Canine osteosarcoma is the most common primary bone tumor found in dogs, representing approximately 85-95% of all bone tumors diagnosed in this species. This aggressive cancer originates from the abnormal production of bone-forming cells, leading to uncontrolled growth and destruction of bone tissue. It is a painful and rapidly spreading disease that can significantly impact a dog’s well-being.
Understanding Canine Osteosarcoma
Osteosarcoma primarily affects large and giant breed dogs, with a higher incidence observed in breeds weighing over 40 kg. While it can occur at any age, it is diagnosed in middle-aged to older dogs. Breeds such as Great Danes, Saint Bernards, Rottweilers, Greyhounds, and Irish Wolfhounds are recognized as having an increased risk.
The tumor most commonly develops in the long bones of the limbs, a location referred to as appendicular osteosarcoma, accounting for 75-85% of cases. Specific sites frequently affected include areas around the knee and forelimb joints. Less frequently, osteosarcoma can arise in the axial skeleton, which includes bones of the skull, spine, ribs, or pelvis.
Symptoms and Diagnosis
The initial signs of canine osteosarcoma often begin subtly, with lameness or limping being the most common early symptom. This lameness may be intermittent at first, sometimes responding temporarily to pain medication, but it progresses to become constant over a period of one to three months. As the tumor grows, noticeable swelling usually develops at the affected bone site, which can feel warm and painful to the touch.
Dogs experiencing significant pain from the tumor may also exhibit more general signs such as lethargy, a reluctance to walk or play, loss of appetite, or even irritability. In some instances, the weakened bone at the tumor site can fracture with minimal trauma, known as a pathologic fracture, which may be the first clear indication of the underlying bone tumor.
Diagnosis begins with a physical examination and X-rays of the affected limb. Osteosarcoma often presents with characteristic radiographic patterns, appearing as areas of bone destruction (lytic lesions) or abnormal new bone growth, sometimes described as a “sunburst” or “moth-eaten” pattern. Blood work, including a complete blood count and serum chemistry, is also performed to assess overall health and organ function.
Staging the cancer is a subsequent step to determine if the disease has spread, as osteosarcoma is highly aggressive and often has microscopic metastasis (spread of cancer cells) by the time of diagnosis, even if not visible. This usually involves chest X-rays to check for visible spread to the lungs, which is the most common site for metastasis. A CT scan or MRI may also be recommended for more detailed imaging of the primary tumor or to detect subtle metastatic lesions. A definitive diagnosis requires a bone biopsy, where a small sample of the affected bone is taken.
Standard Treatment Protocols
For dogs with appendicular osteosarcoma, the primary goal of standard treatment is to prolong life by addressing both the local tumor and potential systemic spread. The primary approach involves a combination of local tumor control and systemic chemotherapy. This multimodal strategy aims to manage the pain from the primary tumor while also targeting cancer cells that may have already traveled to other parts of the body.
Local tumor control primarily involves surgical removal of the affected bone. Amputation of the limb is the most common surgical option, providing immediate and significant pain relief by removing the source of the discomfort. In specific cases, particularly with smaller tumors on the distal radius, limb-sparing surgery may be an alternative, where the cancerous bone segment is removed and replaced with a graft or implant, preserving the limb.
Systemic control is addressed through chemotherapy, which is a necessary component of standard care due to the high likelihood of microscopic metastasis. Chemotherapy aims to destroy these circulating cancer cells, thereby delaying or preventing the development of macroscopic metastatic disease, most commonly in the lungs. Common chemotherapy agents are used.
While chemotherapy can have side effects, many dogs tolerate it well, with severe side effects occurring in less than 5% of patients. Many dogs adapt remarkably well to three legs, often experiencing a substantial improvement in their quality of life post-surgery.
Palliative and Supportive Care
When aggressive treatment protocols are not pursued, either due to advanced disease, other health concerns, or owner preference, palliative and supportive care becomes the focus. This approach prioritizes managing pain and maintaining the dog’s comfort and quality of life, rather than attempting to cure the cancer.
Pain management is a central component of palliative care, often involving a combination of medications to address the severe pain associated with the bone tumor. Non-steroidal anti-inflammatory drugs (NSAIDs) are frequently used, alongside other analgesics. These medications are tailored to the individual dog’s needs to provide the best possible comfort.
Palliative radiation therapy is another option to reduce pain and improve limb function. This involves a smaller number of radiation treatments aimed at shrinking the tumor and alleviating discomfort, rather than eliminating the cancer. While it usually provides pain relief for several months, it does not prevent the spread of the disease. Bisphosphonates are also administered intravenously to inhibit bone destruction and further reduce pain.
Prognosis and Life Expectancy
The prognosis for canine osteosarcoma varies considerably depending on the chosen treatment path and whether the cancer has visibly spread at diagnosis. Without any treatment, the average survival time for dogs diagnosed with osteosarcoma is short, ranging from 1 to 2 months.
For dogs receiving only palliative care, which focuses on pain management through medications, radiation, or bisphosphonates without surgical removal of the tumor, the median survival time is generally around 4 to 6 months. If amputation or limb-sparing surgery is performed as the sole treatment without follow-up chemotherapy, the median survival time is also approximately 4 to 6 months.
The most favorable prognosis is achieved with the primary approach of surgical removal of the primary tumor (amputation or limb-sparing surgery) combined with systemic chemotherapy. With this comprehensive treatment, the median survival time typically extends to 10 to 12 months. A percentage of dogs may live for two years or longer with this aggressive treatment protocol.