Osteosarcoma of the humeral head associated with osteochondritis dissecans in a dog.

A 6-year-old, male Labrador retriever was presented for lameness of the right forelimb. The lameness occurred 5 years after the dog was surgically treated for osteochondritis dissecans (OCD) of the caudal right humeral head. Radiographs revealed an expansile, osteolytic lesion occupying the entire caudal half of the right humeral head, and biopsy confirmed the diagnosis of osteosarcoma. Forelimb amputation was performed. Histopathology of the lesion and associated articular cartilage revealed neoplastic osteoblasts extensively infiltrating the marrow space adjacent to scar tissue associated with the previous OCD lesion. This is the first report of an epiphyseal osteosarcoma in the area of a previous OCD lesion in a dog.