Bone tumors are malignancies that either begin in bone or spread to bone from other parts of the body. They rarely occur in cats.

Primary bone tumors begin in the bones, and are the most common type. These are called osteosarcomas and chondrosarcomas. Secondary bone tumors, those that spread to bones from other tissues, are much more rare.

Seventy-five percent of primary bone tumors occur in leg bones, but they can also occur in ribs, the spine or the skull. Primary tumors commonly appear in only one location in one bone, whereas secondary tumors often occur in multiple sites in multiple bones.

The cause of bone tumors is unknown; however they can occur at previous fracture sites or sites of previous orthopedic procedures.

Bone tumors usually occur in older, male, large and giant-breed dogs. Dogs with bone tumors usually become suddenly lame. The lameness usually is not associated with trauma. The lameness progresses rapidly to non-weight-bearing, with the dog holding the leg up. There is often swelling of the bone at the tumor site, which can be felt by your veterinarian. There is also muscle atrophy of the affected limb. Common sites of primary bone tumors are the shoulder, wrist and knee.

X-rays will show the classic signs of a bone tumor, which are a combination of bone destruction and bone production. Other diseases, such as fungal infections of the bone, can show similar X-ray signs, so definite diagnosis is always done via bone biopsy. That lets us know what type of tumor is present, and what treatment is needed. Bone tumors commonly spread to the lungs, so chest X-rays are always performed to make sure no spread of the bone tumor has occurred to the lungs.

The main treatment of primary bone tumors is amputation of the affected limb. This is done to relieve the intense pain associated with bone tumors. The surgery rarely cures the bone cancer, because microscopic metastasis has usually occurred by the time the diagnosis has been made. Most dogs do extremely well after amputation.

Bone cancers are extremely malignant, with most dogs surviving only 10-12 months when treated with amputation and chemotherapy.

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

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