Osteosarcoma

Osteosarcoma is the most common type of primary bone cancer in dogs accounting for up to 85% of tumors that originate in the skeletal system. It is estimated that at least 6,000 new cases, and perhaps even more than 8,000, are diagnosed in dogs each year in the United States.

Osteosarcomas are seen most often in the appendicular skeleton (long bones of the limbs), most often “near the knee” and “away from the elbow.” However, these tumors can also affect the axial skeleton (cranium, spinal column, ribs). Large and giant breed dogs are at higher risk for bone cancer, especially of the long bones. For example, it is estimated that the relative risk to develop this disease in giant breeds such as Scottish Deerhounds and Great Danes may be as much as 200-times higher than in small and toy breeds. Among dogs at high risk, we also include Rottweilers, Great Pyrenees, Greyhounds, Mastiffs, and others.

The disease usually becomes evident during middle age (~7-10 yr), although bone cancer can affect dogs under 1 year of age. Factors that affect growth rate, such as diets that promote rapid growth in puppies, also appear to influence risk. The tumors are very aggressive and metastatic, so it is a fair assumption that at the time of diagnosis the disease will have already spread beyond the primary site. For this reason, the standard-of-care for bone cancer includes surgery to remove the primary tumor, followed by adjuvant chemotherapy to attack the cells that have left the site. In dogs, approximately 50% survive one year with standard-of-care, less than 30% survive 2 years, and less than 10% reach 3 years.

These statistics are tempered when we consider that one year is approximately 10% of a canine lifetime; however, we feel that new therapies designed to exploit the genetic and biochemical abnormalities of the tumors will help us improve the outcome of dogs with this disease.

Canine Appendicular Osteosarcoma: diagnostic work-up and treatment options by Michael S. Kent, DVM, DACVIM (Oncology), DACVR (Radiation Oncology)

Osteosarcoma is the most common bone tumor in dogs. Although it is mostly a disease of older large or giant breed dogs, it can affect dogs of any size or age. Osteosarcoma may be found in many areas, but it most commonly affects the bones bordering the shoulder, wrist and knee.

The first sign an owner usually sees with this disease is lameness in the affected leg. They may also notice a swelling over the area or their dog may seem painful at the site. Most of the time osteosarcomas and other bone tumors are found when their veterinarian takes a radiographs (“x-rays”) of the affected limb. On the radiograph there is generally a mixed destructive and productive lesion along the bone. A careful physical exam, blood work and urinalysis is also important to assess the overall health of your dog.

Next, radiographs (“x-rays”) of the dog’s chest are taken to see if the tumor has spread to the lungs. It is important to find out whether the cancer has spread (metastasized) to the lungs, as treatment options and prognosis are very different if lung metastasis has occurred. In the initial diagnostic stages, aspirate (sampling of cells) of lymph nodes and any skin masses and an abdominal ultrasound are generally done. Again, these steps are necessary in order to assess the spread of the cancer and health of the dog.

Another diagnostic test that may be performed is a bone scan. This test, which requires an overnight stay in the hospital, will help determine whether the tumor has spread to other bones and how much of the bone where the primary tumor is located is affected.

To obtain a definitive diagnosis, it is necessary to aspirate the tumor with an ultrasound-guided needle or perform a bone biopsy in order to attempt to identify the type of tumor. These procedures require sedation or anesthesia as they would otherwise be painful. There is often a large amount of bony reaction associated with this type of tumor, so it can be difficult to obtain an adequate sample of the actual tumor cells. Additionally, small samples are taken in order to minimize the spread of the tumor cells along the biopsy pathway and to avoid fracturing the bone. For these reasons, the biopsy procedure does not always yield a diagnostic sample. Even though only very small samples are taken, and bone biopsies are performed with the utmost care, this procedure always carries a small risk of fracturing the bone.

The tumor must be removed from your dog’s leg. Aside from the possibility that the cancer will spread, the tumor is painful, and once it has destroyed enough of the bone, even normal activities such as walking or running can cause the bone to break. The standard treatment for an osteosarcoma is the amputation of the affected limb. Most dogs recover quite well from this procedure and are running and playing in a very short time.

Dogs who have other orthopedic problems or who are obese may not be good candidates for amputation. Some owners may also not want to have their dog’s leg amputated. If this is the case, a limb sparing procedure may be possible. In this procedure, the tumor is removed and the bone is replaced either with another bone from your dog or with a bone from a bone bank. This operation cannot be performed in all locations and the tumor must be of a relatively small size at the time of diagnosis.

The most common site where limb spare procedures can be done is for the distal radius, which is the bone just above the wrist of the dog. There is a high complication rate with this procedure and it often involves a more involved recovery than an amputation. Because of the high complication rate, amputation may eventually be necessary.

If neither amputation nor a limb sparing procedure is possible or desirable, another option is to attempt to relieve pain with palliative radiation therapy. Although palliative radiation does not generally increase a dog’s survival time, in about 75% of dogs treated it can help alleviate the pain caused by the tumor. While there are several different protocols used at different facilities radiation therapy is usually given between one and 4 times often spaced out by a week. These treatments are administered while the patient is under general anesthesia.

Chemotherapy is an important follow-up step in the treatment of osteosarcoma. Though there may not be visible evidence of the spread of tumor in the patient, it is estimated that over 90% of dogs with this type of tumor have microscopic spread before amputation or limb sparing procedures are performed. Although chemotherapy has not been shown to be very effective in treating osteosarcoma when there are visible signs of spread, it is very effective in treating microscopic disease. Again different protocols are used at different facilities but most use some combination of cisplatin or carboplatin along with doxorubicin. In most published studies chemotherapy as a follow-up treatment increases the median life expectancy to about one year. The median survival time with amputation alone is about three months. By two years 10-20% of the dogs who have received chemotherapy appear to be free of cancer. Most dogs tolerate the therapy very well and experience few side effects. Although side effects can include vomiting and diarrhea or infection due to decreased white blood cells in a small percentage of patients. If any of these signs do occur it is important that the owners seek veterinary care right away.