Bone Cancer in Dogs

By T. J. Dunn, Jr., DVM

Bone cancer in dogs and cats can be a challenging disorder to overcome. Although rare in cats, bone cancer in dogs is most common in large breeds but can occur in any canine. Achieving and maintaining good health is a balancing act.

There is an invisible chemical ebb and flow, a true harmonic resonance that lives within a healthy animal. And when that vibrant harmony is upset, when the sweet song of life is bumped out of balance, ill effects sweep over the entire individual. Cancer is one notable form of disharmony within an individual.

The hallmark of cancer is uncontrolled cell growth, invasion of cells into nearby structures and sometimes a dispersal to distant organs, which is termed metastatic cancer. And since any cell in the dog’s body has the potential to develop into a cancerous cell, bone cancer dramatically illustrates what can happen when things go wrong.

When a cell turns cancerous by a disruption the cell’s physiology, structure or function, normal neighboring cells usually consume the rogue cell. On other occasions the defective cell simply self-destructs and is swept away. But when conditions are just right -- or wrong from the animal’s perspective -- a modified cell, called a mutant, survives the modification, retains its vigor and reproduces more cells just like itself.

Generation after generation of cells arising from that single mutated cell eventually changes the neighborhood and carves out it’s own territory, spreading its own bad seeds into more and more neighborhoods. Metastatic bone cancer cells break away, hitch hike the blood stream or lymph fluid and travel to entirely new neighborhoods within the dog’s body and begin the malignant process all over again.  

Cancer is also termed neoplasia, which means new growth. A cancerous cell grows faster than normal and divides and multiplies at an abnormal rate; its progeny do likewise. From that one abnormal neoplastic cell more like itself invade and crowd out surrounding tissues. With bone cancer, there are four types of cell lines capable of evolving into a neoplastic condition:

1. Osteosarcoma causing nearly 80 percent of all bone cancers this most common form of bone cancer arises from cells that deposit bony minerals. Aggressive invasion and rapid growth make this form of cancer a dreaded threat.  The X-ray image to the right shows what an osteosarcoma of the humeral head looks like (click on it to enlarge).

2. Chondrosarcomas … these tumors arise from the cartilage joint surfaces at the ends of bone and generally have a less aggressive tendency to invade and spread.

3. Fibrosarcomas … originate from fibrous connective tissue adjacent to bone, are locally invasive into the bone and have a low tendency to spread.

4. Synovial cell carcinomas ... originate from joint tissues and invade the associated bone. These tumors are less aggressive than osteosarcomas.

A definitive diagnosis of bone cancer can only be made via microscopic evaluation of a bone biopsy. Veterinary pathologists classify the degree of malignancy of the cells and likeliness of metastasis to other tissues. Like seeds on the wind, neoplastic cells can be carried by the blood and lymph from the original site of the cancer to distant tissues whereupon a new cancerous growth arises. Called metastatic cancer, whenever distant growths are present in a dog’s body the magnitude of the ill effects on the patient are remarkably increased -- and the chances of a cure drastically reduced.

Most commonly seen in long bones such as the femur, bone cancer has a predilection for larger breeds including the Greyhound, Saint Bernard and Mastiff. Chronic, low-grade lameness with gradually increasing swelling near a joint will alert the veterinarian to the potential for a tumor to be present. X-rays of the affected area often display characteristic changes in a bone that are totally unlike the defects usually associated with arthritis.

On occasion, an apparently normal dog will be presented with a spontaneous, severe lameness. Physical examination and radiographic evaluation, to everyone’s shock, reveals the cause of the break to be due to bone cancer. This break is termed a pathological fracture and there is an example of a pathological fracture in the table below.

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