Osteosarcoma in Dogs: Symptoms and Treatment

Osteosarcoma in dogs is an aggressive, malignant bone cancer and the most common malignant tumor of the skeletal system. It most commonly affects large and giant breeds and can cause pain, swelling, lameness, and weakened bones.

Treatment addresses the painful primary bone tumor and the high likelihood that the cancer has spread to other parts of the body.

What Is Osteosarcoma in Dogs?

Osteosarcoma in dogs is a malignant neoplasia, or bone cancer, that arises 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 affected. 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).

Osteosarcoma in dogs is highly aggressive both locally and distantly. Locally, the cancer starts in the middle of the bone, in the marrow, where primitive bone precursor cells live.

Distantly, most cases have already spread to sites outside the bone cavity, typically the lungs, at the time of diagnosis.

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.

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.

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 6 and 8 years old. However, veterinarians have diagnosed dogs as young as 6 months with osteosarcoma.

What Are the Symptoms of Osteosarcoma in Dogs?

Because osteosarcoma in dogs most commonly affects the limbs, lameness is a common symptom. Dogs may also have limb swelling, a palpable mass, limb asymmetry, and pain when the affected limb is touched or when walking.

Other signs of osteosarcoma in dogs may include:

• Limb soft tissue swelling
• Palpable mass
• Limb asymmetry—one limb may appear larger or abnormal in comparison
• Decreased appetite
• Pain, typically in the limbs when touched or when walking
• Tachycardia (elevated heart rate)
• Dehydration
• Neurologic signs

What Causes Osteosarcoma in Dogs?

Genetics play a large role in the development of osteosarcoma in dogs, especially in large and giant breeds. Osteosarcoma can also develop at previous trauma sites in bone, even without a genetic predisposition.

Genetic Risk Factors

Researchers have identified genetic risk factors in breeds including:

• Scottish Deerhounds
• Rottweilers
• Greyhounds
• Irish Wolfhounds

Previous Trauma Sites

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 Is Osteosarcoma in Dogs Diagnosed?

Veterinarians commonly use radiographs (X-rays) to evaluate suspected osteosarcoma in dogs, but X-rays do not provide a definitive diagnosis.

Cytology or biopsy may be recommended to confirm the diagnosis, along with additional tests to evaluate organ function and cancer spread.

Veterinarians may already suspect osteosarcoma based on a dog's age, breed, and clinical signs.

X-Rays for Osteosarcoma

Your veterinarian 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 with excessive and abnormal bone growth
• “Sunburst” pattern: An area of combined abnormal bone changes that resembles a halo, crown, or sunburst
• Soft tissue swelling: Swelling around the lesion
• Pathologic fractures: Because of highly unstable and decreased bone mass, bones with osteosarcoma may break even without major trauma, especially in 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, such as 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.

Cytology and Biopsy

Additional diagnostic procedures may be needed to obtain a definitive diagnosis.

• Cytology: This relatively noninvasive procedure uses 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.
  
  Veterinarians can also use special stains to test for markers sensitive and specific for osteosarcoma, which can aid in diagnosis. One limitation is that cytology typically samples the perimeter of the mass and may not obtain a diagnostic sample from the center.

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

Additional Testing for Osteosarcoma

Additional tests may be recommended to evaluate your dog's health and the extent of local or distant cancer spread:

• Routine blood work: Used to identify organ function and negative prognostic indicators, such as increased serum alkaline phosphatase, an enzyme that can be elevated with bone production.
• Abdominal ultrasonography: Can be used to determine whether abnormal cell growth has metastasized, or spread, to the abdominal cavity.
• Three-view chest radiographs: Can identify 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: An ultrasound of the heart that can help determine cardiac function in dogs with suspected heart disease, which may make them a poor candidate for treatment.
• Advanced imaging: CT, MRI, and bone scans can fully evaluate the extent of local and distant cancer spread.

Conditions That Can Look Like Osteosarcoma

Other conditions can mimic canine osteosarcoma, including:

• Other primary bone tumors, such as fibrosarcoma and chondrosarcoma
• Secondary bone tumors from metastasis at other sites
• Infectious agents, including fungal or bacterial infections and osteomyelitis

How Is Osteosarcoma in Dogs Treated?

Treatment for osteosarcoma in dogs addresses both the locally painful bone lesion and the high likelihood of distant cancer spread. Treatment options include surgery, chemotherapy, immunotherapy, and palliative care.

The long-term prognosis for osteosarcoma is poor.

Staging Osteosarcoma in Dogs

After diagnosis, veterinarians typically “stage” dogs with osteosarcoma. This involves identifying distant cancer spread, diagnosing concurrent diseases not related to the cancer, and determining the dog's likely prognosis and eligibility for treatment.

Surgery

Removal of the primary bone tumor by amputating the entire limb is the most common consideration for appendicular osteosarcoma.

Amputation is a relatively common procedure in veterinary medicine and is 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 procedures such as removal of a rib or mandible. If removal is not an option, veterinary surgeons may use debulking procedures—removing as much of the tumor as possible—or limb-sparing surgeries.

Chemotherapy

Systemic chemotherapy significantly prolongs survival time after diagnosis. Platinum-based chemotherapy agents, such as cisplatin or carboplatin, and doxorubicin are the standards of care for systemic chemotherapy in dogs with osteosarcoma.

Immunotherapy

Immunotherapy is a developing field that uses the body's own immune system to help treat neoplastic cells.

Palliative Care

If a dog is not a candidate for surgery or chemotherapy, palliative options may help ease pain and possibly increase survival time. These treatments include:

• Traditional radiation therapy
• Stereotactic radiation therapy and high-dose radiation therapy
• Bisphosphate drugs, which inhibit bone destruction and decrease pain
• Aggressive pain management, including NSAIDs, amantadine, gabapentin, and opioids

Recovery and Prognosis of Osteosarcoma in Dogs

The prognosis for osteosarcoma in dogs depends on the specific treatment plan. Survival time varies with amputation alone, amputation combined with chemotherapy, or no treatment.

Prognosis With Amputation

Amputation alone provides short-term pain relief. Most dogs succumb to distant metastasis within months, and only 10% of dogs survive for one year.

Prognosis With Amputation and Chemotherapy

The combination of amputation and chemotherapy increases survival time to approximately one year after diagnosis, with 20% of dogs surviving longer than two years.

Prognosis Without Treatment

Without medical or surgical treatment, the expected survival time is similar to amputation alone, at around four months.

However, without amputation, dogs suffer from severe pain and have a likelihood of pathologic fractures.

Monitoring Dogs With Osteosarcoma

After treatment, veterinarians require physical exams, blood work, and chest radiographs approximately every two to three months to monitor for metastasis and complications from surgery and chemotherapy.

Complications can include infection, immune suppression, gastrointestinal issues, and blood work abnormalities. Some chemotherapeutic agents are toxic to the kidneys and heart, so veterinarians will also monitor for organ damage.