Bone Cancer in Dogs
Bone cancer occurs most often in the bones of the distal radius (above the wrist joint), in the upper arm bone (near the shoulder), in the lower part of the femur (thigh bone) and the upper part of the tibia (the larger of the two leg bones). Less commonly it can appear in the pelvic bones, shoulder blades or spinal bones (vertebrae). The most common symptoms of bone cancer are lameness and pain.
Symptoms can develop over a short period of time, or the lameness can be persistent, not responding to conservative treatment for a presumed sprain or joint injury. In some cases, bony swelling can be seen or felt, sometimes by accident. The involved area may be tender to touch. In other cases, the first sign of bone cancer can be acute pain and lameness from a fracture through the cancerous bone. Less commonly, the first symptoms might be due to spread of the cancer (metastases) to other organs. Other dogs will exhibit loss of appetite, depression or decrease in activity.
Bone cancer can also occur in the skull, facial bones or ribs. Trouble chewing, trouble swallowing, facial swelling or nasal discharge could be signs of bone cancer in the skull or facial bones. A hard lump on a rib could be a sign of bone cancer in the ribs.
In order to diagnose bone cancer (osteosarcoma), you have to think of it. Thinking about it may provide an early diagnosis, which could affect the outcome. The disease occurs more commonly in older dogs, the same group that is more likely to have arthritis or ligament injuries. Young dogs with bone cancer are often diagnosed with sprains or ligament problems. Many dogs develop a lameness that is initially attributed to some other cause, only to be diagnosed later with bone cancer.
Anti-inflammatory medications are often prescribed to treat the lameness, even before x-rays are done. Improvement in symptoms does not exclude the diagnosis of bone cancer. The key to early diagnosis is early imaging. Blood work can help exclude other causes of lameness or systemic illness. It should be done as a baseline reference after the diagnosis is confirmed and can be used to monitor for side effects of medications, especially anti-inflammatories. A blood enzyme called alkaline phosphatase is sometimes checked to assess prognosis. A normal result is better, but a high level does not necessarily mean the dog will do poorly and should not be used to defer treatment of the disease.
The best way to evaluate persistent lameness, pain or swelling is with an x-ray. Characteristic changes of bone cancer can be seen early on. Classic findings include a “lytic” lesion, where the bone appears to have been eaten away, and is no longer smooth, or a “sunburst” pattern, revealing the tumor as it grows outward toward the periphery of the bone and pushes normal bone out of the way. A fracture can also be seen through the tumor. As a rule, osteosarcoma does not cross the joint line and does not affect the bone on the other side of the joint.
While these changes are usually typical of osteosarcoma, the diagnosis may not always be readily obvious. Other types of cancer that can cause lytic lesions in bone include chondrosarcoma, squamous cell cancer, or synovial cell sarcoma. In the majority of cases, the primary veterinarian can interpret the x-rays and diagnose osteosarcoma. If there is any question about the diagnosis, a formal interpretation by a veterinary radiologist might be helpful. In specific cases, bone scan, CT scan or MRI might provide additional information, but these additional tests are usually not necessary.
Many times owners are told the abnormalities could be due to a bone infection, which can be caused by a fungus or bacteria.
The fungal infection most likely to mimic bone cancer is coccidiodomycosis or “valley fever.” This condition is native to the Southwest United States and in rare cases can cause a proliferative bone lesion, which is usually not lytic. It is most often accompanied by evidence of infection in other organs and can affect other bones. Unless the dog has lived in the Southwest, the chances of this particular fungal infection are virtually zero.
There are two forms of bacterial bone infection (osteomyelitis): acute and chronic. There is almost always a history of trauma to the area: open fracture, gunshot injury, bite wound, previous surgery or soft tissue infection. It occurs when there is bacteria in the bloodstream AND the bone has been damaged someway. Just having bacteria in the blood isn’t enough. There can be swelling, lameness, pain and loss of appetite. Fever is a clue to the diagnosis of osteomyelitis in acute cases, but not in chronic ones. X-ray findings include soft tissue swelling, spiculated new bone formation , lysis of bone, thinning of the cortex and even fracture. X-rays can be diagnostic in some cases.
In these cases, the x-rays can be confusing, and both bone cancer and infection can present the same way. When there is a really high suspicion of infection, the veterinarian will do specific blood work and recommend a bone biopsy. The purpose of the biopsy in osteomyelitis is to isolate the infectious organism so proper antibiotic therapy can be prescribed. Antibiotics are typically prescribed for 4-6 weeks if the infection is acute and there is no fracture. If it is chronic and there is seclusion of the infection in a certain area of bone, then surgery is required in addition to long-term antibiotics. Fractures can occur and require bone grafts, which can fail if the infection is not cleared. Chronic osteomyelitis often causes microabscesses that can actually create a tract from the bone through the skin, which makes diagnosing osteomyelitis easier. Treatment is frustrating and expensive, which is why some vets may recommend amputation.
The Next Step
Most cases of bone cancer can be diagnosed with x-rays and the treatment plan can be decided without further testing. Chest x-rays should be done to determine if visible lung metastases are seen, and to assess the heart. Whenever feasible, amputation or resection are recommended, and in many cases chemotherapy is also planned. The current statistics show the greatest survival rates with a combination of amputation and chemotherapy. Even in the presence of lung metastases, some people will proceed with the amputation to provide permanent relief of the pain of bone cancer.
After amputation, the surgical specimen should be sent to pathology for identification of cell type and confirmation of the type of bone cancer. This information can be useful in determining further treatment, including chemotherapy. In addition, some veterinary insurance companies require histologic confirmation of the diagnosis in order to process claims.
If the diagnosis is obvious on x-rays, a biopsy may not provide any additional information. It can cause additional pain and may result in a pathologic fracture. If amputation is being recommended, the entire specimen can be submitted for analysis after surgery to confirm the type of bone cancer. In some cases, however, the x-rays may not be typical for bone cancer and should be reviewed by a veterinary radiologist. If the diagnosis is still not obvious or if there is more than one bone lesion, the veterinarian may recommend a bone biopsy or additional testing.
If the lesion is in the distal ulna or radius and limb-sparing surgery is being considered, the surgeon doing the limb-spare surgery should be the one doing the biopsy, so the biopsy tract is contained within the area that will be removed during surgery.