From the TEXTBOOK OF VETERINARY INTERNAL MEDICINE
Client Information Series
Susan A. Kraegel
Osteosarcoma is the most common type of bone cancer in dogs. This cancer most often affects large, middle-aged dogs weighing more than 40 pounds. Osteosarcoma can occur in any bone (e.g., rib, skull, toe) but the most common sites are the ends of leg bones, especially at the wrist, shoulder, knee, and hip. The causes of osteosarcoma are unknown, but genetics and microscopic injury to the ends of the bone during growth play a role.
Osteosarcoma begins in the bone but spreads to the lungs and other organs early in the course of disease, even before the initial cancer in the bone is detected. This spread of cancer is called metastasis. In 5 per cent of dogs with osteosarcoma, these metastases are visible on a chest x-ray when the dog is first brought in for the bone cancer.
In over 90 per cent of the dogs with osteosarcoma, these metastases are present but are too small to be seen on the initial x-ray. They are termed "micrometastases."
The first sign of ostcosarcoma is generally lameness caused by pain from the cancer. The lameness may come and go and vary in severity from dog to dog. In some dogs the leg may fracture at the cancer site. As the cancer grows, a swelling at the site will also develop, which you may be able to feel or see.
X-rays are the first step in identifying bone cancer. X-rays can only suggest the diagnosis on the basis of the appearance of the bone. X-rays of the chest are also recommended to search for metastases.
Biopsy of the abnormal bone is the only way to diagnose bone cancer absolutely. Side effects of biopsy can include pain, bleeding, and, rarely, fracture of the diseased bone. In 10 to 20 per cent of cases, biopsy may fail to diagnose the cancer, so negative results should be evaluated carefully.
Osteosarcoma is a very aggressive form of cancer in dogs. Oncologists (cancer treatment specialists) have not found a cure. Treatment of both the cancer in the bone and the metastases, however, can give many dogs months to years of good-quality life.
Amputation and chemotherapy are the ideal treatment for dogs with osteosarcoma. The amputation removes the primary cancer and also relieves the bone pain. Most dogs are walking the day after amputation and running soon after. Three-legged dogs have been known to herd cattle and compete in field trials and generally enjoy the same activities as four-legged dogs.
The chemotherapy slows down but does not eliminate the metastases. The chemotherapy currently recommended by most oncologists is usually given once every 3 weeks for four treatments. Most dogs tolerate chemotherapy well and experience only a day or two of mild lethargy or decreased appetite after each treatment. Potentially serious side effects after chemotherapy are uncommon but can include poor to no appetite, vomiting, diarrhea, blood infection, kidney damage, or heart damage. With amputation and chemotherapy, 50 per cent of dogs are alive at 12 months and 15 to 20 per cent are alive at 2 years. Almost all dogs eventually die from osteosarcoma.
Amputation without chemotherapy provides only pain relief. The metastases continue to grow until they cause death. Only fifty per cent of dogs are alive 4 months after amputation alone and 5 to 10 per cent are alive at 1 year.
In rare cases, amputation may not be advised by your veterinarian. Dogs with preexisting disease in other legs or severe obesity may not do well after amputation. One option for these dogs is limb-sparing surgery instead of amputation. This is generally possible only when the cancer is in the front limb in the bone called the radius. With this procedure, the cancerous bone is removed, replaced with bone from a dead dog, and the wrist joint is fused so that it cannot bend. Recovery takes 1 to 2 months and infections are common. Survival times are the same as for amputation. For dogs unable to benefit from surgery at all, radiation therapy may be offered as a treatment to relieve pain. Radiation is given over a 3-week period and reduces but does not eliminate pain in about 30 per cent of dogs. The improvement lasts for about 4 to 5 months in these dogs. This therapy increases the likelihood that a dog will break the limb at the cancer site.
Dogs who do not receive any therapy for osteosarcoma are in pain and have a poor quality of life. This pain continues to progress and, according to people with bone cancer, is severe and uncontrollable. Common painkillers do not work. Euthanasia is generally the only humane choice.