Oncology Update: Osteosarcoma

Kevin Finora DVM, Diplomate ACVIM (Oncology and Small Animal Internal Medicine)

Osteosarcoma (OSA) is the most common bone tumour of dogs. Eighty-five per cent of all bone tumours in dogs are OSA. OSA locates in the appendicular skeleton 75% of time and in other bones (mandible, maxilla, spine, cranium, ribs, nasal cavity and pelvis) 25% of the time. Front limbs are affected twice as often as rear limbs. OSA usually locates “away from the elbow and towards the knee.” OSA is most commonly seen in large and giant breeds. Saint Bernards, Great Danes, Irish Setters, Doberman Pinschers, Rottweilers, German Shepherds and Golden Retrievers are considered at risk breeds. The median age of onset is 7 years. There is a focal incidence noted in young dogs between the ages of 18-24 months. Males are at slightly greater risk compared to females. OSA has been reported to be associated with metallic surgical implants, though this is seen less and less as plate technology advances. OSA appears to have both a mutated tumour suppressor gene (p53) and an over expression of the COX-2 receptor.

Most dogs with OSA present with lameness. The lameness may be acute or gradual in onset. In most cases a presumptive diagnosis is made based on radiographs. The classic radiographic lesion is a lytic/proliferative “sun-burst” pattern. Often a triangular, periosteal, deposition new bone is present on the cortex, at the edge of the lesion. This sign is called Codman’s Triangle and, while not pathognomonic, it is highly suggestive of OSA. OSA will rarely cross a joint space. In situations where the bone lesion has the classic appearance, location and is present in a large breed dog, I will not usually recommend biopsy but will instead move forward with definitive therapy. If the lesions are not classical in appearance or location then I recommend bone biopsy. Small biopsies may lead to misdiagnosis. Therefore, if a biopsy is to be taken a sizable sample must be obtained.

The therapeutic approach to OSA is multifaceted. The first step is for local control of the tumour. In most cases, surgery will involve limb amputation. Amputation is the only way to control pain and is considered an essential step in preserving quality of life. The vast majority of dogs do very well with three limbs. Only dogs with severe orthopedic or neurological problems are not considered candidates for limb amputation. Many clients initially resist limb amputation but most come to accept this treatment. In a JAAHA survey of clients who had had their dog’s limb amputated, client satisfaction with the procedure exceeded 90%, and as many clients reported they would do the surgery again if faced with the same choice. Three view chest films should be taken prior to surgery. In 90% of cases there will be micro-metastatic disease at the time of diagnosis, but disease will be detected radiographically in 10% of cases.

Surgery is followed with chemotherapy treatment. With no therapy of any sort, the average survival time for dogs with OSA is 1-3 months. With surgery alone the average survival time is 3 months. When surgery is combined with chemotherapy 50% of dogs will be alive at one year, and 20% of dogs will be expected to live beyond 2 years. Due to the fact OSA over expresses COX-2, I usually recommend dogs be treated with COX-2 inhibitors following the completion of chemotherapy. Severe GI effects can be seen if COX-2 inhibition therapy is combined with chemotherapy treatments.

Continued areas of research include growth inhibitor therapy to help slow growth and to address non-surgical metastatic disease, radioisotope therapy to target neoplastic foci and continued development of limp sparing procedures. Limb sparing appears to offer no survival advantage and is only recommended when limb amputation is not possible. This technique is restricted to tumours of the radius and ulna only.

OSA is a rare lesion seen mostly in older cats. It commonly occurs in the hind limbs. OSA in cats is far less metastatic compared to dogs. Cats are treated with surgery alone and survival times of 24-44 months can be expected. Chemotherapy does not appear to play a role in the treatment of feline OSA.

Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.

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