Bone tumors can be either malignant or benign. Osteosarcoma and chondrosarcoma are the two most common malignant bone tumors.

Osteosarcoma is by far the most common malignant bone cancer in dogs. This cancer affects dogs of all ages, with a median age of 8 years. It occurs with equal frequency in males and females. Giant breeds, such as the Saint Bernard, Newfoundland, Great Dane, Great Pyrenees, and Bull Mastiffs, are 60 times more likely to develop an osteosarcoma than are dogs weighing less than 25 pounds.{{more}} Large breeds, such as the Irish Setter and Boxer, are eight times more likely to develop osteosarcoma. Toy breeds are rarely, if ever, affected.

Osteosarcoma occurs most often in the bones of the front legs, followed, in order of frequency, by the hind legs, the flat bones of the ribs, and the mandible. Often the first sign is a limp in a mature dog who has no history of injury. Usually this receives little attention until swelling of the leg or a bone mass is observed. Pressure over the tumor causes pain. Fractures can occur at the tumor site.

A few months ago, I saw a classical case of osteosarcoma in a Bull Mastiff belonging to a good friend of mine that had to eventually be put down. The dog was about 8 years of age and the symptoms actually started with a slight limp.

X-rays can strongly suggest the disease, but a definitive diagnosis depends on biopsy of the tumor. Osteosarcoma is an aggressive cancer that quickly spreads to the lungs.

Chondrosarcoma is the second most common malignant bone tumor in dogs. The average age of onset is 6 years. This tumor tends to involve the ribs, nasal bones, and pelvis. It presents as a large, hard, painless swelling in an area containing cartilage. This tumor also metastasizes to the lungs, but is less aggressive than osteosarcoma.

Treatment: Malignant tumors such as osteosarcomas and chondrosarcomas should be treated aggressively. Because these tumors metastasize to the lungs, it is important to obtain a chest X-ray before recommending surgery. The dog should have a complete physical examination, including a blood count and a fine needle aspiration or biopsy of any enlarged lymph node.

Partial or complete amputation is the only effective treatment for osteosarcomas of the limbs. Most dogs are able to get around well on three legs. Although amputation rarely cures the cancer, it does relieve pain and improves the quality of life. The amputation should be performed at least one joint above the involved bone.

Complete surgical removal of chondrosarcomas affords relief, but should not be considered curative.

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