Large and giant breed dogs such as Newfoundlands, greyhounds, St. Bernards, large Labradors, and Rottweilers are predisposed for this form of cancer, which usually develops in geriatric dogs over eight years old. "Rottweilers by far are the most over-represented of the breeds that we see with this type of cancer," says Dr. Fan. "One in eight Rottweilers will develop osteosarcoma in their lifetime."
Osteosarcoma is a primary bone tumor; it originates in bone and can spread, or metastasize, to other parts of the body, especially the lungs.
Dr. Fan explains that the first sign an owner may see is sudden lameness or limping which may or may not be associated with exercise. "Sometimes an owner may notice that his dog is limping after playing, and may suspect the dog sprained its leg."
For a limp, a veterinarian may prescribe anti-inflammatory medications to reduce swelling and pain. If the pain does not go away or worsens with medical treatment, Dr. Fan says that further diagnostics can help determine if the dog has osteosarcoma. "With certain breeds, a veterinarian should suspect osteosarcoma as soon as the dog comes in with a limp."
A veterinarian can take radiographs (x-rays) to look for signs of abnormal bone growth that may indicate osteosarcoma. The characteristic sites for this abnormal growth are the leg bones, as Dr. Fans says, "away from the elbow, toward the knee." In the front leg the lesions would be in the radius (foreleg bone) toward the wrist, and in the humerus toward the armpit. In the hind limbs the lesions would be in the tibia (shin bone) and femur (thigh bone), close to the knee. Occasionally osteosarcoma is seen in the mandible, the lower jaw bone.
Osteosarcoma cannot be diagnosed with radiographs alone, since other types of cancers and other conditions can cause bone problems. Because proper treatment of cancer requires an accurate diagnosis, a veterinarian who suspects osteosarcoma after taking radiographs should refer a patient for a cytology and/or biopsy.
A biopsy, which is the removal of a small chunk of tissue for examination under a microscope, is the standard diagnostic test for most types of cancer. Cytology is a less invasive method that involves inserting a needle into tissue to get a sample of cells. The cells are put onto a glass microscope slide and stained for examination.
Dr. Anne Barger, veterinary clinical pathologist at the University of Illinois Veterinary Teaching Hospital, has developed a new staining technique that can differentiate osteosarcoma cells from other types of cancer cells, making cytology a more accurate diagnostic tool. Cytology is less invasive, less painful, quicker and more appropriate for sampling from thin areas of bone, such as the mandible, that may be too delicate for biopsy.
Proper treatment of osteosarcoma involves surgical removal of the cancerous bone as well as chemotherapy to prevent metastasis to the lungs. If the cancer is caught early, surgery may involve removing just a portion of a bone, but sometimes partial or full amputation of a limb is required. Many dogs can remain active after an amputation.
Veterinary oncologists at the teaching hospital are also investigating a treatment combination of radiation therapy, systemic chemotherapy, and a human drug called pamidronate, that may reduce the need for amputation in dogs with leg tumors.
"Pamidronate increases bone strength and reduces the destructive process associated with bone tumors," explains Dr. Fan. "From what we have seen in the clinic, this combination is likely to be an effective treatment option. We are conducting a clinical trial right now, and should have more definitive results in about 18 months."
When considering treatment options for a pet, it's important to understand that radiation therapy and chemotherapy are used less aggressively in animals than in humans to minimize side effects and ensure that the pet has an enjoyable quality of life.
For more information about osteosarcoma, consult your veterinarian.