Rhabdomyosarcomas with intermediate-filament inclusions and features of rhabdoid tumors. Light microscopic and immunohistochemical study.

A group of 27 rhabdomyosarcomas (RMS) whose histology showed abundant cells containing cytoplasmic intermediate-filament globular inclusions resembling those seen in rhabdoid tumors has been identified among Inter-group Rhabdomyosarcoma Study (IRS) I-III patients (less than 1%). Their histologic subtype was embryonal RMS in 22 and alveolar RMS in 5. One-half of tumors occurred in deep muscles of the extremities, retroperitoneum, or in the pelvis. Immunohistochemical analysis of 12 cases showed the inclusions to be vimentin or desmin positive. Anti-muscle-specific actin antibodies were positive in the cytoplasm of 11 cases, but not in the site of the intermediate-filament inclusions. Seven poorly differentiated neoplasms closely resembled rhabdoid tumors and possessed large nucleoli in most cells along with cytoplasmic inclusions. In contrast to true rhabdoid tumors, their nuclear chromatin was usually coarse. Immunohistochemistry proved useful in distinguishing tumors with early myoblastic differentiation. A positive anti-desmin, when confined to the cytoplasmic inclusions only, should be complemented with other muscle-specific antibodies, especially anti-muscle actin to separate RMS from rhabdoid tumors. The statistical analysis was limited by the small number of cases, but there was no statistical difference in survival when this group of RMS was compared with 996 IRS-II patients as a whole. The distinction of RMS with abundant intermediate-filament inclusions from rhabdoid tumors is of clinical importance because patients with true rhabdoid tumors have a highly unfavorable prognosis.