Dedifferentiated parosteal osteosarcoma of the radius.

A 35-year-old woman presented with a painful swelling in the left distal radius that had been present for 1 year. Radiography and computerized tomography revealed a sclerotic surface lesion that had grown over the year and eroded the cortex. Histological examination demonstrated two distinct components: a cartilaginous low- to moderate-grade osteosarcoma on the surface and a high-grade osteosarcoma in the intramedullary component. This case is uncommon in two aspects: the radius is a rare site for such a tumor and the dedifferentiation was revealed at the time of the first surgery and was not secondary to recurrence.