A case of infantile rhabdomyofibrosarcoma with immunohistochemical electronmicroscopical, and genetic analyses

A case of infantile rhabdomyofibrosarcoma arising on the buttocks of a 15-m onth-old boy is reported with histological, immunohistochemical, electronmi croscopical, and cytogenetic findings. Histological examination showed a pr oliferation of spindle-shaped cells in a fasciculated pattern, with occasio nal rounded rhabdomyoblastic cells with abundant eosinophilic cytoplasm. Im munohistochemically; the tumor cells expressed desmin and MyoD1 but were on ly weakly positive for myoglobin. No clear rhabdomyoblastic features were o bserved by electronmicroscopic examination. Chromosome analysis showed a cl one of 46, XY, der(2)t(2;11) (q37;q13), different from any karyotypic abnor mality in the original report of this neoplasm. Loss of heterozygosity at 1 1p15.5, the most frequent genetic alteration in embryonal rhabdomyosarcoma, was not detected. The low degree of striated muscle differentiation and tu mor localization supported the diagnosis of infantile rhabdomyofibrosarcoma rather than spindle-cell rhabdomyosarcoma in this case. The present case h as been uneventful as of 25 months after surgery. The rather long recurrenc e-free period, which has not been reported in previous cases, may be attrib utable to chemotherapy-induced rhabdoid differentiation of the tumor cells. Copyright (C) 1999 by W.B. Saunders Company.