No tumors displayed areas typical of embryonal or alveolar rhabdomyosarcoma. All tumors exhibited atypical mitotic figures. The two cases studied by electron microscopy both showed evidence for skeletal muscle differentiation. Two patients were alive with no evidence of disease at 12 and 25 years. One patient was dead of disease at 9 years.
Pleomorphic rhabdomyosarcoma is rare but exists in children.
The diagnosis should be considered in pleomorphic sarcomas exhibiting skeletal muscle differentiation, which are otherwise devoid of typical areas or chromosomal changes of embryonal or alveolar rhabdomyosarcoma. Immunohistochemistry is used extensively to distinguish RMS from its mimics. Myogenin and MyoD1, myogenic transcriptional regulatory proteins expressed early in skeletal muscle differentiation, are considered sensitive and specific markers for RMS and are more specific than desmin and muscle-specific actin and more sensitive than myoglobin. Previous studies have focused on expression of myogenin and MyoD1 in small round cell tumors. Nuclear positivity was scored on a three-tiered scale.