Malignant schwannomas presenting as malignant fibrous histiocytomas.

Two soft tissue neoplasms considered to represent malignant fibrous histiocytomas by light microscopy showed typical findings indicative of Schwann cell origin when examined by electron microscopy. These findings included the presence of cells exhibiting long interdigitating cytoplasmic processes, which frequently contained collections of microtubules and were surrounded by thin, frequently interrupted, basal lamina material. In addition, pinocytotic activity at the cell surfaces and immature junctions joining apposing cellular membranes were identified. No evidence of fibroblastic, fibrohistiocytic, or myofibroblastic differentiation was identified ultrastructurally in the neoplastic cells. Our findings stress the need for ultrastructural examination to adequately classify soft tissue sarcomas. The 2 cases presented illustrate that there are some schwannomas that may be inaccurately classified as malignant fibrous histiocytomas if only light microscopy is used. It seems that only by ultrastructural means is it possible to accurately classify these peculiar neoplasms.