POORLY DIFFERENTIATED SYNOVIAL SARCOMA - IMMUNOHISTOCHEMICAL DISTINCTION FROM PRIMITIVE NEUROECTODERMAL TUMORS AND HIGH-GRADE MALIGNANT PERIPHERAL-NERVE SHEATH TUMORS

Synovial sarcoma is a relatively common sarcoma in adults, which in it s classic bimorphic form infrequently poses a diagnostic problem. Mono morphic spindled variants, as well as the less common poorly different iated variants, may be confused with other soft-tissue sarcomas; the p oorly differentiated variant (PDSS), in particular, may be histologica lly indistinguishable from other small, blue, round cell tumors, inclu ding primitive neuroectodermal tumors (PNETs). Detection of the synovi al sarcoma-associated t(X;18) by either cytogenetic or molecular genet ic approaches may be necessary to confirm the diagnosis of synovial sa rcoma in difficult cases. We evaluated 10 cases of PDSS from eight pat ients using a panel of antibodies (including those to intermediate fil ament proteins, nerve-sheath associated markers, and neuronal and neur oectodermal associated markers) in order to better establish the immun ophenotype of this tumor and to help distinguish it from the tumors wi th which it may be confused, particularly PNETs and high-grade maligna nt peripheral nerve sheath tumors (MPNSTs). Our results showed PDSS to have significant immunophenotypic overlap with both PNETs and MPNSTs. In most instances these three entities may be differentiated by a pan el of antibodies that should include those to both low and high molecu lar weight cytokeratins, epithelial membrane antigen, type IV collagen , CD99, CD56, and S-100 protein. Our results also suggest that synovia l sarcoma may be a tumor showing combined neuroectodermal and nerve sh eath differentiation-perhaps because of translocation-associated expre ssion of specific proteins-rather than a carcinosarcoma of soft tissue s or a tumor of specialized arthrogenous mesenchyme.