Pedal osteosarcoma in a donkey

An 18 1/2-year-old castrated male donkey with progressively worsening right forelimb lameness presented with a mass on the distal dorsal aspect of its P3 bone. Grossly, the firm, gritty mass was infiltrative, disrupted the co ntours of the overlying hoof wall, and had mottled and cavitated areas on c ut surface. Histologically, the growth was composed of densely cellular she ets of mildly pleomorphic mesenchymal cells forming irregularly shaped isla nds of poorly mineralized osteoid. The neoplastic mass had patchy areas of necrosis. The diagnostic possibilities considered for this donkey's mass in clude osteosarcoma, osteoma, ossifying fibroma, and fibrous dysplasia. Care ful consideration of the gross and histological characteristics of this don key's mass support a diagnosis of osteosarcoma.