Soft-tissue sarcoma of undetermined histogenesis: an ultrastructural study.

A certain proportion of soft-tissue sarcomas are difficult to classify accurately. In this case report, initial light microscope observations of a soft-tissue tumor suggested a diagnosis of a monophasic synovial sarcoma. However, critical evaluation of the histologic features, results of studies with special stains, and ultrastructural observations indicated that this tumor is best categorized as an undifferentiated soft-tissue sarcoma of "undetermined histogenesis."