Osteosarcoma (Bone Cancer) in Dogs

What is Osteosarcoma (Bone Cancer) in Dogs?

Osteosarcoma is a malignant neoplasia, or cancer, that surface from bone cells. It is the most common malignant tumor of the skeletal system. Although any dog breed can develop osteosarcoma, large and giant breeds are more commonly afflicted. Large breeds typically develop tumors on the appendicular skeleton (legs), while smaller breeds tend to develop osteosarcoma on the axial skeleton (spine, pelvis, skull, or ribs). 

Bone cells, such as osteoblasts and osteoclasts, are responsible for continuously remodeling bones. The body keeps these cells highly organized and controlled. When they go awry, they’re responsible for the destructive and highly aggressive nature of osteosarcoma in dogs. 

First, osteosarcoma is locally invasive. The cancer starts in the middle of the bone, in the marrow, where the primitive bone precursor cells live. The tumor expresses many abnormalities at the molecular level that disrupt normal cell behavior, resulting in unregulated growth and behavior. Instead of maintaining a high degree of order during bone remodeling, the aberrant cells destroy some areas of the bone while producing abnormal amounts of bone in other areas.  

Osteosarcoma is also aggressive distantly, with most cases already having spread to other sites outside the bone cavity, typically the lungs, at the time of diagnosis.  

The development of osteosarcoma in dogs is complicated, but it is believed to have a genetic component. In some breeds, such as Rottweilers, the risk of osteosarcoma may decrease with early spay and neuter; the reason for this is unknown. 

Osteosarcoma is most often diagnosed in dogs between the ages of 6 and 8 years old. However, veterinarians have diagnosed dogs as young as six months with osteosarcoma.  

Symptoms of Osteosarcoma in Dogs

Because canine osteosarcoma most commonly affects the limbs, most dogs show signs of lameness. Other exam findings include:  

• Limb soft tissue swelling 

• Palpable mass 

• Limb asymmetry—one limb larger or abnormal looking in comparison 

• Decreased appetite 

• Painful, typically the limbs when touched or when walking 

• Tachycardia (elevated heart rate)  

• Dehydration  

• Neurologic signs

Causes of Osteosarcoma in Dogs

Genetics play a large role in the development of osteosarcoma, especially in large and giant breeds. Researchers have identified some genetic risk factors in breeds like:  

• Scottish Deerhounds 

• Rottweilers 

• Greyhounds 

• Irish Wolfhounds 

Even without a genetic predisposition, osteosarcoma can develop at previous trauma sites in bone, such as: 

• Radiation therapy sites 

• Previously healed fractures 

• Chronic osteomyelitis (bone infection) 

How Veterinarians Diagnose Osteosarcoma in Dogs

Commonly, radiographs (X-rays) are the most useful diagnostic tool in osteosarcoma, although veterinarians may already suspect osteosarcoma based on age, breed, and clinical signs alone. Your vet will need at least two radiographic views of the affected limb and will look for classic signs of osteosarcoma including:  

• Lytic lesions: Areas that appear moth-eaten, where bone loss or destruction has occurred 

• Productive bone growth: Areas that have excessive and abnormal bone growth 

• “Sunburst” pattern: Area of combined abnormal bone changes that resemble a halo, crown, or sunburst  

• Soft tissue swelling around lesion 

• Pathologic fractures: Due to the highly unstable and decreased bone mass, many bones with osteosarcoma will break even without major trauma, especially with heavy dogs. 

• Location: Osteosarcoma typically forms in the long bones surrounding the knee in the hind legs and away from the elbow in the front legs, i.e., closer to the scapula or wrist. It does not cross joint spaces. 

Although radiographs can be highly suggestive of osteosarcoma, they do not provide a definitive diagnosis. Other conditions can mimic canine osteosarcoma, such as:  

• Other primary bone tumors—fibrosarcoma, chondrosarcoma 

• Secondary bone tumors—metastasis from other sites 

• Infectious agents—fungal, bacterial, osteomyelitis 

To obtain a definitive diagnosis, your vet will need additional diagnostic procedures. The following procedures may be recommended. 

• Cytology of the area is a relatively noninvasive procedure using a small needle to obtain a sample of cells from the lesion. Osteosarcoma cells have a common appearance under the microscope and can display signs of malignancy. Vets can even use special stains to test for markers sensitive and specific for osteosarcoma, which can aid in the diagnosis. The downside to cytology is that it typically samples the perimeter of the mass and may not get a diagnostic sample in the center.  

• Biopsy is the gold standard for diagnosis of osteosarcoma. Your dog will need to be under full or moderate sedation, as this procedure is invasive and painful. A large sample will be obtained from the core of the lesion, and a veterinary pathologist will review the sample microscopically to confirm the diagnosis, identify the specific tumor type, and determine a prognosis.  

• Routine blood work is done to identify organ function and negative prognostic indicators, like an increase in serum alkaline phosphatase, an enzyme that can be elevated with bone production. 

• Abdominal ultrasonography can be used to determine if the abnormal growth of cells has distant metastasis, or spread, in the abdominal cavity. 

• Three-view chest radiographs will indicate obvious, gross metastasis to the lungs. While most cancers have already spread microscopically to the lungs, less than 10% are detectable. Spread to the chest indicates a poor prognosis. 

• Echocardiography, or ultrasound of the heart, can help determine cardiac function in dogs with suspected heart disease, which may make them a poor candidate for treatment. 

• Advanced imaging, such as CT, MRI, and bone scans, can fully evaluate the extent of local and distant cancer spread.  

Treatment of Osteosarcoma in Dogs

Unfortunately, the long-term prognosis for osteosarcoma is poor. The two components to address with treatment are the locally painful bone lesion as well as the high likelihood of distant spread. Typically, after the diagnosis is made, veterinarians will “stage” dogs with osteosarcoma, which involves finding the presence of distant cancer spread, diagnosing concurrent diseases not related to the cancer, then determining a dog’s likely prognosis and eligibility for treatment. Treatment for osteosarcoma varies, and typically involves the following: 

• Surgical management: Removal of the primary bone tumor by amputating the entire limb is the most common consideration when dealing with appendicular osteosarcoma. It is a relatively common procedure in veterinary medicine and usually well tolerated by dogs, who compensate well on three legs. If the tumor originates from a site other than a limb, a dog may require such procedures as removal of a rib or mandible. If removal is not an option, veterinary surgeons utilize debulking procedures (removal of as much as possible of the tumor) or limb-sparing surgeries. 

• Chemotherapy: The use of systemic chemotherapeutic agents significantly prolongs the survival time after diagnosis. Currently, platinum-based chemotherapy agents (such as cisplatin or carboplatin) and doxorubicin are the standards of care for systemic chemotherapy in dogs with osteosarcoma.  

• Immunotherapy is a developing field involving using the body’s own immune system to help treat neoplastic cells. 

• Palliative options: If a dog is not a candidate for surgery or chemotherapy, other options exist to help ease pain and possibly increase survival time. These treatments include: 

  • Traditional radiation therapy (relatively noninvasive) 

  • Stereotactic radiation therapy (more invasive) and high-dose radiation therapy 

  • Bisphosphate drugs, which inhibit bone destruction and decrease pain 

  • Aggressive pain management, including drugs like NSAIDs, amantadine, gabapentin, and opioids 

Recovery and Management of Osteosarcoma in Dogs

Management and prognosis of osteosarcoma depend on a dog’s specific treatment plan. Amputation alone only provides short-term pain relief. Most dogs succumb to distant metastasis within months, and only 10% of dogs will survive for one year. 

The combination of amputation and chemotherapy increases the survival time to approximately one year after diagnosis, with 20% of dogs surviving longer than two years. Without any treatments, medical or surgical, the expected survival time is similar to amputation only, around 4 months. However, without amputation, dogs suffer from severe pain and the likelihood of pathologic fractures.  

Veterinarians treating dogs with osteosarcoma require physical exams, blood work, and chest radiographs approximately every 2-3 months after treatment to monitor for metastasis and complications from surgery and chemotherapy, which can include infection, immune suppression, gastrointestinal issues, and bloodwork abnormalities. Some chemotherapeutic agents are toxic to the kidneys and heart, so veterinarians will also monitor for organ damage.