Periosteal osteosarcoma in the long head of the triceps in a dog

A 2.5-year-old Labrador retriever was evaluated for forelimb lameness. Fine-needle aspirates of a mass in the proximal brachium were suggestive of a mesenchymal tumor. The dog was euthanized and necropsied. Radiographic evaluation and gross and histopathological findings provided a diagnosis of periosteal osteosarcoma in the long head of the triceps.