Canine Cancer: Osteosarcoma - The Most Common Bone Cancer - What Is It?

What is Osteosarcoma?

Osteosarcoma is the most common type of bone cancer; it accounts for up to 85% of all primary bone tumors diagnosed.1 The most common place for osteosarcoma to develop is in the limbs, though the cancer can also originate in other bones such as the skull, ribs, vertebrae, and pelvis—as well as other non-bony tissues such as mammary glands, spleen, liver, and kidneys (extraskeletal osteosarcoma).²

There have been several studies dedicated to finding a connection between a dog's weight/height and their likelihood of developing a bone tumor, and the overwhelming conclusion is that large breed dogs are significantly more likely to develop osteosarcoma than small breeds.³

What makes osteosarcoma stand out from other bone tumors is how quickly it progresses in one localized area—where the tumor first appears and destroys the bone—and how quickly and early in its course it is able to spread to other areas of the body—most commonly the lungs.¹

Risk factors for developing Osteosarcoma

Like most cancers, there is no direct link that explains how or why one dog develops the disease while another won’t. All we can do is look for patterns, and the most notable one is that large dogs are more likely to get this cancer than small dogs.

There are some breeds that seem to be predisposed to developing the cancer, which include: Boxers, Doberman Pinschers, Golden Retrievers, German Shepherds, Great Danes, Great Pyrenees, Greyhounds, Irish Setters, Irish Wolfhounds, Labrador Retrievers, Rottweilers, Saint Bernards, and Weimaraners.²

How is Osteosarcoma diagnosed?

The first step in diagnosing osteosarcoma is to take an x-ray of the affected area. Often a pet parent will notice a swelling on their dog’s limbs, and that the pup is avoiding using that limb.

The vet takes an x-ray to see if the animal has any areas in the bone that look like part of the bone is missing (the way it looks on the x-ray can be described as “moth-eaten”). If the space is noticed on the bone, the next step will be to get a sample of that tissue. A sample is usually removed through a fine needle aspiration (FNA). When collecting cells with an FNA, a small needle is used to remove the sample from the affected area to examine the cells under a microscope.²

Because osteosarcoma spreads so quickly through other parts of the body, and, most commonly, first to the lungs, an x-ray or CT scan of the lungs would be another step in the diagnosing and staging process. Determining if the cancer has spread to the lungs helps to identify what stage the cancer is at, and what type of treatment plan would be the most effective for that patient.¹

Most osteosarcoma tests are performed alongside standard diagnostic tests that allow the veterinarian to be fully confident in the diagnosis and know if, or how, the cancer is affecting the other parts of the body. Using diagnostic tests, the vet can see any outside factors that may impact the appropriate treatment plan, and help stage the cancer more accurately.³

In very specific cases, some veterinary oncologists may see the combination of x-ray results, breed, location, and appearance of the tumor pointing towards osteosarcoma, and consider amputation (of the affected limb) prior to receiving a full diagnosis through biopsy.¹

Signs and symptoms of osteosarcoma

Unfortunately, the first noticeable sign that your pet may have osteosarcoma is pain.

Most dogs who develop osteosarcoma will have it in their limbs, most commonly in the radius/ulna (above the front knee) and the tibia/fibula (below the hind knee). When a dog develops osteosarcoma in those areas (or other parts of the limb’s bones) they will commonly show signs of pain when they use the limb, leading to lameness. This pain often shows itself long before swelling, due to the growing tumor. The pain in the limb comes from the cancer eating away at the healthy bone tissue to make space for the tumor to grow.²

At this point, pet parents will often take their dog to the vet, but because there may be no other signs pointing to osteosarcoma at this stage, the vet will likely prescribe a pain killer for the pet to help them manage the pain. However, painkillers don’t remain effective for long, and it’s usually seen that after about a week on the medication, the drugs will no longer have an effect on a dog’s pain levels. This is usually when swelling becomes apparent, and that is when your family vet will refer you to a veterinary oncologist. The swelling may be soft or hard, and it will get worse over time.³

For dogs who develop osteosarcoma outside of the limbs (extraskeletal)—in bones in other places in the body—locating the development is slightly more difficult. For some dogs, the first sign of something wrong will be swelling in the affected area, the most common of these being the ribs, skull, jaw, or other bony areas.⁴

For any dog developing osteosarcoma, there are common signs that they may not be feeling healthy as their body tries to process and fight what is going on inside: irritability and aggression (usually linked with increased pain), loss of appetite, weight loss, sleeplessness, and lack of interest in activities that normally excite (like walks or play). Dogs developing the cancer in their jaws may have trouble eating due to unseen pain. Dogs who develop the bone cancer in their skull may show symptoms of neurological traumas such as seizures, a harder time keeping balance, and frequent confusion.^1,4

What does treatment look like for dogs with osteosarcoma?

For many dogs, the first line of action for osteosarcoma is amputation of the affected limb. For a majority of dogs, the pet parents have a harder time adjusting to the amputation than they will. Dogs are incredibly adaptable, and the change from having the pain of the osteosarcoma in the bone to not having that particular leg will be a relief to them.¹

There are some cases where amputation is not the best option, such as in dogs with neurological problems happening alongside osteosarcoma, dogs with severe arthritis, or dogs that have developed the cancer in a bone other than a limb. For these dogs, the first step is to get the tumor removed through surgery and to heal the affected bone.³

For some dogs, a non-amputational surgery will involve removing the tumor and replacing the affected bone with a custom titanium implant. Though this method can be effective, there is still a risk that the tumor may relapse in the same bone in the future.

There are also some advanced radiation treatments that can be used to treat an affected area that is not safe, nor convenient, to approach surgically. Stereotactic radiation (SRS/SRT) can be helpful for patients who have not endured a significant amount of bone loss. It is a highly accurate type of radiation that focuses high doses of the treatment directly into the osteosarcoma cells.⁴

After the primary affected area has been given the attention it needs, the patient will need to be treated for the secondary spread. Because of osteosarcoma’s advanced metastasis, the likelihood for spread even after the primary tumor has been removed is still very high. Many dogs will be put on a chemotherapy or radiation protocol to protect their bodies from developing osteosarcoma again, and/or in new locations. Once that treatment protocol is completed, there is still a chance that over time the cancer will present itself again, so your vet will usually develop a check-in schedule to run standard tests on your pet to check for signs of the cancer returning.⁵

It is important to note that chemotherapy medicines for dogs do not impact their quality of life in the same way that chemotherapy drugs do to humans. Many dogs in chemotherapy treatment are able to be just as happy as they were before their diagnosis. Though there are a few common side effects, the chances of severe side effects occurring is less than 5%.

The most important part of your pet’s recovery from osteosarcoma is that their pain is adequately managed. Your vet should be able to create a pre-treatment and post-treatment pain management plan to help your pet adjust to the new factors their life will hold.

Osteosarcoma Prognosis — how long do they have?

Depending on the type of treatment chosen, the time that treatment is given, the location of the tumor, and the breed and age of the patient, there are many different prognoses for dogs diagnosed with osteosarcoma.

Without any treatment, the average survival time is about two months. Most dogs are not able to bear the pain and discomfort that comes with osteosarcoma longer than that, and their quality of life drops dramatically as pain medications become ineffective.

If the affected limb is amputated, the average survival time increases to 6.5 months with around 2% of all patients living for longer than 2 years.

When the chosen treatment is palliative care—either chemotherapy or radiation—to assist in creating a higher quality of life while also lessening the pain, the average survival time is around 6 months.

A combination of chemotherapy and amputation of the affected limb brings the average survival time up to just under 1 year, with around 20% of patients surviving with a good quality of life for more than two years.

Dogs who have limb-conserving surgery in combination with chemotherapy have close to the same survival rates as dogs with chemotherapy and amputation.³

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