The most common bone tumor in dogs is a type of cancer called osteosarcoma. These are tumors that originate from bone cells and are highly aggressive in nature. They are most commonly seen in older, large breed dogs. At the time of diagnosis, approximately 90% of dogs with osteosarcoma have metastasis (spread of cancer to other areas in the body). Thankfully, in most of these dogs, the metastasis is early and too small (microscopic) to be detected with imaging. In only 10% of dogs, metastasis is large enough to be detectable at the time of diagnosis. The most common metastatic site is the lungs. Another common metastatic sites are other bones. This type of cancer only spreads to the lymph nodes in 4% of cases.
Definitive diagnosis of this cancer type involves getting a tissue biopsy either at the time of surgical removal of the tumor, often with an amputation, or with a biopsy before surgical amputation.
Alternatively, we may be able to get a higher suspicion of a cancer diagnosis prior to initiation of treatment with a fine-needle aspiration cytology. Because of the small number of cells present with this method, it is less likely to be as informative as a bone biopsy.
In some cases, a definitive diagnosis is not warranted prior to amputation such as when our clinical suspicion is high or when your pet has intractable pain which can occur when your pet’s leg is broken due to the destruction caused by the cancer. In these cases, we will recommend an amputation, which will provide a diagnosis and immediate relief from pain.
Prior to radiation, limb-spare surgery, or chemotherapy, a cytology or histology diagnosis is recommended or required, depending on the facility. Here at WVRC, we require a diagnosis before giving chemotherapy.
Unfortunately, the therapies available to us do not cure this cancer type but can greatly extend their life while providing a very good quality of life. Treatment for osteosarcoma focuses on 2 aspects: 1. Alleviating the associated and very severe bone pain, 2. Slowing down metastatic disease.
Because osteosarcoma is so incredibly painful, for the majority of dogs we recommend amputation, which is the only definitive was of completely resolving your pet’s pain.
In dogs in which amputation is not possible, there are palliative measures that can be taken to decrease the amount of pain they are experiencing. The best way to manage pain in any pet with a bone tumor is palliative radiation therapy with a combination of oral pain medications and an injection of a bisphosphonate (see below). Many veterinary oncologists believe that bone pain caused by cancer is never truly managed adequately with pain medications alone. Therefore, we recommend combining radiation therapy with additional medications to provide the best outcome. The various methods to control pain are detailed below:
1. Radiation therapy – Pain is palliated in 75-90% of dogs for a duration of 2-3 months. Response to treatment is rapid, with improvement generally observed in the first 7-21 days. The patient is placed under brief anesthesia for each dose of radiation to make sure that he or she is positioned properly. Significant side effects due to radiation therapy are unlikely with this protocol. We do not currently offer radiation therapy here at WVRC.
2. Bisphosphonate injection – This medication is used for osteoporosis and bone metastasis in people and works by decreasing additional bone destruction, thereby decreasing pain. It is given injectably every 28 days.
3. Pain medications – Multi-modal pain management (the use of multiple classes of pain medications) can also be used to try to control the amount of pain your pet is experiencing.
With oral pain medications alone, the average dog will live approximately 1-2 months, eventually becoming extremely painful and unresponsive to the pain medications. Their survival time can be improved by several months when radiation therapy +/- biosphosphonates is/are added to their pain medication regimen.
For those dogs that undergo an amputation/limb spare surgery but no chemotherapy, the survival time is generally 3-5 months following diagnosis. While their pain is well managed, they eventually succumb to metastatic disease (spread of cancer).
For those dogs where we combine amputation/limb spare surgery with chemotherapy, we quadruple their life span, extending their life to approximately 1 year with about 25% of dogs living to 2 years or longer.