Soft tissue tumor with prominent leptomeric fibrils and complexes.

A recurrent soft tissue tumor from the groin in a 37-year-old male is described. The tumor cells contain many leptomeric structures, both fibrils and complexes, but none of the other characteristic features of striated muscle: alternating parallel thick and thin myofilaments with Z-line material. A review of one rhabdomyoma and six rhabdomyosarcomas demonstrates that leptomeric structures are present in large amounts in the rhabdomyoma but are very infrequent in only three of the six rhabdomyosarcomas. A careful search for leptomeric structures is indicated when the ultrastructural diagnosis of either rhabdomyoma or rhabdomyosarcoma is being considered.