In this study, we report a rare case of leiomyosarcoma with osteoclast-like giant cells occurring in the hand. A 58-year-old man underwent excision of the right palmar tumor in another hospital, and the pathological diagnosis was giant cell tumor (GCT) of the tendon sheath. Because the tumor recurred twice, he visited our hospital. We suspected recurrence of the GCT of the tendon sheath, and performed excision of the tumor. Pathologically, the tumor was composed of fascicles of spindle cells, showing typical features of smooth-muscle differentiation, as well as marked nuclear pleomorphism and high mitotic figures with some atypical mitoses. In order to differetiate other tumors with osteoclast-like giant cells, we performed immunohistchemical examination. Immunohistochemically, many tumor cells tested positive for desmin, muscle specific actin, and α-smooth muscle actin. Eventually we diagnosed this tumor as leiomyosarcoma with osteoclast-like giant cells. Initially this case was thought to be malignant transformation of the GCT of the tendon sheath, but after careful revision of the slides and immunohistochemical examination, diagnosis was changed to leiomyosarcoma with osteoclast-like giant cells.
