Use of anti-skeletal muscle antibody from myasthenic patients in the diagnosis of childhood rhabdomyosarcomas

Rhabdomyosarcoma (RMS), a common soft tissue tumor in children, may often be difficult to distinguish from Ewing's sarcoma, neuroblastoma, and malignant lymphomas. Confirmation of the skeletal muscle origin of RMS depends partly on the demonstration of striations in tumor cells that are usually undetectable in poorly differentiated tumors. A number of tissue markers (e.g., myoglobin and desmin) are currently being used to establish the origin of RMS. However, most of these markers lack specificity and have relatively low sensitivity. We have investigated the specificity and sensitivity of anti-skeletal muscle antibody (ASMA) from patients with myasthenia gravis in the diagnosis of childhood RMS. Out of eight cases of childhood RMS (four embryonal and four alveolar) examined, two showed striations with hematoxylin and eosin and four with phosphotungstic acid hematoxylin. Myoglobin was detected in five tumors; only well-differentiated tumor cells contained myoglobin. Anti-desmin antibody and ASMA reacted with cells in all the eight tumors whether or not the tumor cells were well differentiated. Anti-skeletal muscle antibody did not react with nine lymphomas, four Ewing's sarcomas, four neuroblastomas, four osteogenic sarcomas, four lipomas, eight duct carcinomas of the breast, and eight squamous cell carcinomas of the lung. Eight leiomyomas and four leiomyosarcomas of the uterus were compared for their reactivity with anti-desmin antibody and ASMA. All the tumors stained with anti-desmin antibody and none with ASMA. The results show that ASMA is useful in the diagnosis of childhood RMS and is a more sensitive reagent than anti-myoglobin antibody. Unlike anti-desmin antibody, it can distinguish skeletal muscle tumors from smooth muscle tumors.