Malignant giant cell tumor of synovium and locally destructive pigmented villonodular synovitis: ultrastructural and immunohistochemical study and review of the literature.

The first reported case of an intraarticular malignant giant cell tumor of synovium studied with electron microscopic and immunohistochemical examination is presented, together with a case of diffuse intraarticular pigmented villonodular synovitis with extensive bone destruction. The malignant case was dominated by uniform cells positive for histiocytic markers, the fine structure showing a gradual change from cells dominated by organelles serving a secretory function to cells with phagocytic activity. The reported cases of giant cell tumor of the tendon sheath indicate that the pertinent histologic changes regarding malignancy are an increase in cell polymorphism and in the number of mitoses, and a decrease in the number of multinucleated giant cells.