Soft Tissue

Clinical Features

• Primarily affects young adults aged 20 to 40 years; occasionally seen in children

• Presents as a rapidly growing solitary mass; may be painful

• Inconsistently associated with recognized previous trauma (10% to 15%)

• Can involve any site; flexor aspect of forearm, chest, and back are common sites

• Spontaneous regression is expected

Gross Pathology

• Located in the deep dermis or subcutis; occasionally occurs intramuscularly

• Round to oval, nodular, well-circumscribed mass; usually smaller than 3 cm

• Cut surface may be fibrous, myxoid, or cystic

Histopathology

• Usually well circumscribed but occasionally infiltrative

• “Tissue culture” appearance with long fascicles of spindled cells with a whorled growth pattern; extravasated red blood cells are a helpful feature ( Figure 17.1 )

  

• Newer lesions have a loose, feathery collagenous stroma with myxoid or microcystic appearance, whereas older lesions are less cellular and more densely collagenized

• Zonal pattern with cellular periphery and loose, feathery center that may be cystic

• Scattered inflammatory cells, typically lymphocytes and macrophages

• Occasional mitotic figures; no abnormal mitotic figures

• Variants

  • Intravascular fasciitis

    • Primarily affects children and adolescents

    • Involves arteries and veins

  • Cranial fasciitis

    • Affects infants younger than 1 year

    • Involves the scalp and skull

  • Ossifying fasciitis

    • Often shows osseous metaplasia

    • Periosteal location

    • Similar to myositis ossificans but lacks triphasic zonal pattern

Special Stains and Immunohistochemistry

• Vimentin and smooth muscle actin (SMA) positive

• Immunohistochemistry does not help to exclude other myofibroblastic or smooth muscle proliferations

Other Techniques For Diagnosis

• Translocation t(17;22)(p13;q13.1), producing a MYH9-USP6 fusion demonstrable by molecular or in situ hybridization studies, typically cryptic by karyotype

  • Only seen in nodular fasciitis, not its variants

Differential Diagnosis

Clinical Features

• Typically occurs in adults (usually about 50 years of age)

• Firm, palpable, rapidly growing subcutaneous or intramuscular nodule; may be painful

  • Proliferative fasciitis

    • Most common site is forearm, followed by leg and trunk

    • Often associated with a history of trauma

  • Proliferative myositis

    • Commonly located in the flat muscles of the trunk and shoulder girdle

Gross Pathology

• Poorly circumscribed, gray-white soft tissue mass

• Typically measures 1 to 3 cm in diameter

• Proliferative myositis is commonly a pale, gray, scarlike induration involving muscle and overlying fascia

Histopathology

• Ill-defined lesions characterized by large myofibroblasts that have large vesicular nuclei, prominent nucleoli, and abundant eosinophilic cytoplasm (ganglion-like cells) admixed with immature spindle cells in a matrix composed of varying proportions of mucoid material and collagen

• Often numerous mitotic figures in spindled and ganglion-like cells; they are not atypical ( Figure 17.2 )

  • Proliferative fasciitis

    • Typically grows along fibrous septa between fat lobules

  • Proliferative myositis

    • Endomysial and epimysial growth separates bundles of atrophic skeletal muscle, creating a checkerboard pattern

  

Special Stains and Immunohistochemistry

• Ganglion-like cells are often nonreactive toward muscle markers and react with vimentin only

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Commonly affects young, athletic adults; usually involves the extremities

• Uncommon in children

• Presents as a solitary, tender mass; often associated with a history of trauma (>50% of cases)

• Radiographic findings show characteristic zonal ossification, with rapid mineralization evident on sequential studies

Gross Pathology

• Well-circumscribed, gray-yellow lesions with gritty areas

• Some cases have central blood-filled spaces and are regarded as a variant of aneurysmal bone cyst occurring outside of bone ( Figure 17.3 )

  

Histopathology

• Typically shows a triphasic pattern with distinct zonation

  • Central cellular region

    • Resembles nodular fasciitis

    • Cells have bland nuclear features and a variable mitotic rate

    • Occasional multinucleated giant cells

  • Intermediate region is composed of immature osteoid

  • Peripheral zone is composed of mature, “purposeful” lamellar bone

    • Cartilage may be present

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• USP6 translocation is detectible by fluorescence in situ hybridization (FISH) in some cases

Differential Diagnosis

Clinical Features

• Also referred to as atypical decubital fibroplasia

• Occurs over bony prominences or other pressure points in debilitated patients

• Almost exclusively seen in late adulthood and rarely in younger patients

• More commonly found in females

Gross Pathology

• Poorly circumscribed, multinodular mass up to 10 cm in diameter

• May have overlying ulceration

Histopathology

• Typical zonation pattern

  • Central necrotic region

    • Liquefactive or coagulative necrosis with fibrin deposition ( Figure 17.4 )

      

  • Peripheral fibroblastic and vascular proliferation

    • Granulation tissue-like with plump endothelial cells

    • Atypical fibroblasts with abundant eosinophilic cytoplasm and ganglion-like features

    • Vascular thrombosis and fibrinoid necrosis

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Usually presents as a deeply seated mass located in the lower subscapular area

• Almost exclusively seen in late adulthood and rarely in younger patients

• More commonly found in females

Gross Pathology

• Firm, rubbery soft tissue mass with ill-defined margins

• Cut surface is gray-white and glistening with entrapped foci of fat

• Focal cystic degeneration often seen

Histopathology

• Poorly defined lesion composed of thickened, coarse slightly basophilic elastic fibers and scant fibroblastic cells embedded in a heavily collagenized stroma ( Figure 17.5A )

  

• Entrapped mature adipose tissue is typically seen

Special Stains and Immunohistochemistry

• Verhoeff–van Gieson elastic stain: highlights elastic fibers ( Figure 17.5B )

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Presents as a small, slow-growing, subcutaneous nodule or thickening

  • Palmar fibromatosis (Dupuytren contracture)

    • Palmar surface of the hand; may result in contractures

    • Almost exclusively in adults, males affected more than females

    • Often bilateral, especially in alcoholics

  • Plantar fibromatosis (Ledderhose disease)

    • Plantar, non-weight-bearing area of the foot

    • Occurs in both children and adults

    • Often multinodular

  • Penile fibromatosis (Peyronie disease)

    • Dorsal aspect of the shaft of the penis

    • Exclusively seen in adults

Gross Pathology

• Single or multiple, gray-white, firm nodules or scarlike tissue in the subcutis

Histopathology

• Proliferative and involutional phases

• Proliferative phase shows variably cellular fascicles of bland, spindled cells often arranged in a nodular pattern ( Figure 17.6 )

  

• Occasionally prominent giant cells in plantar lesions

• Mitotic figures may be seen

• Involutional or residual phase shows paucicellular, densely collagenized tissue

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Rapidly growing, painless subcutaneous mass in young children, sometimes congenital

• Common sites include trunk, shoulder, axilla, and groin

• Most cases occur within the first 2 years of life

Gross Pathology

• Poorly defined deep dermal or subcutaneous mass

• Gray, firm cut surface with yellow flecks

• Usually 2 to 5 cm but may be larger

Histopathology

• Triphasic appearance comprising an admixture of fibrous tissue, adipose tissue, and bundles of immature mesenchymal cells ( Figure 17.7 )

  

• Often has a stellate configuration and infiltrates surrounding fat

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Previously referred to as infantile fibromatosis, nondesmoid type

• Occurs in childhood, between birth and second decade; males affected more than females

• Slowly growing, painless mass most commonly presenting in an extremity or on the trunk; rare cases in the head and neck

• May cause isolated macrodactyly

Gross Pathology

• Poorly defined subcutaneous mass with admixed adipose tissue

• Usually 1 to 3 cm

Histopathology

• Bands of bland spindled cells and collagen traversing through mature adipose tissue ( Figure 17.8 )

  

• Infiltrative borders

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Also known as juvenile aponeurotic fibroma or Keasbey tumor

• Most commonly affects children but may also occur in adults

• Presents as a slow-growing, painless mass, usually on the palmar or plantar surfaces of the hands or feet, rarely in other locations

Gross Pathology

• Poorly circumscribed, firm, gray-white, rubbery nodule usually smaller than 3 cm

• Gritty cut surface

Histopathology

• Bland oval plump fibroblasts in a heavily collagenized stroma

• Foci of stippled to confluent amorphous calcifications surrounded by rounded chondrocyte-like cells ( Figure 17.9 )

  

• Infiltrative margins with extension into adipose tissue

• Osteoclast-like giant cells may be associated with calcification

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Most commonly affects the hands of young to middle-aged adults; slight male predominance

• Presents as a slow-growing, painless mass, usually on the preaxial digits or wrist but may affect foot or knee joint

Gross Pathology

• Circumscribed, firm, gray-white, lobulated nodule attached to the tendon, usually smaller than 3 cm

• Fibrous, often multinodular cut surface separated by clefts

Histopathology

• Spindled to stellate cells embedded in a collagenous stroma, sometimes myxoid

• Clefted, pseudovascular spaces separate the nodules ( Figure 17.10 )

  

• Degenerative cytologic atypia may be present; giant cells are rare

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Also known as desmoplastic fibroblastoma

• Most commonly presents as slow growing painless mass in subcutaneous or deep tissues of upper extremities

• The majority occur in late adulthood but can occur in childhood

Gross Pathology

• Well-circumscribed mass, usually less than 5 cm ( Figure 17.11A )

  

• Cut surface gray-white with a lobulated or whorled appearance

Histopathology

• Hypocellular spindle to stellate cell mass embedded in a loose collagenous stroma ( Figure 17.11B )

• Mitotic figures are typically not seen

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Also known as infantile congenital myofibromatosis or congenital myofibromatosis in children

• Most common fibrous tumor of infancy

• About 90% occur within the first 2 years of life; however, adults may be affected

• Myofibroma refers to a solitary lesion (common), whereas myofibromatosis denotes multiple skin and soft tissue lesions with variable visceral involvement

  • Solitary subcutaneous nodules typically involve the head and neck but can occur anywhere

  • Multicentric form may involve the lungs, heart, bones, and gastrointestinal (GI) tract

Gross Pathology

• Cut surface is rubbery gray-white with a lobulated or whorled appearance

• May have central necrosis or cyst formation

• Margins may be well defined or focally infiltrative

• Size from 0.5 cm up to 8 cm

Histopathology

• Typically shows a biphasic pattern or zonal phenomenon

  • Peripheral areas show fascicular or whorled growth of plump, spindled cells with eosinophilic cytoplasm (myofibroblasts)

  • Central areas of the lesion are more cellular with oval cells and a staghorn-appearing, hemangiopericytoma-like vasculature ( Figure 17.12 )

    

• Variable mitotic activity but no atypical division figures

• Scattered lymphoplasmacytic infiltrate typically present

• Polypoid protrusion into vascular spaces is typical at the edge of the lesion

• Focal areas of hemorrhage, calcification, and necrosis may be seen centrally

• May be well circumscribed or infiltrative

Special Stains and Immunohistochemistry

• SMA and MSA positive

• Desmin variable

• Immunohistochemistry does not help to exclude other myofibroblastic or smooth muscle proliferations

Other Techniques for Diagnosis

• Noncontributory aside from ruling out other selected lesions such as infantile fibrosarcoma

Differential Diagnosis

Clinical Features

• Benign lesion of childhood and early adulthood that has a strong association with desmoid-type fibromatosis and familial adenomatous polyposis (Gardner syndrome)

• Poorly defined, plaquelike soft tissue mass in superficial and deep tissues of back and paraspinal region, head and neck, extremities, and chest

Gross Pathology

• Ill-defined firm mass with a white-gray, rubbery cut surface

• Ranges in size from 1 to 12 cm

Histopathology

• Sheets of densely hyalinized bundles of collagen containing scant, small spindle cells ( Figure 17.13 )

  

• Collagen fibers are separated by cracks or clefts

• Infiltrative borders are seen with entrapped connective tissue

Special Stains and Immunohistochemistry

• CD34 positive

• β-Catenin: most are positive with nuclear labeling

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Also referred to as aggressive or deep fibromatosis

• Relatively common neoplasm that typically occurs in adolescents and young adults, but age range is wide

• Comprises a group of proliferative tumors that present as deep-seated masses

• Shoulder region, chest wall, thigh, and mesentery are favored sites

  • Musculoaponeurotic fibromatosis

  • Abdominal fibromatosis

  • Mesenteric fibromatosis

    • Lesions are associated intimately with muscular aponeuroses

    • Rectus muscle is the favored location

    • Occurs almost exclusively in women who are pregnant or postpartum

    • Found in mesentery of the bowel or retroperitoneum

    • Often associated with previous history of abdominal surgery

    • May be associated with Gardner syndrome (familial adenomatous polyposis, mesenteric fibromatosis, osteomas, and multiple epidermal inclusion cysts)

Gross Pathology

• May appear well defined but actually has infiltrative margins

• Often grows along fascial planes

• Firm tumor that often has a gritty cut surface

• Sectioning reveals a glistening, white, trabeculated surface

Histopathology

• Composed of uniform-appearing, spindle-shaped fibroblasts and abundant collagen ( Figure 17.14 )

  

• Infiltrative margins

• Occasional mitotic figures are present in more cellular areas

• Inconspicuous vasculature

• Myxoid matrix may be seen, primarily in abdominal fibromatosis

Special Stains and Immunohistochemistry

• β-Catenin: positive nuclear immunoreactivity

• SMA positive

Other Techniques for Diagnosis

• Recurrent chromosomal abnormalities include trisomies 8 and 20 and loss of 5q, not usually needed for diagnosis; CTNNB1 gene sequencing may identify mutations with prognostic relevance.

Differential Diagnosis

Clinical Features

• Benign fibrous tumor that occurs predominantly in adolescents and young adults

• Most common in subcutaneous and deep soft tissues of extremities, trunk, groin, and neck but has been described in many locations, including viscera

Gross Pathology

• Typically a circumscribed solid mass, 3 to 5 cm, but may be larger

• Cut surface is solid, firm, and gray-white

Histopathology

• Hypocellular, sclerotic tissue with a sparse lymphoplasmacytic infiltrate and discrete calcifications ( Figure 17.15 )

  

• Calcification may be psammomatous or dystrophic

• Germinal center formation may be seen at lesion periphery

Special Stains and Immunohistochemistry

• Noncontributory

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Previously known as inflammatory pseudotumor and plasma cell granuloma

• Most often occurs in children and young adults but has a wide age range

• Commonly seen in the lung; the most frequent extrapulmonary sites are mesentery and omentum, but it can involve any location

• Systemic symptoms and signs may be present, including fever, weight loss, anemia, increased erythrocyte sedimentation rate, and elevated C-reactive protein levels

Gross Pathology

• Typically circumscribed, but nonencapsulated; often multinodular

• Cut surface is solid, firm, and gray-white

Histopathology

• Variably cellular tumor comprised of spindle cells and mixed inflammatory cells in a myxoid or collagenized background ( Figure 17.16 )

  

• Some lesions contain large histiocytoid ganglion-like cells

• May be hypocellular and resemble scars

• Mitotic figures may be numerous

Special Stains and Immunohistochemistry

• Variably positive for smooth muscle markers

• ALK-1 protein present in about 40% of cases, more frequently in childhood tumors

Other Techniques for Diagnosis

• Rearrangement of ALK locus at 2p23 by molecular or cytogenetic analysis

Differential Diagnosis

Clinical Features

• Typically occurs in middle-aged adults but has a wide age range

• Includes cases previously called hemangiopericytoma (except nasal type)

• Presents as a localized, slow-growing, painless mass

• Most commonly involves the pleura; extrapleural sites include subcutaneous and deep soft tissues, orbit, retroperitoneum, mediastinum, pericardium, and other locations

Gross Pathology

• Ranges in size from 1 to 27 cm

• Typically well circumscribed with a firm, tan-white cut surface; sometimes multinodular

• Focal necrosis, hemorrhage, and cystic degeneration may be seen

Histopathology

• Characterized by uniform spindle cells haphazardly arranged in a collagenized background ( Figure 17.17 ); collagen focally surrounds individual cells

  

• Hypercellular and hypocellular, collagenous areas (so called “patternless pattern”)

• “Hemangiopericytoma-like” open, staghorn-shaped, and hyalinized blood vessels

• Epithelioid areas may be present

• Low mitotic activity (<4 mitotic figures/10 high-power fields)

• Criteria for malignancy include dense cellularity, numerous mitotic figures, obvious cytologic atypia, necrosis, and infiltrative growth; “dedifferentiation” (transformation to high-grade sarcoma) has been reported

Special Stains and Immunohistochemistry

• CD34 positive in about 85% of cases, CD99 and bcl-2 positive in about 75% of cases

• Nuclear immunoreactivity for STAT6 in 98% of cases

  • STAT6 staining may be lost with transformation to high grade

Other Techniques for Diagnosis

• Genomic inversion at 12q13 leading to fusion NAB2-STAT6 in majority of cases

Differential Diagnosis

Clinical Features

• Hemosiderotic fibrolipomatous tumor (HFLT; also known as hemosiderotic fibrohistiocytic lipomatous lesion ) and myxoinflammatory fibroblastic sarcoma (MIFS) are discussed together due to their clinicopathologic and molecular similarities; hybrid tumors have been reported

• Presents as a localized, slow-growing, infiltrative mass of the distal extremities

  • HFLT is most common in the subcutis of the dorsum of the foot and ankle

  • MIFS is usually on the hands and is locally infiltrative into synovium and tendons

• Adults are affected more often than children, and HFLT is more common in women

Gross Pathology

• Ranges in size from 1 to 20 cm and has infiltrative borders

• Tan to yellow cut section but may be gelatinous or fatty; hemorrhage may be present

Histopathology

• HFLT is characterized by spindle cells admixed with mature adipose tissue

  • Abundant hemosiderin-laden macrophages and scattered inflammatory cells are present in the spindle cell component ( Figure 17.18A )

    

• MIFS is heterogenous with a myxoid component containing pseudolipoblasts, neutrophils and lymphocytes, and a bland spindle cell proliferation ( Figure 17.18B )

  • Large ganglion celllike or Reed-Sternberg–like cells with prominent nucleoli are present in MIFS

  • Low mitotic activity (<3 mitotic figures/10 high-power fields)

Special Stains and Immunohistochemistry

• CD34 immunoreactivity in the spindle cell component in both HFLT and MIFS

• SMA, desmin, and S100 are negative

Other Techniques for Diagnosis

• Presence of t(1;10) (p22;q24) producing a TGFBR3-MGEA5 fusion has been reported in both HFLT and MIFS (as well as pleomorphic hyalinizing angiectatic tumor)

Differential Diagnosis

Clinical Features

• Also known as Evans tumor. One variant originally termed “hyalinizing spindle cell tumor with giant rosettes”

• Typically seen in young adults but can occur in children and elderly

• Deep soft tissue mass most often in proximal extremities or trunk

• May be present for several years before diagnosis

Gross Pathology

• Usually a large and nonencapsulated but well-circumscribed mass

• Cut surface is firm, white, or tan, sometimes with a myxoid appearance

Histopathology

• Bland fusiform cells with variably collagenous to myxoid stroma ( Figure 17.19 )

  

• Collagenized areas are arranged in short haphazard fascicles

• A whorling growth pattern, myxoid nodules, and a prominent arcade of hyalinized vessels may be present

• Rosettes may be present, characterized by hyalinized nodules cuffed by tumor cells

• Mitotic figures are absent or sparse

• May be seen as a hybrid with sclerosing epithelioid fibrosarcoma or less commonly with an undifferentiated round cell component

Special Stains and Immunohistochemistry

• Immunoreactive toward MUC4

• Variably positive for EMA, SMA, desmin, and CD34

Other Techniques for Diagnosis

• Presence of t(7;16)(q34;p11), producing an FUS-CREBL2 fusion, can be demonstrated by cytogenetic or molecular analysis in most cases

Differential Diagnosis

Clinical Features

• Also known as myofibrosarcoma

• Distinctive low-grade tumor with myofibroblastic differentiation

• Tumor of middle-aged adults, rarely reported in children

• Most commonly involves head and neck

Gross Pathology

• Firm mass with white cut surface and poorly defined margins

Histopathology

• Moderately cellular spindle cell lesion arranged in fascicles and whorls ( Figure 17.20 )

  

• Modest nuclear hyperchromasia and mild cellular pleomorphism

• Infiltrates adjacent tissues

• Mitotic figures are variable in number

Special Stains and Immunohistochemistry

• Desmin, MSA, SMA: at least one is positive

Other Techniques for Diagnosis

• Noncontributory

Differential Diagnosis

Clinical Features

• Occurs primarily in children younger than 2 years; about 25% are congenital

• Most common on extremities, followed by trunk and head and neck

• May mimic a vascular lesion both clinically and radiographically; large “hemangiomas” in young children should undergo biopsy if they enlarge

Gross Pathology

• Infiltrative borders

• Firm, fleshy, lobulated mass, often large

• Cut surface is gray-white to tan-yellow

Histopathology

• Cellular tumor is characterized by apposed, spindle-shaped fibroblasts arranged in interlacing fascicles or a herringbone pattern ( Figure 17.21 )

  

• Frequent mitotic activity is seen, sometimes with atypical forms

• Necrosis and hemorrhage are common

• Myxoid or collagenous stroma may be seen

• May have focal hemangiopericytoma-like vasculature

• Scattered chronic inflammatory cells and focal extramedullary hematopoiesis are seen

Special Stains and Immunohistochemistry

• Negative for epithelial, muscle, and neural markers as well as CD34, bcl-2, and CD99

Other Techniques for Diagnosis

• Up to 90% of cases have t(12;15)(p13;q26) that creates a fusion gene, ETV6-NTRK3 ( TEL-TRCKC ): this may be cryptic on conventional karyotyping and requires reverse transcription polymerase chain reaction or fluorescent in situ hybridization studies

Differential Diagnosis

Clinical Features

• Rare tumor of middle-aged to elderly adults

• Located in deep tissue of extremities, trunk, or head and neck; rarely other locations

• A similar histologic pattern is seen in transformation of dermatofibrosarcoma protuberans

• May be seen as a post-irradiation neoplasm

Gross Pathology

• Firm, lobulated mass usually 3 to 10 cm in diameter

• Small tumors may be well circumscribed

• Cut surface is gray-white to tan-yellow with hemorrhage or necrosis

Histopathology

• Variably hyperchromatic spindle cells with eosinophilic or amphophilic cytoplasm, may show a herringbone growth pattern

• Variable mitotic activity

• Lacks significant pleomorphism

Special Stains and Immunohistochemistry

• Negative for epithelial, muscle, and neural markers as well as CD34, CD99, bcl-2, STAT6, and nuclear β-catenin. Preserved expression of H3K27me2

Other Techniques for Diagnosis

• Noncontributory except to rule out other tumors, especially monophasic synovial sarcoma t(X;18)

Differential Diagnosis

Clinical Features

• Distinctive variant of fibrosarcoma

• Tumor of middle-aged adults, but has been reported in children

• Common locations include deep soft tissue of extremities, trunk, chest wall, or head and neck; may be painful

Gross Pathology

• Firm, oval, or lobulated soft tissue mass ranging in size from 2 to 20 cm

• Cut surface is gray-white; may have myxoid or cystic areas

Histopathology

• Nests or cords of uniform, round to oval tumor cells with eosinophilic to clear cytoplasm embedded in a densely hyalinized stroma ( Figure 17.22 )

  

• May have fascicular, myxoid, or cystic areas and hemangiopericytoma-like vasculature

• Mitotic figures are infrequent

• Some cases have an associated low-grade fibromyxoid sarcoma

Special Stains and Immunohistochemistry

• Most cases are positive for MUC4, variably for EMA. Typically negative for cytokeratins