. Solid growth of closely packed, large polygonal cells with
abundant eosinophilic or granular cytoplasm and eccentrically located
nuclei.

Treatment and Outcome

Marginal excision
Up to 42% local recurrence

Vascular Benign Soft Tissue Tumors

Hemangiomas

Hemangiomas, which are most commonly seen in the skin,
are seen in the orthopaedic context predominately as intramuscular
angiomas or less commonly as synovial hemangiomas.

Figure 11-32 (A) Intramuscular hemangioma. Large vascular channels lined by flat endothelial cells surrounded by skeletal muscle. (B) Hemangioma. Cavernous vascular spaces lined by flat endothelial cells and focally containing red blood cells.

Pathogenesis and Pathophysiology

Although theories of neoplasm, reactive
processes, and malformations have been put forth, these are currently
thought to most likely be simply malformations.

Epidemiology

Intramuscular angiomas are among the most
common of soft tissue tumors, particularly in adolescents and young
adults, while the other variants are uncommon to rare.
Intramuscular angiomas have a female predominance.

Histopathology (Fig. 11-32)

Various combinations of variably thick-walled vascular channels with hemosiderin deposition
Variable amounts of fatty tissue within intramuscular angiomas

Classification (Table 11-4)

Synovial hemangioma
Intramuscular angioma
Venous hemangioma
Arteriovenous hemangiomas

Diagnosis

Clinical Features

Intramuscular angioma causes classic activity-related fluctuation in size and pain even over the course of a single day.
- Spongiform, compressible mass
- Slowly enlarging

Arteriovenous malformations result in shunting.

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Table 11-4 Features of Hemangioma Subtypes

Hemangioma Subtype	Peak Age	Occurrence	Most Common Location	Clinical Symptoms
Synovial hemangioma	Children and adolescents	Rare	Knee	Joint swelling and pain, mechanical symptoms
Intramuscular angioma	Adolescents and young adults	Common	Deep, thigh	Activity-related pain and swelling
Venous hemangioma	Adults	Uncommon	Superficial or deep, extremities	Soft tissue mass
Arteriovenous hemangioma	Children and young adults	Uncommon	Superficial or deep, head and neck > extremities	Shunting manifestations

Physical examination reveals audible bruit, palpable thrill.
Clinical manifestations of shunting: limb hypertrophy, heart failure, consumption coagulopathy (Kasabach-Merritt syndrome)

Radiologic Features (Fig. 11-33)

Plain radiographs may reveal phleboliths.
MRI findings typically diagnostic
- Characteristic serpentine pattern of blood vessels
- Intermediate on T1-weighted images, bright on T2-weighted images; heterogeneous
- Intermixed fatty signal on established lesions

Diagnostic Work-up

Clinical examination combined with plain
radiographs and MRI is usually diagnostic and obviates the need for
biopsy except in unusual circumstances.
Also see Chapter 2.

Treatment and Outcome

Only sufficiently symptomatic lesions
warrant treatment consideration; the remainder should be observed or
treated with nonsteroidal anti-inflammatory medications and compressive
devices.
Surgical excision
- Small synovial and intramuscular hemangiomas amenable to marginal en bloc excision
- Especially if single expendable muscle involved
Embolization and sclerotherapy
- Larger lesions for which surgical excision is not feasible but sufficiently symptomatic to warrant treatment
- Contraindicated if neurologic symptoms or compression
Recurrence rate 30% to 50% following excision unless entire involved muscle excised

Angiomatosis

Angiomatosis represents involvement of a large segment
of the body by hemangioma(s). This may manifest as extensive muscle
involvement or involvement of multiple tissue levels, including skin,
subcutaneous tissue, muscle, and bone.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: adolescents and young adults
Histopathology is same as for hemangioma.

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Common sites: lower extremity > chest wall > upper extremity
Skin involvement may be evident by varicosities.
Same symptoms as described for hemangioma

Radiologic Features

Same as for hemangiomas but more extensive (Fig. 11-34)

Treatment and Outcome

Often too extensive for adequate excision
High rate of local recurrence following attempted excision

Lymphangioma

Lymphangioma, also referred to as cystic hygroma, is similar to hemangioma except in its absence of blood components.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: birth to childhood
Histopathology: variably sized, thin-walled lymphatic vessels (Fig. 11-35)
Genetics: associated with Turner syndrome

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Figure 11-33
This intramuscular hemangioma of the proximal forearm shows classic
radiographic features. Phleboliths can be seen on the plain radiograph (A), and the sagittal (B) and axial (C) T1-weighted MR images and axial T2-weighted (D) MR image show serpiginous vascular structures intermixed with fat signal.

Classification

Cystic versus cavernous subtypes

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Cystic: neck, axilla, groin
Cavernous: upper trunk, extremities

Radiologic Features

Cystic signal characteristics with nonenhancing, fluid-filled, well-defined structures on MRI
Cystic structures on ultrasound

Treatment and Outcome

Marginal excision
High rate of local recurrence

Perivascular Benign Soft Tissue Tumors

Glomus Tumors

Glomus tumors are tumors of the smooth muscle cells that
make up the normal glomus body and are characterized best by their
clinical presentation as small painful nodules predominately found in a
subungual location.

Pathogenesis and Pathophysiology

Etiology unknown
Peak age: young adults predominate
Subungual lesions more common in women

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Figure 11-34
Angiomatosis involves multiple tissue levels within a large segment of
or an entire limb. This patient had undergone amputation for
complications of angiomatosis of the lower extremity. Note the
phleboliths within the soft tissues and involvement of the bone as well.

Figure 11-35 Lymphangioma. Thin-walled vascular channels lined by flat endothelial cells and filled by proteinaceous material.

Figure 11-36 (A)
Glomus tumor. Nests of small, uniform, round cells with centrally
located nucleus and amphophilic or slightly eosinophilic cytoplasm
surrounded by capillary blood vessels. (B) Glomus tumor showing strong immunoreactivity with smooth muscle actin stain.

Histopathology (Fig. 11-36)

Three components in varying amounts
- Glomus cells
  - Small, uniformly round cells
  - Centrally placed round nuclei
  - Prominent cell membranes define cytoplasm.
- Vascular structures
- Smooth muscle cells

Genetics

Familial cases: autosomal dominant pattern of inheritance
Subungual glomus tumors associated with NF-I

Classification

Typical glomus tumors have three histologic variants:
- Solid glomus tumor: glomus cells predominate
- Glomangioma: vascular structures predominate
- Glomangiomyoma: smooth muscle cells predominate
Glomangiomatosis: extremely rare clinical variant similar to angiomatosis but characterized by glomus histology
Symplastic glomus tumors: rare histologic variant with atypia but benign behavior
Malignant glomus tumors: 1% of glomus tumors

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Subungual region (hand, wrist, foot) predominates.
Superficial in vast majority
Small reddish-blue nodules
Associated with chronic pain in severe paroxysms
Exposure to cold or minor pressure often exacerbates pain.

Radiologic Features

Ultrasound may detect tumors as small as 3 mm.
MRI: bright on T2-weighted images, dark on T1-weighted images; enhancing

Treatment and Outcome

Complete excision is the only treatment.
Local recurrence in 5% to 50%

Chondro-Osseous Benign Soft Tissue Tumors

Soft Tissue Chondroma

Soft tissue chondroma, otherwise known as chondroma of
the soft parts, is an extraosseous, extrasynovial mature hyaline
cartilage tumor.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: middle-aged (may involve any age)
Histopathology: mature hyaline cartilage lobules, S100 positive (Fig. 11-37)
Genetics: 12q13-15 structural rearrangements involving HMGA2 gene locus

Classification

Chondroblastic chondroma: chondrocytic cells in lacunae predominate
Fibrochondroma: prominent fibrosis
Osteochondroma of soft tissue: prominent ossification centrally
Myxochondroma: prominent myxoid change

Figure 11-37 Soft tissue chondroma composed of well-defined lobules of mature cartilage embedded in a fibrous stroma.

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Fingers (two thirds), hands, toes, feet predominate.
Painless small lumps rarely >3 cm

Radiologic Features

Plain films show variable mineralization.
MRI shows hyaline cartilage signal (lobulated, dark on T1-weighted images, bright on T2-weighted images).

Treatment and Outcome

Marginal excision
Recurrence unusual

Tumors of Uncertain Differentiation

Intramuscular Myxoma

Intramuscular myxoma is a benign, low-cellularity myxoid tumor characterized clinically by its somewhat distinctive

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radiographic appearance in showing very hypointense signal compared to
muscle on T1-weighted MRI. It may occur as a solitary soft tissue mass
or may be accompanied by fibrous dysplasia lesions, as in Mazabraud
syndrome.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: 40 to 70 years

Histopathology (Fig. 11-38)

Uniformly bland cellularity with small bland spindle cells
Bland extracellular myxoid background
Infiltrating borders

Genetics

Point mutations in GNAS1 gene common both in Mazabraud syndrome and isolated myxomas

Classification

Cellular myxoma: histologic variant with increased cellularity and vascularity

Diagnosis

Clinical Features

Deep, painless soft tissue mass
Thigh, shoulder, buttocks, upper arm
Usually 5 to 10 cm
Mazabraud syndrome: myxomas and fibrous dysplasia (monostotic, polyostotic, or Albright syndrome)

Figure 11-38 Intramuscular myxoma. Hypocellular neoplasm composed of very bland-looking spindle cells in an abundant myxoid background.

Radiologic Features (Fig. 11-39)

Homogenous, hypointense to muscle on T1-weighted images, hyperintense on T2-weighted images
Variable patterns of enhancement (peripheral and/or central) dependent on cellularity
Peritumoral edema and fat cap

Diagnostic Work-up

This process may be highly suspected based on MRI, so biopsy may be unnecessary.
Also see Chapter 2.

Treatment and Outcome

Marginal excision
Recurrence unlikely

Juxta-Articular Myxoma

The juxta-articular myxoma arises close to large joints and frequently has associated cystic changes.

Pathogenesis and Pathophysiology

Etiology is unknown.
Peak age: range 9 to 83 years

Histopathology

Similar to cellular myxoma variant of intramuscular myxoma
- Bland spindle cells
- Myxoid background
Cystic-like ganglion spaces

Genetics

Lacks the GNAS1 mutations seen in intramuscular myxomas

Diagnosis

See Chapter 2 for diagnostic work-up.

Clinical Features

Frequently painful or tender soft tissue mass
Present for weeks to years
May be associated with meniscal tear
May be incidental finding during knee or hip arthroplasty

Radiologic Features

Similar to intramuscular myxoma
Distinguished by juxta-articular location

Treatment and Outcome

Marginal excision
Local recurrence in up to one third

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Figure 11-39 This intramuscular myxoma within the quadriceps shows characteristically hypointense signal on T1-weighted sagittal (A) and axial (C) MRI sequences and hyperintense signal on T2-weighted coronal (B) and axial (D) sequences.