[Case of synovial sarcoma in an infant].

The case of a 4-month old boy with monophasic synovial sarcoma of the left thigh is reported. He died of pulmonary metastasis 11 months after surgical removal of the primary tumor. Histologically, the tumor was composed of densely packed, small spindle cells; there were occasional islands and clusters of polygonal cells. Ultrastructurally, the closely packed polygonal cells displayed occasional desmosomelike attachments between apposing cytoplasmic membranes of neighboring cells. Many small abortive duct-like intercellular spaces with elongated cytoplasmic filopodias were seen. These findings were suggestive of epithelial differentiation of some of the tumor cells. In general, synovial sarcoma is a disease of young adults and is rare in children.