S-100 PROTEIN REACTIVITY IN SYNOVIAL SARCOMAS - A POTENTIALLY FREQUENT DIAGNOSTIC PITFALL - IMMUNOHISTOCHEMICAL ANALYSIS OF 100 CASES

S-100 protein, a calcium-binding protein, has a wide distribution with in human tissues, and antisera directed against S-100 protein are used often in diagnostic surgical pathology. Their usefulness, however, is hampered by their low specificity. The monophasic, spindle cell varia nt of synovial sarcoma is a soft-tissue neoplasm that can be difficult to diagnose because of its resemblance to other spindle cell tumors, especially malignant peripheral nerve sheath tumor (malignant schwanno ma, MPNST). Classically, the cells in synovial sarcoma show at least f ocal positive immunoreactivity for epithelial membrane antigen (EMA) a nd cytokeratin, but, generally, they are regarded as negative for S-10 0 protein. Conversely, MPNSTs are, in their great majority, EMA and cy tokeratin negative but S-100 protein positive in at least 50% of the c ases. We have undertaken a systematic study of a large series of 100 s ynovial sarcomas, of which 29% showed focal positive immunoreactivity for S-100 protein in addition to EMA or cytokeratin. Positivity was se en in monophasic fibrous (27% of cases), biphasic (42% of cases), and poorly differentiated tumors. These results show that S-100 protein re activity in malignant spindle cell soft-tissue neoplasms is not synony mous with nerve sheath differentiation, and the use of additional mark ers, including EMA and cytokeratins, is strongly recommended to avoid potential misdiagnosis.