Primary leiomyosarcoma of bone. An immunohistochemical and ultrastructural study.

Primary leiomyosarcoma of bone is extremely rare. A 60-year-old woman had a mass in the right femur that was studied immunohistochemically and by electron microscopy. Human smooth-muscle actomyosin was detected in tumor cells, but human skeletal-muscle myoglobin and lysozyme (muramidase) were not. Electron microscopy of the tumor showed findings suggestive of a smooth-muscle origin, such as myofilaments, dense bodies, pinocytotic vesicles, and basement membrane. The results were diagnostic of leiomyosarcoma rather than rhabdomyosarcoma, fibrosarcoma, or malignant fibrous histiocytoma, which are similar neoplasms. We believe that ours is the first case of primary leiomyosarcoma of the bone proved by immunohistochemistry.