IMMUNOHISTOCHEMICAL DEMONSTRATION OF GLYCOPROTEIN P30 32(MIC2) (CD99)IN SYNOVIAL SARCOMA - A POTENTIAL CAUSE OF DIAGNOSTIC CONFUSION/

Immunohistochemical demonstration of the MIC2 gene product (CD99) repr esents a valuable diagnostic tool that makes identification of periphe ral primitive neuroectodermal tumors easier. Different commercially av ailable antibodies have been raised that show a high sensitivity along with a specificity that is widely regarded as acceptable although not absolute. MIC2 expression has been reported in lymphoblastic lymphoma , occasionally in several other types of sarcoma, and in some neuroend ocrine tumors and ependymomas. The recent personal observation of immu nopositivity for MIC2 gene product in some synovial sarcomas that had been difficult to diagnose prompted us to undertake a systematic study of a larger series of 50 synovial sarcomas, of which 62% showed posit ive immunoreactivity for either 013 (Signet) or 12E7 (Dako MIC2). This is the highest incidence of CD99 positivity reported to date in any t ype of sarcoma other than the Ewing's/primitive neuroectodermal tumor group. Immunopositivity for CD99 was common in all morphologic subtype s of synovial sarcoma. As poorly differentiated synovial sarcoma may o verlap morphologically with primitive neuroectodermal tumor, careful h istologic examination along with evaluation of a wider panel of differ entiation markers is mandatory to avoid this potential diagnostic pitf all, which has both therapeutic and prognostic implications.