Bone Cancer in Dogs and Cats (Osteosarcoma)

August 3, 2017 /

If your dog or cat has been diagnosed with bone cancer, this article will help break down what it is and what treatment might look like.

Bone cancer, medically known as osteosarcoma, is one of the worst types of cancer affecting pets. Due to the aggressive nature of the disease and the significant pain that it causes affected patients. If your pet has been diagnosed with bone cancer, we are so sorry to hear this. We've created this article to help break down what exactly bone cancer is in dogs and cats, what treatment options are available, and other helpful information.

What is Canine & Feline Osteosarcoma (Bone Cancer)

Osteosarcoma is malignant cancer of bone cells. Malignant cancers are generally considered to have a worse prognosis than benign cancers due to the tendency to be both locally invasive and have a high potential for metastasis (spreading to other body systems). Benign cancers typically have a better prognosis, as they are less likely to metastasize but can be locally invasive. Osteosarcoma is the most common bone cancer affecting dogs. Other cancers affect bone, either by having origin in bone or spreading there. Osteoma is a benign tumor of bone cells that stays localized but can still cause pain, swelling, and other related problems. Malignant cancers of glandular tissues, such as adenocarcinoma, can have origin in the liver, kidney, thyroid, prostate, mammary and other tissues and metastasize to bone. Any bone in the body can be affected by osteosarcoma, but long bones like the humerus (bicep bone) and femur (thigh bone) are common sites of diagnosis. Such bones are part of the appendicular skeleton, along with the scapula (shoulder blade), clavicle (collar bone, just a vestigial bone in dogs and cates), radius and ulna (forelimb bones), and tibia and fibula (shin bones). The bones of ribs, skull, sternum (breastbone), and vertebrae (backbone) make up the axial skeleton. According to Malignant Bone Tumors in the Dog, in an expansive study of 1,215 cases, 82% of osteosarcomas involve the appendicular skeleton, while 18% affected the appendicular skeleton. There is also a trend for osteosarcoma to occur away from the elbow (higher up on the humerus) and near the knee (further down the femur from the hip). The distal radius (portion closer to the knee) and the proximal humerus (portion closer to the shoulder) are respectively the first and second most-common sites where canine osteosarcoma is diagnosed.

What are the Causes of Bone Cancer in Dogs and Cats

There are no known causes of bone cancer, but there are correlations in certain dog breeds (and their mixes), sizes and ages. Large and giant-breed dogs like the Boxer, Doberman Pinscher, Great Dane, Irish Setter, Labrador Retriever, Mastiff, Rottweiler, Saint Bernard, Scottish Deerhound, and others are well-documented as susceptible breeds. The mixes of these breeds may have similar tendency to develop osteosarcoma. Some breeds have a trend for males or females to be more commonly-affected by osteosarcoma. The male German Shepherd, Labrador Retriever, and Old English Sheep Dog show a greater indigence of the disease. The female Great Dane, Rottweiler, and Saint Bernard have a greater tendency for osteosarcoma. Large and giant-sized dogs are more known to have osteosarcoma than small and medium-sized dogs. Giant-sized dogs, which weigh more than 90 pounds, account for 29% of the cases of osteosarcoma. Large-sized dogs, with weights ranging from 60 to 90 pounds, make up 55% of osteosarcoma cases. Medium-sized dogs, having weight ranges from 30 to 60 pounds, comprise of 11% of osteosarcoma cases. Small-sized dogs weighing less than 30 pounds are the luckiest of the bunch as they only account for 5% of osteosarcoma cases. Adult and senior dogs develop osteosarcoma more commonly than juveniles (puppies). Malignant Bone Tumors in the Dog indicates “the average age of onset of osteosarcoma in the dog is about 7 and 1/2 years, with a range of 1 to 15 years.” Dogs having had bone fractures may be at greater risk for osteosarcoma with those undergoing surgical repair with an implant (pin, plate, etc.) or external fixation. Cats are less commonly affected by osteosarcoma than dogs, but the disease is the number one feline bone tumor. Fortunately for our feline friends osteosarcoma is less aggressive in their species than in canines.

What Symptoms Do Pets Show When Having Osteosarcoma

Cancer cells have abnormal DNA and aren’t able to turn off their replication like normal cells, which is one of the reasons why tumors form and cause damage to tissue at the sites of growth. With osteosarcoma there is a continuous process where malignant cancer cells damage normal bone and a corresponding effort by the body to repair microfractures by laying down new bone and other connective tissue. At the microscopic stage, where tumors are smaller than a few millimeters (microscopic disease), osteosarcoma causes no apparent pain. When cancer cells further divide and a tumor grows to a size detectable on physical exam or diagnostic testing (macroscopic disease) pets experience pain and clinical signs are seen. Clinical signs of osteosarcoma include:

Lameness (limping) - Reduced weight-bearing on the limb affected by osteosarcoma, which can also make the pet appear less able to walk, run, jump, play, stand up, lie down, and more.
Pain - When cancer cells become tumors the damage caused to normal bone causes pain. A pet having osteosarcoma may also lick at the area in attempt to self soothe or be resistant to having the affected site touched.
Swelling - As osteosarcoma grows tissue swelling will appear at the affected site. Swelling can also correspond with pain and lameness and can occur suddenly if the microfractures coalescence into larger fractures (pathologic fractures). Pathologic fractures also cause bruising at the surface which appears red or black.
Lethargy - A pet suffering from osteosarcoma may seem to lack energy, as any movement stimulating the affected site will cause pain. As a result, patients are less-willing to move and prefer to be still in order to reduce painful sensations. Osteosarcoma can also cause behavior changes, such as withdrawal from interaction with other pets and people, lethargy, decreased appetite, aggression, and others.

How to Diagnose Bone Cancer in Dogs and Cats?

Besides clinical signs, the medical history provided by the owner and the veterinarian’s assessment of a pet's breed, size, age, and trends of illness and wellness are combined with diagnostic testing. Diagnostics like radiographs (x-rays) are generally the first step in differentiating if a pet is suffering from osteosarcoma or a non-life-threatening ailment like osteoarthritis, intervertebral disc disease (IVDD), and others. Radiographs commonly show a pattern appearing like fire bursting off the surface of the sun (sunburst) or a moth has eaten away parts of the normal bone. Pathologic fracture can also be seen as a displacement of or lack of apposition of normal bone or swelling of adjacent soft tissue structures. If osteosarcoma occurs near a joint it generally does not cross from one joint to another. Although such findings immediately raise concern for osteosarcoma, there are other types of cancers that can appear similar to osteosarcoma like chondrosarcoma (malignant cancer of cartilage), squamous cell carcinoma (malignant cancer of the cells covering bone), and synovial cell carcinoma (malignant cancer of the inner lining of joints), and bone infections (bacterial, fungal, etc.). Advanced imaging techniques like CT scan (“cat scan”), MRI, and nuclear imaging (“bone scan”) can also be used to help detect cancers like osteosarcoma when they may not be readily apparent on radiographs. Unfortunately, once bone cancer is diagnosed there is a high likelihood it has already metastasized to other tissues even if it is not apparent on radiographs. The lungs are the most-common organs where metastases occur, but lymph nodes and other tissues can be affected. According to Washington State University “most (approximately 90-95%) of dogs with OSA are considered to have metastasis at the time of their diagnosis, although metastasis will only be evident at the time of diagnosis in approximately 10% of dogs and in others the metastatic tumors are considered to be microscopic.” Blood testing (chemistry, CBC, T4, etc.) and urinalysis are also used to gauge the assessment of overall health, as there may be a need for an osteosarcoma patient to undergo anesthesia for treatment (surgery, radiation, etc.) or have chemotherapy, it's crucial to know baseline organ function.

What Kind of Treatments Are Used to Treat Osteosarcoma in Dogs and Cats?

There are a variety of treatments available for bone cancer in dogs and cats, with the primary focus of alleviating patient pain. As osteosarcoma is such severe disease commonly affecting multiple body parts, it’s best that our animal companions have a consultation with a veterinary oncologist so that treatment best-suited to the patient’s needs can be prescribed. As veterinary oncologists work exclusively in the realm of cancer treatment they can lend an experienced perspective to cancer care and have the latest information on clinical trials. Your primary veterinarian can refer you to a veterinary oncologist or one can be found through the American College of Veterinary Internal Medicine (ACVIM). Surgery Since bone cancer in dogs and cats commonly forms on weight-supporting bones like the front limb’s humerus and hind limb’s radius, amputation is the top recommended treatment to save your pet from experiencing severe pain caused by ongoing damage to normal tissues from cancerous bone cells. Pain is a constant sensation at the site impacted by osteosarcoma and is not solely experienced when the patient bears weight on the affected leg or the area is touched. Amputation ceases the severe pain that ultimately leads most owners to making a quality of life decision to pursue euthanasia and can extend a pet’s pain-less life for weeks to months pending the degree to which metastasis has occurred. Although the recommendation that a pet undergoes limb amputation sounds vastly life-altering to owners, doing so greatly improves the patient’s quality of life and can reduce the chance of metastasis. Since dogs and cats carry themselves on four limbs, they generally move around quite well when an osteosarcoma-affected front or rear leg is surgically removed. Even with three legs, most dogs move better and more-comfortably than when dragging around a cancer-burdened leg. If an owner wants to strive to keep the pet’s front or rear leg, then limb-sparing surgery can be pursued in which the osteosarcoma-affected bone is removed and replaced by a bone graft. In order to increase stability in the limb, the joint nearest to the surgery site is fused so it cannot flex or extend. Chemotherapy Chemotherapy is also another tool used to fight osteosarcoma as it treats microscopic disease. Intravenous chemotherapy like platins (Cisplatin, Caroplatin, etc.), anthracyclines (Doxorubicin, etc.), and others damage cancer cells to inhibit their division. Patients having osteosarcoma can also benefit from bone-strengthening medications like Bisphosphonate (Pamidronate, etc.) that inhibit osteolysis (bone damage). Radiation Treatment Radiation can be used to kill cancer cells, inhibit bone destruction, shrink tissue swelling, and minimize pain. To undergo radiation treatment (RT), patients are fully anesthetized for a low number (palliative) or many (definitive) sessions. Palliative RT can significantly improve patient discomfort in only a few sessions and is generally the more-common treatment osteosarcoma patients receive. Stereotactic Radiation Therapy (SRT) is an emerging option that can have a curative effect for osteosarcoma in only three to four sessions. Immunotherapy The Penn Vet Cancer Center and Dr. Nicola Mason are developing the next generation of cancer treatment in the form of immunotherapy. Immune Therapy for the Treatment of Osteosarcoma involves a bacteria-based vaccination which stimulates the patient’s immune system to identify and eliminate bone cancer cells. Anti-Inflammatory and Pain-Numbing Medications Medications that reduce inflammation or have a pain-numbing effect are the top choices and can include non-steroidal anti-inflammatory drugs (NSAIDs), Tramadol, Gabapentin, Amantadine, Codeine, and others. Generally, combinations of drugs that reduce inflammation and numb pain are used and the need for multiple pain-numbing drugs grows as the osteosarcoma progresses and discomfort worsens. Complementary and Alternative Medicine (CAM) There are also complementary and alternative medicine (CAM) treatments that can reduce pain and inflammation along with improving a patient’s sense of well-being. Acupuncture (needles, laser, electrostimluation, etc.), aquapuncture (injecting acupuncture points with liquids like Vitamin B12, homeopathics, joint support medications, etc.), massage, Chinese medicine food energy therapy, Reiki, and others are CAM options that may benefit osteosarcoma patients. Dietary supplements and herbs can also have anti-inflammatory, blood-moving, immune system boosting, anti-cancer, joint-supporting, and other effects. Turmeric, a root which is commonly used as a spice in cooking, contains curcumin which has anti-inflammatory and anti-cancer properties. Omega fatty acids from fish or vegetarian sources can have an anti-inflammatory effect and reduce cachexia (wasting of body tissues). Although controversial in veterinary medicine, cannabidiol (CBD) products can inhibit pain, improve appetite, and generally make a pet feel better despite. Lifestyle Modification A pet’s environment and lifestyle must also be considered as part of the treatment process. Getting up onto and down from elevated surfaces, traversing stairs, and participating in high-impact activities (running, jumping, etc.) should be avoided, as doing so puts more stress on osteosarcoma-affected body parts and can lead to pathologic fractures. Putting gates at the top and bottom of stars, blocking access to beds and couches, and covering slippery surfaces with traction can help keep your home safer for osteosarcoma-afflicted pets. Supportive harnesses and carts can also be used to aid mobility and reduce weight-bearing on an osteosarcoma-affected limb. Veterinarians that work in the realm of physical rehabilitation typically have the most awareness of which mobility-supporting devices work best for a pet’s condition, size, level of activity, etc.

Can Bone Cancer in Dogs and Cats Be Prevented?

There is no known preventative treatment for osteosarcoma, but there are lifestyle measures that can be taken that may help to minimize exposure to agents that can stimulate cancer development. Human-grade, whole-food diets and treats lacking chemical preservatives (BHA, BHT, Ethoxyquin, etc.), artificial colors (4-Methylimidazole containing caramel color, etc.) and flavors, and other ingredients that have been known to correlate with cancer (see California’s Office of Environmental Health Hazard Assessment’s list of Chemicals Known to Cause Cancer or Birth Defects) should be chosen over feed-grade, commercially-available pet foods and treats having potential to contain such substances. My patients eating whole-food, human-grade meals (such as The Honest Kitchen’s low carb recipes like the Limited Ingredient Turkey Recipe, Grain Free Turkey Recipe and Grain Free Beef Recipe for dogs and the Grain Free Dehydrated Cat Food and Grain Free Wet Foods for cats) and treats generally are healthier, have improved digestive and immune system function, and tend to show fewer side effects from chemotherapy, radiation, and anesthesia. Give your pet water that is the same quality as you drink yourself. Filtration systems can remove heavy metals, chemicals, microorganisms, and other substances that are carcinogenic or can cause illness. Additionally, avoid putting pesticides, herbicides, and other chemicals into the environment frequented by your companion canine or feline. Doing so can also reduce the risk to your human family members, so pets and people both win while striving to prevent exposure to man-made toxins placed in our shared homes and yards. I hope your pet stays cancer free on a lifelong basis. If your canine, feline or other species of pet has been diagnosed with or undergone treatment for cancer feel free to share your perspective in the comments section.

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