Immunohistochemistry of soft tissue tumors. Myoglobin as a tumor marker for rhabdomyosarcoma.

Myoglobin, found exclusively in striated muscle, can be considered a fetal antigen. This study investigated its usefulness as a tumor marker for rhabdomyosarcoma (RMS) using an immunoperoxidase technique. Eight-nine percent (89%) or 27 rhabdomyosarcomas contained immunoreactive myoglobin. In contrast, all other soft tissue tumors (54), carcinomas (10), and numerous normal tissues including smooth muscle were negative. Among RMS, staining correlated directly with cytoplasmic differentiation regardless of histologic type. Importantly, six of nine small cell RMS with no initially appreciated cytoplasm were positive, although focally. Since the three negative RMS had only limited tissue available for study, it remains possible that all RMS would exhibit rare positivity if sizeable specimens were tested. Myoglobin should be considered an extremely sensitive and specific tissue tumor marker for rhabdomyosarcoma of considerable clinical importance.