Synovial sarcoma in cerebellum: a case report and literature review.

Synovial sarcoma is a tumor of unknown origin and is extremely rare in the central nervous system. We present a case involving an unusual cerebellar synovial sarcoma in a male infant. Neuroimaging revealed a large, solid, gadolinium-enhancing mass located in the parenchyma of the right cerebellar hemisphere and associated with multiple cyst formation. Histologically, the tumor was composed of uniform spindle cells with indistinct borders and numerous mitotic figures. The tumor cells were observed to form dense cellular sheets, but in some areas the tumor showed a hemangiopericytomatous vascular pattern consisting of tumor cells arranged around dilated, thin-walled blood vessels. Immunohistochemistry showed that vimentin, CD99 and Bcl-2 were diffusely positive in most cells, and focal reactivity for cytokeratin (AE1/AE3) and S-100 protein was also observed. The tumor cells were, however, negative for CK19, EMA, CD34, synaptophysin, GFAP, desmin, myogenin, and smooth muscle actin. Cytogenetic analysis using fluorescence in situ hybridization demonstrated the translocation t(X;18)(p11;q11). A diagnosis of primary cerebellar monophasic synovial sarcoma was made. To our knowledge, this is the first report of a synovial sarcoma in brain parenchyma. The present case indicates that it is essential to select the appropriate immunohistochemical panel and-especially-perform molecular analysis to accurately diagnose intracranial spindle cell tumors.