Alveolar and poorly differentiated rhabdomyosarcoma. A clinicopathologic, light-microscopic, ultrastructural and immunohistochemical analysis.

Eighteen poorly differentiated, small and dark cell malignancies afflicting young individuals without light-microscopic evidence of a rhabdomyoblastic differentiation or a growth pattern characteristic of rhabdomyosarcoma were analyzed and compared with a series of 30 alveolar rhabdomyosarcomas of varying differentiation, where the diagnosis could be established light-microscopically. The study comprised clinical data, light and electron microscopy and immunohistochemistry, using a battery of mono- and polyclonal antibodies against intermediate filaments, myoglobin, epithelial membrane antigen, neuron-specific enolase, S-100 and leucocyte common antigen. All 30 alveolar rhabdomyosarcomas were positive for desmin, while a minority were positive for myoglobin, using monoclonal antibodies. In 8 of the 18 small and dark cell malignancies, support for a rhabdomyoblastic differentiation was obtained by a positive staining for desmin. In only 3 of these 8 cases was there ultrastructural evidence of rhabdomyosarcoma. The results of the investigation indicate that immunohistochemistry is a more useful tool than electron microscopy in the diagnosis of poorly differentiated rhabdomyosarcoma and that the criteria for the diagnosis of poorly differentiated rhabdomyosarcoma may need to be reformulated.