Hepatoid tenosynovial giant cell tumor - a rare morphologic variant case report

We report a morphologically rare type of tenosynovial giant cell tumor (TSGCT), localized type, occurring in a 49-year-old man. Imaging examination revealed multiple nodular lesions around the right knee joint. The largest one extended to both intra- and extra-osseous region of the right distal femur. Histologically, the tumor consisted of epithelioid mononuclear cells and they looked like to have abundant eosinophilic cytoplasm reminiscent of hepatic tissues. In some areas, however, typical histologic features of TSGCT were observed. Electron microscopy revealed that the eosinophilic cytoplasm-like substance was intercellular fibrinous material surrounding the mononuclear tumor cells. Immunohistochemically, mononuclear tumor cells and multinucleate giant cells were positive for CD68 (Kp1) and some of the mononuclear tumor cells were also positive for desmin. Finally, we made the diagnosis of hepatoid TSGCT.