Bone Tumors in Dogs: Benign or Malignant

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

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, and Great Pyrenees, are 60 times more likely to develop an osteosarcoma than are dogs weighing less than 25 pounds. 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.

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 nodes.

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. New surgical techniques that preserve the leg are currently being done at some veterinary referral centers.

Chemotherapy in addition to amputation increases the survival time for osteosarcoma, but not the cure rate. Radiation therapy may be considered if the cancer is metastatic or far advanced, but is also not a cure. Osteosarcoma of the mandible is treated with radiation therapy, to which it is moderately responsive. Radiation is also used for palliation of pain.

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