It is unknown why some dogs develop osteosarcomas, but one theory suggests that the rapidly growing cells found at the growth plates in the bones are genetically at a greater risk of mutation. Another theory is that the tumors develop at the site of trauma. The increased cellular activity at the site of a fracture or trauma could result in the development of cancer cells. The reality is that both of these may be true and there may be other causes not yet discovered. The symptoms of osteosarcomas are often closely associated with their location.
Most osteosarcomas develop on the limbs of dogs below the elbow or near the knee.
The tumors usually form at or near the growth plates. Affected dogs will often have a pronounced bone swelling. X-rays often reveal a characteristic bone pattern that, coupled with history and breed, may indicate the development of an osteosarcoma. These tumors often produce pain in the joint that can first be detected as lameness in the affected limb. Because of this high incidence of metastasis, all dogs with osteosarcomas are treated as if they have metastasis to the lungs regardless of the findings on the initial lung x-rays.