Osteosarcomas account for only 5% of all canine tumors, but 80-90% of malignancies involving the bone. Much more common in large breed dogs, osteosarcoma is an aggressive cancer of the bone that often requires amputation of the affected limb coupled with chemotherapy to provide temporary relief from this aggressive disease.
Which dogs are at risk for developing osteosarcomas?
Osteosarcomas generally affect older large or giant breed dogs. The giant breeds at greatest risk for developing osteosarcoma include Great Danes, St. Bernards, Great Pyrenees, Newfoundlands, Bernese Mountain Dogs, and Irish Wolfhounds. Large breeds such as Rottweillers, Labradors, Golden Retrievers, Shepherds, Dobermans, Weimaraners and Boxers are also at an increased risk. It is not a very common tumor in small breed dogs and rarely occurs in cats. Dogs that weigh over 80 pounds have been shown to be at least 60 times more likely to develop an osteosarcoma than dogs weighing less than 75 pounds. While older dogs more commonly develop osteosarcomas, there does appear to be an increased incidence in one to two year old dogs as well. Male dogs have an increased incidence of osteosarcomas.
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.
What are the symptoms of osteosarcomas?
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. Up to 90% of these tumors will have metastasis to the lungs at the time of diagnosis but because of the small initial size of the metastases, less than 10% will initially show up on a chest x-ray. 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. Osteosarcomas will occasionally show up at different locations and likewise other tumor types can initially appear to be an osteosarcoma. Because of this possibility, a biopsy is always recommended. Fungal bone infections can produce similar symptoms and appearance on an x-ray, so a fungal culture is often performed to help clarify the diagnosis.
What is the treatment for osteosarcoma?
Osteosarcoma is an aggressive, highly metastatic cancer that requires an aggressive treatment protocol. Once the tumor has been positively identified as an osteosarcoma, the affected limb is usually amputated. In rare cases where the tumor is in the right location, some limb sparing surgeries have been performed but that is not usually the case. After the amputation, a course of chemotherapy is usually begun. The most successful drugs have been carboplatin and cisplatin. Carboplatin is more expensive but safer and easier to administer. Doxorubicin is sometimes used as well. A qualified veterinary oncologist is often the best source of information and he or she will be aware of the newest chemotherapy protocols. The life expectancy of a dog with a properly identified and treated osteosarcoma varies greatly, but can approach a year or longer.
Is osteosarcoma preventable?
It does not appear that osteosarcoma is preventable. Because of some strong breed correlations, any breed line that has a history of osteosarcoma should be examined closely prior to breeding. Unfortunately we do not completely understand the cause of osteosarcoma but, hopefully as our knowledge improves, we can continue to provide more effective treatments and early diagnostic tests.
Villalobos, A. Osteosarcomas in the Clinical Setting. Veterinary Practice News. April; 2000.
Garrett, L. Update on Canine Appendicular Osteosarcoma. Veterinary Practice News. February; 2000.
Villalobos, A. Managing osteosarcoma in Dogs and Cats. Veterinary Practice News. June; 2000.
Ettinger, S. Textbook of Veterinary Internal Medicine. Published by Saunders. Philadelphia; 1989.