Orbital rhabdomyosarcoma: case report with immunohistochemical detection of muscle cell type intermediate filament proteins.

A 4 1/2-year-old girl had a rapidly growing tumor in the orbit. An extirpated mass consisted of small round or spindle neoplastic cells containing hyperchromatic mitotic nuclei. The cytoplasm had many lipids and glycogens with few organelles. A light and electron microscopic examination revealed an exceptional tumor cell with distinct cross striations compatible with typical rhabdomyoblast; occasional cells exhibited a few bundles of thin filaments scattered in the cytoplasm. Immunohistochemical studies for muscle cell-related proteins by avidin-biotin-peroxidase complex methods revealed positive reactions of the tumor cells to the antiserum against myoglobin, myosin, desmin and vimentin, providing evidence for muscle cell derivation of the neoplastic cells.