Low-grade central osteosarcoma of the rib.

We report a case of low-grade central osteosarcoma of the rib, mimicking fibrous dysplasia protuberans or parosteal osteosarcoma. A 13-year-old boy presented with a densely ossified exophytic tumor in the left sixth rib. The surgically resected rib revealed an ossified tumor that had developed within bone and had formed an exophytic mass. Microscopic examination revealed parosteal osteosarcoma-like low-grade central osteosarcoma. Low-grade central osteosarcomas rarely affect the rib, and exophytic tumor formation is also rare. This tumor required differentiation from fibrous dysplasia protuberans and parosteal osteosarcoma because of its clinical presentation.