Intracellular myoglobin--a specific marker for skeletal muscle differentiation in soft tissue sarcomas. An immunoperoxidase study.

Intracellular myoglobin represents an excellent marker for specific characterization of normal (adult and fetal) and malignant skeletal muscle cells in paraffin sections. With an immunoperoxidase indirect sandwich technique for detection of intracellular myoglobin, positive staining was observed in 13 of 17 rhabdomyosarcoma specimens including 5 of 7 of the alveolar type, 5 of 5 of the embryonal type, and 3 of 5 of the pleomorphic type. Initial fixation in Zenker's-acetic acid solution gave optimal staining, but satisfactory results were obtained with fixation in formalin, Bouin's, and B5 solutions. Other types of sarcomas (13 cases) and other types of tumors (24 cases) that sometimes mimic rhabdomyosarcoma on histologic examination gave negative results. The immunoperoxidase method affords a sensitive and specific method for identifying rhabdomyoblasts in tissue on the basis of intracellular myoglobin and is of use in distinguishing rhabdomyosarcomas from other sarcomas and from malignant tumors of other types.