Undifferentiated small round-cell tumors of childhood: the immunocytochemical demonstration of myogenic differentiation in fine-needle aspirates.

The recognition of myogenic differentiation is not always possible using routine light or electron microscopic techniques. In this article, we describe our experience with two small round-cell neoplasms occurring in young children, each of which exhibited an undifferentiated light microscopic appearance. In both cases, myogenic features were revealed by immunocytochemical methods applied to fine-needle aspiration (FNA) biopsies. Each was immunoreactive for desmin and vimentin and failed to react with antibodies to leukocyte-common antigen. Moreover, formalin-fixed tissue sections of one case were negative for cytokeratin and epithelial membrane antigen. Sporadic reactivity for neuron-specific enolase and Leu-7 antigen was observed in occasional cells in FNA specimens, but synaptophysin was not identified. The rapidity of the method and the reliability of the immunocytochemical findings, when compared with routine cytologic evaluation, emphasize the diagnostic utility of immunocytochemical analysis when FNAs of pediatric soft-tissue tumors are obtained.