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SPINDLE CELL TUMOR / SARCOMA SUBTYPING

Subtyping of spindle cell tumors and sarcomas requires integration of histology, clinicopathologic parameters,

and results of IHC and/or FISH studies. Three subgroups of spindle cell tumors/sarcomas can be identified:

1. Tumors showing cell-specific differentiation (e.g., leiomyoma/ leiomyosarcoma, peripheral nerve sheath tumors)

2. Sarcomas with specific translocations (e.g., PNET/Ewing sarcoma, synovial sarcoma)

3. Sarcomas with single gene alterations, i.e., mutations, deletions, and amplification (e.g., rhabdoid tumor, dedifferentiated

liposarcoma, etc.)

IHC studies are most efficacious in identifying group 1, while a combination of IHC and FISH studies, or sometimes FISH studies

alone, are most efficacious in identifying the latter two groups.

The following cell type-specific IHC markers can be useful in the identification of sarcomas showing cell-specific differentiation:

Markers of epithelial differentiation in soft tissue tumors: Some sarcomas display true epithelial differentiation

(e.g., synovial sarcomas and epithelioid sarcomas) and concomitant cytokeratin expression. Other sarcomas (e.g., leiomyosarcomas,

melanomas, and angiosarcomas) can display ‘anomalous’ cytokeratin expression.

Markers of smooth muscle differentiation: The principal markers of smooth muscle differentiation are antibodies to the

intermediate filament, desmin, and to muscle-specific actin isoforms (e.g., antibodies 1A4 and HHF-35). Such antibodies are most

useful in identifying tumors such as leiomyosarcomas, although smooth muscle actin expression can be seen in other tumors showing

myofibroblastic differentiation.

H&E SM actinA case of low-grade leiomyosarcoma showing strong expression of smooth muscle actin identified by the 1A4 clone.

H&E 1A4Case of abdominal fibromatosis showing characteristic myofibroblastic differentiation, with peripheral ‘tram-tracking’ pattern

of smooth muscle actin expression.

Markers of myofibroblastic differentiation: Myofibroblasts are distinguished from true smooth muscle cells by their

‘partial’ muscle immunophenotype, usually smooth muscle actin positive and desmin negative. Furthermore, smooth muscle actin is

generally expressed in a characteristic peripheral, wispy pattern. Soft tissue tumors characterized by myofibroblastic differentiation

include nodular fasciitis and inflammatory myofibroblastic tumors; however, myofibroblastic differentiation can be seen in a wide

range of soft tissue tumors.

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SPINDLE CELL TUMOR / SARCOMA SUBTYPING

Markers of nerve sheath differentiation: The principal markers of nerve sheath differentiation are antibodies to S100

protein and the p75-NTR nerve growth factor receptor. They are most helpful in confirming the diagnosis of benign and malignant

nerve sheath tumors. None of these markers, however, is specific for nerve sheath differentiation and must be used in combination

with other markers. p75-NTR is also an excellent marker of spindle cell and desmoplastic melanomas, which are generally negative for

expression of melanoma-specific antigens.

H&E p75-NTRCase of peripheral nerve sheath tumor showing expression of the p75-NTR nerve growth factor receptor.

Markers of endothelial differentiation: Endothelial markers include von Willebrand factor (the co-factor of the clotting

factor VIII, often erroneously referred to as ‘F. VIII’), CD34, CD31, and the FLI-1 gene product, with varying sensitivities and

specificities. As a practical issue, it is best to employ both a highly specific (e.g., CD31) and a highly sensitive (e.g., FLI-1) marker to

assess the presence of endothelial differentiation in histologic and clinical settings where the diagnosis of a vasoformative tumor is

being entertained.

H&E CD31Case of spindle cell angiosarcoma showing expression of the endothelial-restricted marker, CD31.

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SPINDLE CELL TUMOR / SARCOMA SUBTYPING

Other useful IHC markers in identifying soft tissue tumors:� Type IV Collagen: While not specific for a cell or tumor type, the presence and even pattern of expression of type IV

collagen can be helpful in identifying specific soft tissue tumor subtypes, as only a subset of sarcomas produce a type IV

collagen; furthermore, the pattern of type IV collagen expression can provide evidence to help identify selected spindle cell

tumors.

����������� is normally a cell membrane-associated protein. However, alterations in the APC/beta catenin pathway result

in abnormal nuclear localization of this protein. Only a restricted subset of spindle cell tumors, including fibromatoses,

endometrial stromal tumors, solitary fibrous tumors, and synovial sarcomas, show this unique distribution of beta catenin.

� DOG1 and CD117 (c-kit) are highly sensitive and specific markers for gastrointestinal stromal tumors. DOG1 is

particularly useful in identifying the subset of c-kit negative gastrointestinal stromal tumors (GISTs).

���� � is a marker of a unique subset of spindle cell tumors. In addition to vascular tumors, CD34 is expressed in

dermatofibrosarcoma protuberans, solitary fibrous tumor/hemangiopericytoma, and a subset of nerve sheath tumors,

among others.

������� is a highly sensitive and specific marker for synovial sarcoma. This protein, initially identified with gene expression

profiling, is best used as a ‘screening marker’ for synovial sarcoma and, if positive, followed by confirmatory FISH for the

SYT translocation. Other spindle cell tumors may occasionally show high-level expression of TLE-1 such as peripheral

nerve sheath tumors.

������� is a tumor supressor gene product, the loss of which has been found to be a characteristic finding of the following

tumors: rhabdoid tumors, atypical teratoid/ rhabdoid tumors of the CNS, epithelioid sarcoma, MPNST, myxoid

chondrosarcoma, and soft tissue myoepitheliomas.

������� is the glycoprotein to which the monoclonal antibody HMB-45 is directed. In addition to being a marker of

melanoma and clear cell sarcoma, gp100 expression also characterizes the group of tumors known as PEComas (PEC =

perivascular epithelioid cell), a family of spindle to epithelioid tumors that includes angiomyolipoma, sugar tumor of the

lung, pulmonary lymphangioleiomyomatosis, clear cell myomelanocytic tumor of the falciform ligamentum teres, and

others.

TLE-1Synovial sarcoma showing uniform expression of the nuclear protein, TLE-1.

Beta cateninNuclear localization of beta catenin is a characteristic feature of the cells of abdominal fibromatosis.

CD117 (c-kit)Strong uniform expression of c-kit in a case of gastrointestinal stromal tumor.

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SPINDLE CELL TUMOR / SARCOMA SUBTYPING

TUMOR TRANSLOCATIONFUSION GENE FORMED

TYPE

Alveolar rhabdomyosarcoma

t(2;13)(q35;q14) PAX3-FOXO1 Breakapart (FOXO1)

Angiomatoid fibrous histiocytoma

t(12;22)(q13;q12)t(2;22)(q33;q12)t(12;16)(q13;p11)

EWSR-ATF1EWSR-CREB1FUS-ATF1

Breakapart (EWSR)Breakapart (FUS)

Desmoplastic small round cell tumor

t(11;22)(p13;q12) EWSR-WT1 Breakapart (EWSR)

PNET/Ewing sarcoma

t(11;22)(q24;q12)t(21;22)(q24;q12)t(7;22)(q22;q12)and others

EWSR-FLI-1EWSR-ERGEWSR-ETV1EWSR partner for most

Breakapart (EWSR)

Extraskeletal myxoid chondrosarcoma

t(9;22)(q22;q12) EWSR-NR4A3Breakapart (EWSR)

Myxoid/round cell liposarcoma

t(12;16)(q13;p11)t(12;22)(q13;q12)

FUS-DDIT3EWSR-DDIT3

Breakapart FUSBreakapart EWSR

Synovial sarcoma t(X;18)(p11.2;q11.2)SS18-SSX1SS18-SSX2SS18-SSX4

SYT breakapart

Chromosomal TranslocationsApproximately one-third of all sarcomas contain chromosomal translocations which result in the juxtapositioning of two different

genes (one from each translocation partner) that form a ‘fusion gene’. An example is the EWSR-WT1 fusion which results from the

t(11;22)(p13;q12) translocation found in desmoplasic small round cell tumor. For detection of these translocations in tissue sections

of the tumor, two different types of probes or probe sets can be employed: (a) dual fusion, dual color FISH probes, which identify the

chromosomes participating in the translocation; a ‘positive’ result is one in which normally disparate red and green signals ‘fuse’ as a

result of the translocation into a single yellow spot; (b) breakapart probes, in which two different-colored probes span the expected

breakpoint region in one of the partner genes, with a ‘positive’ result yielding disparate (e.g., red and green) spots that would normally

be a single yellow spot.

FISH for SYT translocations of synovial sarcoma: The histologic diagnosis of synovial sarcoma can be difficult as these

tumors often resemble other spindle cell sarcomas. IHC studies are usually helpful in characterizing these tumors, but many of the

marker studies (TLE-1 and beta catenin) used to confirm this diagnosis are not completely specific. Translocations involving the SYT

gene on chromosome 18q11.2 are unique to, and ultimately define, synovial sarcomas, and confirmation of this diagnosis can be made

with high sensitivity and specificity using dual color, ‘breakapart’ FISH studies. In these assays, translocations involving the SYT gene

lead to a breakapart of the normal red-(yellow)-green overlapping signal, resulting in distinct red and green signals, as illustrated below,

which confirm the presence of the SYT translocation characteristic of synovial sarcoma.

FISH for translocations of the SYT gene characteristic of synovial sarcoma

H&E Monophasic synovial sarcoma