IMMUNOHISTOCHEMISTRY OF MYOD1 IN ADULT PLEOMORPHIC SOFT-TISSUE SARCOMAS

Adult pleomorphic rhabdomyosarcomas are difficult to distinguish from other pleomorphic soft tissue sarcomas. Immunostaining with the monocl onal antibody MyoD1 5.8A has recently shown promise as a sensitive and specific marker of rhabdomyosarcomas of childhood. To determine the u sefulness of MyoD1 in distinguishing adult pleomorphic rhabdomyosarcom a from other pleomorphic sarcomas, we studied 21 cases of pleomorphic sarcoma, including six cases diagnosed as rhabdomyosarcoma. Immunostai ning was accomplished using monoclonal antibodies against MyoD1, desmi n, alpha-smooth-muscle actin, and muscle-specific actin on cryostat se ctions of frozen tumor. All cases of pleomorphic rhabdomyosarcoma stai ned positively with both desmin and MyoD1. Five cases diagnosed as non rhabdomyosarcomatous pleomorphic sarcomas stained positively with desm in, but only one showed weak MyoD1 positivity. Three of these five cas es were also positive with alpha-smooth-muscle actin. The results of t his study suggest that the sensitivity and specificity of the MyoD1 an tibody in the differential diagnosis of adult pleomorphic soft tissue sarcomas approaches that seen in pediatric rhabdomyosarcomas.