Juxtacortical dedifferentiated chondrosarcoma from a primary periosteal chondrosarcoma.

In this report, we describe dedifferentiation in a primary periosteal chondrosarcoma in the proximal tibia of a 73-year-old man. The diagnosis of primary periosteal chondrosarcoma was made by the characteristic radiographic, gross, and microscopic features of the tumor. The presence of dedifferentiation, however, in the form of malignant fibrous histiocytoma, was an unexpected histologic finding. Although rare, dedifferentiation is a well-recognized occurrence in secondary chondrosarcomas arising from precursor osteochondromas. This event has not been previously documented in the setting of primary periosteal chondrosarcoma.