Malignant diffuse-type tenosynovial giant cell tumor of the buttock.

Malignant diffuse-type tenosynovial giant cell tumor (D-TSGCT) is an unusual sarcoma. We report a case of malignant D-TSGCT located in the left buttock. A 58-year old woman noticed a small mass at her left buttock 3 months previously. The mass tended to enlarge rapidly, and became 6 cm in diameter. Tumor resection was performed. Grossly, the tumor showed a solid growth pattern and whitish appearance with hemorrhage and necrosis. Microscopically, the tumor was composed of a proliferation of short spindle and oval mononucleated cells with numerous osteoclast-like multinucleated giant cells, which occasionally showed coagulative necrosis. In addition, tumor cells had high mitotic activity and atypical mitoses. Immunohistochemically, the mononucleated cells were positive for CD163 and focally positive for CD68 (clone KP-1 and PG-M1), CD4, smooth muscle actin and S100 protein. Osteoclast-like multinucleated giant cells were positive for CD68 (clone KP-1 and PG-M1) and CD4. Pulmonary metastases were found 6 months after the operation. These findings indicate that this lesion is consistent with malignant D-TSGCT.