Maglignant giant cell tumor of tendon sheath.

A benign, but aggressive, giant cell tumor of tendon sheath developed over a period of 20 years into a metasasizing, histologiclly malignant giant cell tumor. Ultrastructure of the malignant tumor showed the same five cell types as described in giant cell tumors of tendon sheath. Even the same crystals were identified in the osteoblast-like and osteoclast-like cells. It therefore appears reasonable to assume that giant cell tumors of tendon sheath indeed are neoplasms with a malignant potential and not an inflammatory with partial osseous differentiation. No ultrastructural similarities with fibrous histiocytoma were apparent.