Canine Osteosarcoma
Case of the month

Diesel, a 9 year old male French Mastiff presented for a 3 day history of right forelimb lameness. His owners reported that Diesel was running around chasing squirrels in the backyard and when they let him back in the house noticed he was favoring his forelimb. Diesel was otherwise doing well but on the morning of presentation did not want to eat his breakfast and seemed to be more lethargic. Due to the persistent lameness his owners brought him in to be examined. On presentation Diesel was bright and alert but preferred to sit rather than walk around. On ambulatory exam Diesel exhibited a weight bearing right forelimb lameness. No swelling was palpated but Diesel attempted to bite when his right forelimb was extended forward. The rest of his general physical examination was unremarkable. Diesel had a history of a torn cruciate ligament in his right stifle that was surgically repaired 4 years prior. Otherwise he had no significant medical history. 

Radiographs were performed and revealed a swelling at the proximal aspect of his right humerus with lysis of the bone and a moth eaten appearance. Chest radiographs were performed and did not reveal any abnormalities. Bloodwork was performed and was within normal limits. A suspicion of osteosarcoma was discussed with Diesel’s owner and he was referred to an orthopedic surgeon for consultation. Three days later, Diesel underwent surgery to amputate his right forelimb. Chemotherapy was offered but his owners decided not to pursue any further treatment, as they were interested in just keeping Diesel as comfortable as possible. Diesel recovered wonderfully from surgery and learned to get around on three limbs with ease. Five months later, Diesel returned for an exam as he was not doing well. He was very lethargic, had no appetite, and seemed to have increase respiratory effort at home according to his owners. Chest radiographs revealed a large number of metastatic lesions in his lungs. At this time due to his poor quality of life and underlying cancer diagnosis, Diesel’s owners elected euthanasia.

Canine Osteosarcoma
Osteosarcoma (OSA) is the most common primary bone tumor and accounts for more than 85% of malignant bone tumors in dogs. Tumors arise from the central region of bones and expand outwards disrupting the normal bone structure. OSA can affect the appendicular skeleton (the limbs) or axial skeleton (head and trunk), less commonly. The most common location for appendicular OSA is the metaphysis (the long narrow portion of the long bones) and it more commonly occurs in the forelimb than the hind limb. The most common location for axial OSA is the mandible and maxilla (bones of the mouth) and less commonly affects the vertebrae, skull, and ribs. Large or giant breed dogs are at a higher risk of developing OSA; these include Saint Bernards, Great Danes, Irish Setters, Dobermans, Rottweilers, German Shepherds, Golden Retrievers, and others.
The most common clinical signs are progressive lameness and/or swelling at the primary site. Sometimes there is association with a recent mild trauma so owners often believe their dog may have suffered a sprain or fracture.  Dogs typically exhibit some degree of pain on physical examination. 

Diagnosis of OSA involves radiographs of the affected limb. Radiographs will show abnormalities (lysis, destruction of bone, abnormal bony proliferation) that are consistent with OSA. Although radiographs do not provide a definitive diagnosis, they usually provide very high suspicion. If OSA is suspected, radiographs of the chest are often performed at the same time to look for lung metastasis (as this is the most common place for the tumor to spread). Less than 10% of dogs with OSA will have metastasis at the time of diagnosis, but 90% will die of metastatic disease. Bloodwork is often recommended to screen for concurrent illness, which may affect treatment plans. Often times physical examination, history, and radiographs are enough to make a diagnosis, but if there are doubts, a cytology or bone biopsy can be performed. Bone biopsy and cytology are typically not needed.

Treatment & Prognosis
If staging tests do not reveal metastatic disease, treatment includes resection of the primary tumor followed by systemic chemotherapy.  Due to location and aggression of these tumors, resection usually involves amputation of the affected limb. Limb sparing surgery is a possibility, but a patient needs to fit certain criteria to be a candidate and it has its own complications. Even if owners are not interested in pursuing chemotherapy, amputation is often recommended to provide comfort to the patient, as the tumor can be extremely painful. Median survival time for animals that undergo amputation only is approximately 6 months; those who undergo chemotherapy is approximately 1 year. Surgical approaches to axial OSA can be more difficult due to location and prognosis varies pending location as well. Radiation therapy is a consideration in these cases as well.