EXTRASKELETAL MYXOID CHONDROSARCOMA - AN IMMUNOHISTOCHEMICAL REAPPRAISAL OF 39 CASES

Extraskeletal myxoid chondrosarcoma is an uncommon soft-tissue tumor t hat occurs mainly in the lower limbs of adults. The differential diagn osis includes other myxoid sarcomas, such as myxoid liposarcoma and my xofibrosarcoma, chordoma, and mixed tumors of adnexal origin. Immunohi stochemistry plays a key role in the differential diagnosis, and S-100 protein immunopositivity along with keratin immunonegativity are ofte n said to be important clues. Since it has been our experience that S- 100 protein immunoreactivity in extraskeletal myxoid chondrosarcoma is , in fact, much less common than generally believed, we undertook a sy stematic immunohistochemical analysis of a series of 39 cases. Complet ely negative immunoreactivity for S-100 protein (bath monoclonal and p olyclonal) with positive built-in controls was observed in >80% of cas es. Aside from S-100 staining in seven cases, we also found very focal staining for epithelial membrane antigen (six of 34), keratin (one of 37), and glial fibrillary acid protein (one of 30). Because S-100 pro tein immunoreactivity represents a common finding in many of the lesio ns included in the differential diagnosis (i.e., myxoid liposarcoma, c hordoma, and mixed adnexal tumors), we conclude that in the presence o f a myxoid neoplasm exhibiting positive S-100 protein immunoreactivity , myxoid chondrosarcoma is a relatively less likely diagnosis.