Osteosarcoma is the most common type of bone cancer in domestic dogs. It is an aggressive, malignant form of the disease that starts in the bones and spread to other parts of the body if not detected and treated. This type of cancer tends to metastasize quickly to the lungs and is usually fatal.
Osteoblasts are the young cells that are responsible for forming bone. In dogs with osteosarcoma, the immature cells begin to grow out of control within the bone. The mass of cells grows outward, destroying healthy bone as it grows. This causes abnormal bone growths that are painful, swollen, hard, and eventually, cause lameness. These tumours usually form at or near the joints in a dog’s legs, although they can form on any bone. When the cancer grows in the leg bones, it’s called appendicular osteosarcoma. When it grows in any other bone, it’s called axial osteosarcoma.
Cancerous bone growths are not as strong as normal, healthy bones and break very easily. When this happens, it’s called a pathological fracture. Pathological fractures do not heal.
No one knows for sure what causes osteosarcoma. Some people believe that blunt bone injury, previous fractures, or surgery can be a risk factor. Others believe that the fast growth of large and giant breed pups, combined with high-impact running, jumping, and play can increase the chances of developing osteosarcoma. Other possible causes include ionizing radiation, bone transplants, chemical carcinogens, foreign bodies, and bullets. Research done in 2002 revealed that there is a possible link between early sterilization and development of this type of cancer.
There may also be a genetic predisposition, since some family lines appear to be more at risk.
There is no real way to prevent osteosarcoma. Avoiding trauma, reducing surgery where possible, and providing a high-quality diet can help. Regular vet checkups can help to identify any problems at an early stage.
Osteosarcoma most commonly affects the radius, ulna, and humerus in the forelegs, although this cancer can also develop in the femur, fibula, and tibia in the rear legs. Far less commonly, bones in the face, jaw, vertebrae, and ribs may be affected. Initial symptoms may be slight lameness or limp. The dog may show favour to that limb, or begin to lick the area. Depending on the location of the tumour, other symptoms include pain, weakness, pain when opening the mouth, swelling around the area, progressive lameness, pathologic fractures, swollen upper or lower jaw, lack of appetite, weight loss, nasal discharge, swelling and pain along the spine or ribs, difficulty chewing or eating, and respiratory distress.
If you notice any of these symptoms, take your dog to the vet immediately.
Dogs at Risk
Middle-aged and older dogs develop this type of cancer more often, although young dogs can be affected. The average age of onset of osteosarcoma is about eight years. Females and males are both affected, but males appear to be more at risk. Spayed or neutered dogs develop osteosarcoma more often than intact dogs, particularly if the surgery took place before the dog was one year old. For an as-yet unknown reason, large and giant breeds are much more prone to developing this type of cancer than smaller breeds.
Testing and Diagnosis
In many cases, osteosarcoma is diagnosed when the dog is taken in for an x-ray to diagnose a potential bone fracture. X-rays of cancerous bone look moth-eaten and ragged. Other times, if there are other symptoms of a problem and the dog goes to the vet for a checkup, a full history, physical exam, and blood tests should be performed. X-rays of the affected area will show evidence of the cancer. Once an initial diagnosis of bone cancer is made, chest x-rays can determine if the cancer has spread to the lungs. Further x-rays or a CT scan may be performed to see if the cancer has spread to other bones. Unfortunately, it is very likely to have spread, because this is an extremely fast-growing and malignant type of cancer.
To definitively diagnose osteosarcoma, microscopic examination of biopsied bone samples must be done.
If the dog’s lymph nodes are swollen, fine needle aspiration samples may be taken for analysis to determine if the lymph system has been affected.
Treatment and Prognosis
Treatment and prognosis depend upon the extent of the cancer when it is diagnosed. The goals of treatment are to prolong the dog’s life and to relieve pain. If it is caught very early, it is possible to amputate the affected limb to halt progress of the disease. Chemotherapy and radiation are also possible treatment options. Amputation not only improves the dog’s quality of life, but can also greatly improve prognosis. Even in cases where prognosis is grave, amputation of the affected limb effectively reduces the pain the dog will experience in the last months of its life.
Surgical removal of the affected bone is always ideal, but not always possible.
Many dogs are euthanized when this type of cancer is diagnosed because the pain is so severe.
Unfortunately, prognosis is grave in most cases, simply because the cancer spreads so quickly. Most dogs only live a couple of months once diagnosis has been made.