Extraskeletal osteosarcoma arising in myositis ossificans

A 53-year-old woman had extraskeletal osteosarcoma that developed from a so ft tissue bony mass present on the volar aspect of the left wrist for 4 yea rs. Initially, the bony mass was soft and movable, but during the first yea r it became hard and fixed. The patient had no history of trauma. Because t he lesion did not grow or cause any symptoms, the patient did not come to t he hospital until 4 years after she first noticed the lesion. Radiologicall y, the bony mass had features characteristic of mature myositis ossificans, showing "egg-shell" ossification. A nonmineralized soft tissue mass occurr ed between the surface of the radius and the bony shell. Histologically, a highgrade osteosarcoma was present between the surface of the radius and th e well-differentiated bone tissue, which included fatty and hematopoietic m arrow. All the findings indicated that our patient had an extremely rare ca se of malignant transformation of myositis ossificans.