Radiology–pathology conference: osteosarcoma in a cartilaginous exostosis of the femur

Osteochondromas are common lesions in the metaphyseal segments of the long bones and are known to be able to degenerate into chondrosarcoma. We present the case of a 20-year-old patient with an osteosarcoma at the base of a cartilaginous exostosis and discuss the causal relationship between the two lesions on the basis of the radiologic and pathologic findings.