Skeletal muscle actin as tumor marker in the diagnosis of rhabdomyosarcoma in childhood.

Immunoaffinity-purified and tissue-specific antibodies against carp skeletal muscle actin have been assessed for their usefulness in diagnosing rhabdomyosarcoma. Routinely processed formaldehyde-fixed tissue and the avidin--biotinyl--peroxidase complex technique were used. Thirty-six tumors of patients varying in age from less than 1 year to 17 years were diagnosed as rhabdomyosarcoma on the basis of routine histological stains or electron microscopy, and on clinical grounds. Among them were 20 poorly differentiated tumors. All moderately and well-differentiated rhabdomyosarcomas and the majority of the poorly differentiated tumors (13 of 20) showed positive immunostaining for actin. Positive staining was observed in all three types of rhabdomyosarcoma, i.e., embryonal, alveolar, and pleomorphic. Besides rhabdomyosarcomas, the only other positive neoplasms were those that contained rhabdomyoblastic differentiation such as malignant "triton" tumors and malignant mixed müllerian tumors. Our results indicate that antibodies against skeletal muscle actin are a powerful tool for diagnosing rhabdomyosarcoma and that they can be used to distinguish the poorly differentiated forms from other types of small round cell tumors in childhood such as neuroblastoma, Ewing's sarcoma, and malignant lymphoma. The results are discussed in the light of the embryogenesis of cross-striated skeletal muscle.