Canine Osteosarcoma: Bone Cancer in Dogs

Although many types of bone cancer can develop in dogs, osteosarcoma (OSA) is the most common one. Usually occurring in the legs, osteosarcoma represents about 75-80 percent of malignant canine bone tumors, though less than five percent of canine tumors overall. Osteosarcoma tends to develop in the legs, possibly because it develops from cells that are involved in the manufacture of new bone, a process that occurs more often and more rapidly in long bones. Nevertheless, osteosarcoma may also develop elsewhere, such as in the spine, bones of the cranium, and rib cage. The incidence of osteosarcoma is higher in larger dog breeds than smaller dogs. Though it may appear in a dog of any age, osteosarcoma tends to affect older dogs, which is in sharp contrast with human osteosarcoma which usually strikes children and young adults, the most famous case being Ted Kennedy Jr. (son of the late Senator Ted Kennedy), who survived, but lost his leg to the disease.

Basic Definitions: Tumors, Cancer, Benign, Malignant, Metastasis, Sarcoma, Carcinoma
The word tumor refers to a growth that is abnormal compared to the surrounding tissue. Abnormally rapid and uncontrolled reproduction of cells can lead either to a benign tumor, or a malignant tumor. Malignant tumors are those which are more aggressive, do more hard to the tissue in which they are located, and usually (although there are a few exceptions) have a high chance of metastasis -spreading to remote areas of the body and seeding new tumors there. A sarcoma is a malignant tumor that develops either in connective tissue (which includes bone and cartilage tissue among others), or in muscle tissue. A carcinoma is a malignant tumor that develops in epithelial tissue, the type of tissue which lines surfaces. Since bones have epithelial tissue as part of their lining, carcinomas as well as sarcomas are possible.

Types of Malignant Tumors that can occur in Canine Bones
Osteosarcoma, the most common bone cancer, develops from cells that essentially are cousins of the cells that manufacture the bone material itself, the cells known as osteoblasts. Although sometimes dog owners are told osteoblasts actually produce the cancer, this is not exactly true. In fact, there are several subcategories of osteosarcoma, including osteoblastic, chondroblastic, fibroblastic, and many more. These are names of different types of cells that are present in bone, and the tumors are named accordingly. If the cells of a tumor look like osteoblasts, it is called osteoblastic. If they look like chondroblasts -the cells which produce cartilage- it is called chondroblastic, and so on. It is known, however, that sarcomas in bone result not from osteoblasts, chondroblasts, and the others per se, but from the stem cells that produce them. Stem cells are generalized cells which can develop into various cell types. In bone and cartilage, one type of stem cell that is present during development of the embryo can develop into several connective cell types as it reproduces. If cell reproduction goes a certain way, osteoblasts are produced. If it goes another way, chondroblasts are produced. But if a line of stem cells is somewhere along the way to producing osteoblasts and something goes wrong, cancer cells may be produce that look a lot like osteoblasts, but don’t act like osteoblasts. This is why it is more correct to say that the cells of osteoblastic osteosarcoma are cousins of the bone-making cells that we call osteoblasts.

Now, chondrosarcoma is the second most common malignant tumor that can develop in dog bones. As you may have guessed from the name, chondrosarcoma is a sarcoma of cartilage cells. Though less common than osteosarcoma, it is nearly as aggressive. As in the case of osteosarcoma, the diseased cells all begin as the same type of stem cells that also produced healthy osteoblasts and chondroblasts and a variety of other cells. Chondrosarcoma tends to occur in flat bones and the rib cage, since these are places where there is more cartilage compared to bone, in contrast to long bones which have cartilage concentrated at the ends. Sarcomas of different types can occur in other areas of bone too, for instance in the linings of joint capsules. Hemangiosarcoma and fibrosarcoma are two other sarcomas that can develop in dog bones. In the case of carcinomas, a subtype called squamous cell carcinoma can develop in the outer cell layer of canine bones, known as the periosteum. But again, bone cancer in dogs most often means osteosarcoma.

Etiology of Canine Osteosarcoma
What causes osteosarcoma, both in humans and dogs? Nobody is exactly sure, though many have speculated about possible mechanisms. Some have hypothesized cell damage due to ionizing radiation or carcinogenic chemicals as a possible causes. Nutritional factors have been studied, since the tumor usually develops near the region of the long bones where growth takes place (the growth plates). Other possibilities include trauma to the bones, and damage resulting from implants. Studies have been conducted to determine whether hormonal factors may be at play, perhaps in connection with neutering of the dogs. Genetic predisposition is another possibility; the presence of abnormal copies of a gene known as p53, which normally inhibits tumor development, correlates with increased osteosarcoma incidence.

Presentation and Diagnosis of Canine Osteosarcoma
Osteosarcoma in dogs typically presents as swelling, not always with pain initially, though pain will develop at some point. The dog is lame in the affected leg. The leg may fracture. On account of the pain, the dog will lose his or her appetite and lose weight, become lethargic, and often develop insomnia and be generally irritable. Unfortunately, by the time these symptoms appear, the tumor already has destroyed much of the bone in which it began. Even worse, in more than 90 percent of cases, the tumor already has metastasized to other bones and to the lungs.
X-rays can reveal visual features that are typical of an osteosarcoma in a bone. Together with clinical signs, usually this is enough to confirm a diagnosis. If needed though, a biopsy can be taken. In biopsy. a sample of the tumor is removed surgically, then examined by a pathologist.

Treatments for Canine Osteosarcoma
If osteosarcoma is detected early, there are treatments that not only can extend a dog’s life but can reduce the dog’s suffering. The treatments include surgery and chemotherapy, which usually means that the limb is amputated, followed by chemotherapy. If the cancer is in an early stage, amputation can actually save the dog’s life by preventing it from metastasizing to other parts of the body. Approximately 60 percent of dogs treated with amputation and chemotherapy for osteosarcoma are alive one year later, and forty percent two years later. In unusual cases in which only a tiny, isolated part of a bone is affected, it is possible to remove the tumor without amputation. Radiation treatment is used only to shrink an osteosarcoma in cases when it is so big that surgery is not possible.