Giant cell tumor of the tendon sheath composed largely of epithelioid histiocytes.

Giant cell tumor of the tendon sheath (GCTTS) is a relatively uncommon lesion. GCTTS composed largely epithelioid histiocytes are very rare. In the literature, the author could not find such cases. A 73-year-old man presented with a mass of right thumb, and resection of the mass was performed. Grossly, the mass was encapsulated and yellowish, and measured 1.5 x 2 x 2 cm. Microscopically, the mass was composed of cellular and hypocellular zones. The former was composed of spindle cells and osteoclast-like giant cells, while the latter of epithelioid clear histiocytes. The area of the former was 20%, and the latter 80%. Pigment was seen in the former elements. Mitotic figures were seen in 3/per 30 high power fields (HPFs) in the former element and 2/per 30 HPFs in the latter element. Histochemically, the pigment was hemosiderin positive with Prussian blue staining. Immunohistochemically, both the elements were negative for cytokeratin (CK) CE1/3, CK CAM5.2, CEA, HMB45, alpha-smooth muscle antigen, p53, CD10, TTF-1, and CDX2. Both the elements were positive for CD68 and Ki-67 (cellular element 30% and hypocellular element 20%). The histiocytes of the hypocellular element and osteoclast-like giant cell of the cellular element were positive for CD45. S100-protein positive Langerhans cells and CD45-positive lymphocytes were scattered. The pathological diagnosis was GCTTS. In the author's experience, GCTTS composed largely epithelioid histiocytes are very rare. In the literature, the author could not find such cases. Thus, the author reports herein this case.