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OSTEOSARCOMA:
BONE TUMORS IN THE DOG

By Rick Snook, D.V.M.

The bones of a dog can be the site of development for several different kinds of cancer (neoplasia). The most common type seen is a tumor originating from the bone itself called osteogenic sarcoma (osteosarcoma) which means bone forming tumor. Eighty percent of primary bone tumors in the dog are osteosarcomas and represent approximately 7% of total malignancies seen in canine patients. It is estimated that osteosarcoma occurs in over 8,000 dogs per year and this figure may be rising due to the increasing popularity of large and giant breed dogs.

Osteosarcoma is a primarily a disease of big dogs and is very uncommon in dogs weighing less than 25 pounds. The exact cause of osteosarcoma is not known at this time. The tendency of the tumor to occur in major weight bearing bones of large and giant breeds may point to multiple minor trauma as an inciting factor. Case studies have shown the tumor to be more size than breed related. The incidence of osteosarcoma is higher in males than females, with the exception of the St. Bernard, I which females are more often affected. Studies also show the tumor is twice as likely to show up in the prone limbs compared to the rear. The two most common locations are just above the wrist (distal radius) and just below the shoulder (proximal humerus). Osteosarcoma is a highly metastatic tumor. Approximately 90% of affected dogs will develop metastasis or spread of the rumor within one year of diagnosis. The most common site of secondary tumor development are the lungs.

Dogs with osteosarcoma will usually develop a mild lameness that may or may not be associated with an episode of mild trauma. Patients are often presented to veterinarians by owners for treatment of a possible mild sprain or strain. As the tumor develops, the lameness worsens and swelling in the araa of the affected bone becomes apparent. The onset of limb pain in dogs with osteosarcoma is most likely due to the micro fractures that occur as the tumor invades and replaces the normal bone.

The diagnosis of osteosarcoma in canine patients involves radiographs (x-rays) and bone biopsy. Radiographic evidence of the tumor development in the bone is present early in the course of the disease. Due to the aggressive malignant behavior of the tumor, destruction of the bone an invasion of surrounding soft tissue is often quite rapid. Because several other bone diseases can have a similar radiographic appearance to osteosrcoma, a definitive diagnosis requires a bone biopsy to confirm the presence of malignant tumor cells.

The long-term prognosis for dogs with osteosarcoma is very poor. This is due in part to the early and rapid spread (metastasis) to other parts of the body. Survival in patients with untreated osteosarcoma is rarely longer than six months. Studies are currently under way to develop treatment protocols that would improve survival times for patients with this highly malignant form of cancer. The combination of surgicals (amputation or limb sparing), radiation and chemotherapy appear to be the most promising, but even with combination therapy survival is rarely more than 50% at one year from diagnosis.

Dr. Rick Snook is a veterinarian who practices in Santa Fe.

Note: This article first appeared in the Winter/Spring 1997 issue.

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