Extraskeletal Ewing's sarcoma. Diagnosis of a case by fine needle aspiration cytology.

A retroperitoneal mass was subjected to fine needle aspiration biopsy for cytologic evaluation. The aspirate consisted of a monotonous population of undifferentiated tumor cells whose cytologic appearance was consistent with a small-cell (Ewing's) sarcoma. The differential diagnosis of extraskeletal Ewing's sarcoma from other small-cell neoplasms, such as lymphoma, neuroblastoma and other soft-tissue sarcoma, cannot be made solely on the basis of morphologic studies. As in this case, histochemical studies and consideration of the clinical features are needed to make the final diagnosis.