Malignant giant cell tumor of tendon sheath. Report of a case.

In a patient with pigmented villonodular synovitis of the right knee joint, there occurred a malignant giant cell tumor of tendon sheath. There was clinical evidence of metastasis after the second local recurrence and the recurrent tumors were studied enzyme cytochemically and electron microscopically. Ultrastructurally, the malignant tumor consisted of three principal cell types; histiocyte-like cells, fibroblast-like cells, and intermediate cells, with unique attendance of myofibroblasts. This may be the first report of the presence of myofibroblasts in malignant giant cell tumor of tendon sheath. Enzyme cytochemistry revealed various functional properties of histiocytes.