Bone tumors are malignancies that either begin in bone or spread to bone from other parts of the body. Primary tumors are ones that begin in bone. The most common primary bone tumor in dogs is an osteosarcoma, followed by a chondrosarcoma. Seventy-five percent of primary tumors occur in leg bones, but they can also occur in the skull, ribs, or spine. Primary tumors commonly appear in only one location in one bone, whereas secondary tumors that occur due to metastasis can occur in multiple sites. Although tumors can develop at the site of the previous fracture or orthopedic surgical procedure, the cause of most bone tumors is unknown.

Bone tumors typically occur in older, male, large and giant breed dogs. Dogs with bone tumors usually become suddenly lame. The lameness associated with bone tumors progresses rapidly to a non-weight bearing lameness, with the animal holding the leg up. There may also be swelling of the limb at the tumor site and muscle atrophy in the rest of the leg.

Sudden onset of dramatic lameness with no known trauma and an older large dog strongly suggest the possibility of a bone tumor. X-rays may show classic signs of a bone tumor, such as combined bone destruction and bone production. A bone biopsy is usually recommended to confirm the presence of the tumor, to determine the tumor type, and to provide information that affects the treatment options. Laboratory tests and chest X-rays are done to look for tumors elsewhere and to assess the status of other organs.

The main treatment of primary tumors is amputation of the affected leg. In the case of osteosarcoma, amputation is done to relieve the intense pain associated with the tumor. The surgery rarely cures the disease, because microscopic metastasis has usually occurred by the time of diagnosis. When amputation is not an option, chemotherapy, or radiation therapy may reduce the pain caused by the tumor and improve the dog's quality of life for a short period of time.

Most dogs do extremely well after amputation. Osteosarcoma is highly malignant, with most dogs surviving only 12 months after the initial diagnosis and treatment with amputation and chemotherapy. Bone tumors are very painful, and euthanasia should be considered if specific treatment is not an option.

Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum. 

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