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 monoclonal antibody MyoD1 5.8A has recently shown promise as a sensitive and specific marker of rhabdomyosarcomas of childhood. To determine the usefulness of MyoD1 in distinguishing adult pleomorphic rhabdomyosarcoma from other pleomorphic sarcomas, we studied 21 cases of pleomorphic sarcoma, including six cases diagnosed as rhabdomyosarcoma. Immunostaining was accomplished using monoclonal antibodies against MyoD1, desmin, alpha-smooth-muscle actin, and muscle-specific actin on cryostat sections of frozen tumor. All cases of pleomorphic rhabdomyosarcoma stained positively with both desmin and MyoD1. Five cases diagnosed as nonrhabdomyosarcomatous pleomorphic sarcomas stained positively with desmin, but only one showed weak MyoD1 positivity. Three of these five cases were also positive with alpha-smooth-muscle actin. The results of this study suggest that the sensitivity and specificity of the MyoD1 antibody in the differential diagnosis of adult pleomorphic soft tissue sarcomas approaches that seen in pediatric rhabdomysarcomas.