Tumors of skeletal muscle.

The most common tumors arising in muscle are soft tissue sarcomas, fibromatoses, and hemangiomas. Rhabdomyosarcoma is primarily a tumor of childhood and adolescence and arises most commonly in extramuscular sites. Most intramuscular rhabdomyosarcomas are alveolar. Increased diagnostic accuracy and the recognition of malignant fibrous histiocytoma have changed understanding of adult, intramuscular, pleomorphic rhabdomyosarcoma. Immunohistochemistry is playing an increasingly important role in the diagnosis of rhabdomyosarcoma, and the correlation between the histologic features and clinical behavior of rhabdomyosarcoma is under investigation. Because of their diversity and overlapping histologic features, muscle tumors are a challenge for the pathologist and require intensive study by current techniques.