Osteosarcoma Part One

Osteosarcoma (OSA) is the most common bone cancer in dogs, accounting for about 85% of bone cancer cases. The bottom line on OSA is that metastasis is a problem: 90% of patients will die from the metastasis within the 1^st year when amputation is the only treatment. Those are grim statistics, but it is the bone pain from the tumor itself that creates the biggest challenge to most clients I meet. In an upcoming post I will focus on the decision to amputate and the myths I hear. But first, let’s get background info out of the way.

Which bones are most common to develop osteosarcoma?

Osteosarcoma can develop in any bone in the body, but three-quarters develop in the limbs, with the front legs twice as likely to develop osteosarcoma as the hind legs. OSA tumors are usually found at the end of the bone (called the metaphysis), and this can help to distinguish them from bone tumors that have spread from other primary cancers, which are usually found in the middle of the bone (called the diaphysis). The most common locations for OSA are the top of the shoulder (top of the humerus bone), the wrist (bottom of the radius bone), and the knee (bottom of the femur bone or the top of tibia bone). Another common site is the bottom of the tibia bone at the ankle or hock joint. OSA can sometimes be found in the middle of bones and in body bones, but this is less common.

Why is osteosarcoma considered to be aggressive?

Osteosarcoma (OSA) tumors grow fast and metastasize quickly. Only 10 to 15% of dogs with osteosarcoma already have detectable lung metastasis when they are first diagnosed, but a whopping 90% have micrometastasis (undetectable spread). This incredibly high rate of micrometastasis makes systemic chemotherapy treatments just as important as tumor removal. Treating the primary bone tumor does not address the metastasis. Still, despite this aggressive behavior locally in the bone and the metastasis, OSA is a highly treatable tumor, in my opinion.

Tumors can also metastasize to other bones, but this is less common. The regional lymph nodes can also be involved, although this only occurs in 5% of dogs.

Which dogs get osteosarcoma?

Limb osteosarcoma is usually seen in middle-aged and older dogs, age seven to nine, but we also see a smaller peak incidence in dogs between the age of eighteen months and two years. Axial OSA can be seen in any breed at any time, but limb OSA is usually seen in the front limbs of large and giant breeds, including Great Danes, Saint Bernards, Irish Setters, Rottweilers, German Shepherds, Golden Retrievers, and Doberman Pinschers. Breed is not as important as height and weight; heavier and taller dogs are more likely to develop OSA.

What causes osteosarcoma?

Many different factors have been found that may influence the incidence of this disease, although the exact cause of OSA is unknown. One theory is that small “micro-fractures” occur in the long bones as they bear the weight of the dog’s body. These tiny fractures and subsequent multiple minor injuries in cells can lead to signals that increases the chances of mutant cells and developing malignancies.

There is an also association with surgical metallic implants (like those used to repair a fracture), bone infection, bone trauma, and even fractures without internal fixation has also been found. Radiation exposure — previous radiation treatment that bone was included in the radiation field — can lead to OSA later in life.  This late complication of ionizing radiation is rare and  usually happens three to five years after radiation treatments for other cancers that a bone was included in the radiation field.

Sex hormones have recently been shown to protect against OSA development. The interesting study was of Rottweilers who were spayed or neutered before one year of age: they were four times more likely to develop OSA later in life.

A number of molecular and genetic factors have been implicated in OSA development. Some genes researched include mutated p53 (a tumor suppresser gene), retinoblastoma, PTEN, and possibly c-Kit alterations. Angiogenesis, new blood vessel formation, can also play a role in OSA development and progression. Other growth factors, cytokines, and hormone signaling systems, alterations in matrix metalloproteinases,

What are the symptoms of osteosarcoma?

OSA bone tumors can cause pain, lameness or limping, weakness and even severe bone fractures in the primary site. The risk of a pathological fracture, a sudden and painful bone break, is real, which is why amputation is the most prudent immediate course to take. We will talk about treatment including amputation more in a future blog.

As I said above, despite the aggressive behavior locally in the bone and the high metastastic rates, osteosarcoma is a highly treatable tumor in my opinion. Next, we will talk about getting a diagnosis and treatment.