Osteosarcoma in Dogs
What is osteosarcoma?
Osteosarcoma (OSA) is the most common malignant bone tumor in dogs, and is a painful and aggressive disease. OSA develops from bone cells and most commonly occurs in the long bones (the upper and lower bones of the legs). Less commonly (in about 20% of cases), it occurs in the skull, ribs, spine, or bones in the feet. Cats rarely develop OSA; however, in dogs it is one of the more common malignant tumors.
What increases the risk for developing osteosarcoma?
Multiple factors, such as genetic and environmental influences, influence the risk of a dog developing OSA. The primary determining risk factor is size or height, as OSA primarily affects large and giant breed dogs. Some studies have shown that intact (female dogs that are not spayed and male dogs that are not neutered) dogs have a higher risk of developing OSA, and some studies have also shown that spaying or neutering at earlier ages (prior to 1 year of age) may increase risk.
What are signs of osteosarcoma?
The most common sign is limping or lameness, often with swelling or pain at the site of the bone tumor. It is common for patients to be treated for limping for a period of time prior to diagnosis. In some cases, OSA can weaken the bone, leading to fracture of the bone (called pathologic fracture), in which case dogs may present with a sudden, severe lameness and are unable to bear weight on the affected limb.
How is a diagnosis of osteosarcoma made?
Often the suspicion for OSA arises based on the presence of limping and painful swelling of the limb. Radiographs (x-rays) of the area of concern commonly show a characteristic combination of areas of bone destruction and new bone production that the tumor causes. However, x-rays alone are not enough to achieve a definitive diagnosis, so a tissue sample, either by a needle aspirate, a needle biopsy, or a surgical biopsy, is required to make a diagnosis. These sampling procedures require sedation or anesthesia and are important to determine if the bone abnormalities are from OSA or other conditions that may cause changes on x-rays, including infection, bone cysts, or other cancer types.
Another important diagnostic test to be performed if OSA is suspected is thoracic radiographs (chest x-rays) to look for evidence of spread of cancer (also called metastasis) to the lungs. Although this is present at the time of diagnosis in a minority of patients, finding evidence of cancer spread to the lungs would change the treatment plan and prognosis, so it is important to evaluate.
How is osteosarcoma treated?
Osteosarcoma causes local bone pain, but also has a very high rate of spread (metastasis). Therefore, it is important for a Board-Certified Veterinary Oncologist to treat both the local tumor by removing the tumor, as well as giving medical therapy to prevent or delay spread of the tumor.
For most dogs with a localized tumor of one of the long bones, amputation of the affected limb is advised as the first treatment. Dogs tolerate amputation very well and can get around surprisingly quickly following surgery. Another option for some patients is an advanced surgical procedure called a limb spare procedure, in which the tumor is removed but the leg is kept (“spared”) by replacing the defect that remains after the tumor is removed with another material, bone, or a surgical plate. This is typically only possible for tumors of the lower front leg.
Amputation achieves multiple goals for dogs with OSA:
- Immediately removes the source of bone pain and improves quality of life.
- Allows a definitive diagnosis, as the leg is submitted for biopsy after the procedure.
- Reduces the amount of cancer present in the body.
What to expect after amputation:
Dogs carry most of their weight in their front legs, so amputation may affect their gait (the way they walk), depending on which leg is amputated. Dogs that have a front leg removed end up with a “rocking horse” gait, whereas dogs that have a hind leg removed end up with very minimal change to their gait.
Because OSA affects middle aged to older dogs, many patients do have osteoarthritis in other joints by the time of diagnosis, so special attention should be paid to ensuring that the joints on the remaining 3 limbs are comfortable; supplements to maintain joint health should be considered.
Connecting with other families that have been through the amputation process can help. Numerous resources for families considering or going through the amputation process with their pet can be found online and by consultation with their veterinary care team.
Following surgery for OSA, medical therapy is advised to slow disease spread in the body (metastasis). Medical therapies that may be recommended include chemotherapy and immunotherapy. Most medical therapies for OSA are very well-tolerated, with dogs maintaining an excellent quality of life during treatment.
If surgery is not elected or not possible, then typically it is recommended to pursue treatments that help to reduce the amount of pain that the tumor is causing. Options may include radiation therapy, medical treatments with compounds called bisphosphonates to reduce bone destruction and pain, and oral pain medications.
What is the prognosis for dogs with osteosarcoma?
The prognosis for dogs diagnosed with OSA depends on location of the tumor and the combination of treatments elected. When surgery is not elected, prognosis depends on the ability of treatments to successfully control pain. Pain can typically be controlled successfully for 4-6 months with radiation, or 1-2 months with medical therapy alone; however, it should be noted that pathologic fracture is a risk, and should be monitored for closely. While treatment with oral pain medication alone is an option, pain cannot typically be adequately controlled with this type of therapy alone.
Most dogs that have surgery following by medical therapy have an excellent quality of life for around a year, and they are happy and active during and after treatment.
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