Malignant giant cell tumor of soft parts. An ultrastructural study of four cases.

Four cases of malignant giant cell tumor of soft parts (MGCT) were studied ultrastructurally. Most of the cells in three cases were identified as undifferentiated mesenchymal cells and mononuclear monohistiocytic cells. The osteoclastlike cells seemed to arise from fusion of monohistiocytic cells. Immature fibroblastic cells, occasionally containing filaments with densities, were also present. They were scarce in three cases and predominant in the fourth. It was concluded that MGCT is a mesenchymal sarcoma with a bimorphic--monohistiocytic and fibroblastic--differentiation. MGCT is therefore considered a special variant of malignant fibrous histiocytoma.