Osteosarcoma (Bone Cancer) in Dogs

What is Osteosarcoma (Bone Cancer) in Dogs?

Osteosarcoma, a form of bone cancer, is a malignant tumor originating from bone cells in dogs. It stands as the most prevalent malignant tumor affecting the skeletal system of canines. While osteosarcoma can occur in any breed, it predominantly afflicts large and giant breeds. Typically, large breeds manifest tumors on their appendicular skeleton (limbs), whereas smaller breeds tend to develop osteosarcoma on their axial skeleton (skull, spine, pelvis, or ribs).

The genesis of osteosarcoma lies within the bone cells, including osteoblasts and osteoclasts, which are responsible for the constant remodeling of bones. Normally, the body regulates these cells meticulously. However, when they malfunction, osteosarcoma emerges with its aggressive and destructive characteristics. Initially, the cancer emerges in the bone marrow, where primitive bone precursor cells reside. Abnormalities at the molecular level lead to uncontrolled growth and behavior of the tumor cells. Consequently, the bone’s structure is disrupted, with some areas being destroyed while others exhibit abnormal bone production.

Moreover, osteosarcoma demonstrates a propensity for distant metastasis, often spreading beyond the bone cavity, particularly to the lungs, by the time of diagnosis. The development of osteosarcoma is multifaceted, with genetics believed to play a significant role. Interestingly, certain breeds like Rottweilers may have a decreased risk of osteosarcoma with early spaying and neutering, although the underlying reasons remain unclear.

Typically diagnosed between the ages of 6 and 8 years old, osteosarcoma can also affect younger dogs, with cases reported in dogs as young as six months old.

Symptoms and Types

The symptoms of osteosarcoma in dogs primarily manifest in the limbs, which are the most commonly affected areas. Common signs include:

Lameness
Swelling of soft tissue in the limb
Detectable mass
Asymmetry in the limb size or appearance compared to others
Reduced appetite
Pain, especially when the limbs are touched or during movement
Elevated heart rate (Tachycardia)
Dehydration
Neurological symptoms

Causes

The development of osteosarcoma in dogs is significantly influenced by genetics, particularly in large and giant breeds. Certain genetic risk factors have been identified in breeds such as:

Scottish Deerhounds
Rottweilers
Greyhounds
Irish Wolfhounds

Even in the absence of a genetic predisposition, osteosarcoma may arise at sites of previous trauma in the bone, including:

Locations where radiation therapy was administered
Areas of previously healed fractures
Instances of chronic osteomyelitis (bone infection)

Diagnosis

Veterinarians rely heavily on radiographs (X-rays) as the primary diagnostic tool for osteosarcoma in dogs, particularly when clinical signs, age, and breed indicate a possibility of the disease. At least two radiographic views of the affected limb are necessary, with veterinarians observing for typical signs of osteosarcoma:

Lytic lesions: Areas displaying bone loss or destruction resembling a moth-eaten appearance.
Productive bone growth: Abnormal and excessive bone growth in certain areas.
“Sunburst” pattern: A distinctive pattern resembling a halo, crown, or sunburst caused by combined abnormal bone changes.
Soft tissue swelling surrounding the lesion.
Pathologic fractures: Bones affected by osteosarcoma are highly unstable and have decreased bone mass, making fractures common even without significant trauma, especially in larger dogs.
Location: Osteosarcoma typically occurs in the long bones surrounding the knee in hind legs and away from the elbow in front legs, closer to the scapula or wrist. It does not extend across joint spaces.

While radiographs can strongly suggest osteosarcoma, they alone cannot confirm the diagnosis definitively. Other conditions may mimic osteosarcoma, including:

Other primary bone tumors like fibrosarcoma and chondrosarcoma.
Secondary bone tumors originating from metastasis.
Infectious agents such as fungal or bacterial osteomyelitis.

To obtain a definitive diagnosis, veterinarians may recommend additional diagnostic procedures:

Cytology: A relatively noninvasive procedure using a small needle to sample cells from the lesion. While cytology can reveal common characteristics of osteosarcoma cells under a microscope and may use special stains for specific markers, it may not always yield a diagnostic sample from the center of the mass.
Biopsy: Considered the gold standard for diagnosis, biopsy involves obtaining a sample from the core of the lesion under sedation. A veterinary pathologist reviews the sample to confirm the diagnosis, identify the tumor type, and determine prognosis.
Routine blood work: Conducted to assess organ function and identify negative prognostic indicators such as increased serum alkaline phosphatase, an enzyme associated with bone production.
Abdominal ultrasonography: Used to detect distant metastasis in the abdominal cavity.
Three-view chest radiographs: Indicate gross metastasis to the lungs, although microscopic spread to the lungs is common and may not be detectable in less than 10% of cases.
Echocardiography: Helps assess cardiac function in dogs with suspected heart disease, which could impact treatment candidacy.
Advanced imaging like CT, MRI, and bone scans: These techniques provide a comprehensive evaluation of local and distant cancer spread.

Treatment

Regrettably, the long-term outlook for dogs with osteosarcoma remains bleak. Treatment aims to address both the locally painful bone lesion and the high probability of distant metastasis. Typically, veterinarians conduct staging to assess the extent of the disease, including identifying metastasis and other unrelated health issues, to determine prognosis and treatment eligibility. Treatment approaches for osteosarcoma include:

Surgical Management: The primary treatment involves surgically removing the bone tumor, often through limb amputation for appendicular osteosarcoma cases. Dogs generally adapt well to three-legged mobility. In cases where the tumor originates from non-limb sites, procedures such as rib or mandible removal may be necessary. If complete removal isn’t feasible, surgeons may opt for debulking or limb-sparing surgeries.
Chemotherapy: Systemic chemotherapy significantly extends survival post-diagnosis. Platinum-based agents like cisplatin or carboplatin, along with doxorubicin, are the standard chemotherapy drugs for osteosarcoma in dogs.
Immunotherapy: This emerging field utilizes the body’s immune system to combat neoplastic cells.
Palliative Options: For dogs ineligible for surgery or chemotherapy, palliative treatments can help manage pain and potentially extend survival. These options include:
- Traditional radiation therapy, which is relatively noninvasive.
- Stereotactic radiation therapy, which is more invasive, and high-dose radiation therapy.
- Bisphosphate drugs, which reduce bone destruction and alleviate pain.
- Aggressive pain management using NSAIDs, amantadine, gabapentin, and opioids.

By addressing both the local tumor and potential metastasis, veterinarians aim to improve the quality of life and extend survival for dogs diagnosed with osteosarcoma.

Recovery and Management

The management and prognosis of osteosarcoma hinge upon the specific treatment plan tailored for each dog. Amputation alone offers only temporary relief from pain. Sadly, most dogs face distant metastasis within months, with only a 10% survival rate at the one-year mark.

Combining amputation with chemotherapy extends survival to around one year post-diagnosis, with 20% of dogs living beyond two years. For dogs without any treatment, whether medical or surgical, survival rates mirror those of amputation alone, averaging about 4 months. However, without amputation, dogs endure severe pain and face a high risk of pathologic fractures.

Veterinarians overseeing the treatment of dogs with osteosarcoma conduct regular physical exams, blood work, and chest radiographs every 2-3 months post-treatment. This monitoring helps detect metastasis and assess any complications stemming from surgery and chemotherapy, including infection, immune suppression, gastrointestinal issues, and abnormalities in blood work. Given that some chemotherapy drugs pose risks to the kidneys and heart, veterinarians also closely monitor for signs of organ damage.

Osteosarcoma in Dogs FAQs

Can bone cancer in dogs be prevented?

Currently, there are no known methods to prevent bone cancer in dogs.

What is the life expectancy of dogs diagnosed with osteosarcoma?

Following diagnosis, the survival time typically ranges from 4 months to 1-2 years, contingent upon the treatments administered.

Is bone cancer in dogs associated with pain?

Yes, bone cancer in dogs is exceedingly painful, necessitating the use of multiple pain relievers for management.

What are the initial signs of osteosarcoma in dogs?

Frequently, limping and swelling are among the initial indicators of osteosarcoma, particularly in large breed dogs.

What characterizes the final stages of osteosarcoma in dogs?

In the advanced stages, osteosarcoma tends to spread systemically, often affecting the lungs. Eventually, the bone weakens and may fracture under normal stresses like walking or standing, prompting veterinarians to strongly recommend amputation whenever feasible.