Osteosarcoma is the most common primary bone tumor in dogs. Large and giant breeds have the highest risk. Osteosarcoma is reportedly estimated to occur in more than 10,000 dogs each year in the U.S. Most commonly, osteosarcoma occurs on the limbs but can also occur in other parts of the skeleton. While amputation is the most frequently used treatment option for limbs, many dogs may not be ideal candidates for this type of surgery. Dogs with pre-existing neurologic or orthopedic disease may not be suited for life on three legs. Additionally, owners may be opposed to amputation or be interested in a treatment option that will preserve the limb for as long as possible.
Stereotactic body radiation therapy (SBRT) combined with chemotherapy is a definitive-intent treatment option that precisely delivers a high dose of radiation over a condensed number of days. We have been treating dogs with osteosarcoma with SBRT since the installation of the Varian Trilogy linear accelerator at the Flint Animal Cancer Center in November 2007. Most dogs are treated with three doses of radiation therapy; however, our plans are adaptable and individualized to meet patient needs. Treatment plans are designed to limit the radiation prescription to only the affected bone and soft tissue while sparing the surrounding normal tissues.
SBRT requires patients to have a CT scan that is performed at the Colorado State University Veterinary Teaching Hospital, so that each patient can be placed in a customized semi-rigid immobilization device, and an individualized radiation plan can be created. At the time of the CT scan, the radiation oncology team and the radiologists assess the degree of destruction that has occurred in the bone and determine if SBRT is an appropriate treatment choice. If there is a large degree of lysis or destruction of the bone, there is a greater concern for a fracture, and another treatment option may be recommended. Fracture of the affected limb can occur in up to 40% of treated patients as the bone becomes weakened by the tumor. These types of fractures are historically managed by amputation as they do not heal; however, we are currently investigating other methods of stabilizing and strengthening the bone to decrease the risk of fracture.
The majority of dogs experience a significant decrease in pain and increased limb usage within 1-3 weeks of treatment, but improvement can begin as early as a few days following therapy. Most dogs do not experience any side effects of radiation; however, about 20% will see some hair loss or reddening of the skin in the treatment field, and only about 10% of treated dogs will have more significant side effects requiring medical intervention or pain medication. The expected survival is 8-11 months with adjunctive chemotherapy.
In 2017, we updated our linear accelerator with the capability of RapidArc®. RapidArc® allows for faster delivery and more precise targeting of radiation to the tumor. This upgrade enables the gantry, or the part of the linear accelerator where the radiation beam comes out, to move 360° around a patient and deliver dose continuously throughout that time. Before this update, predesignated angles were selected, and the gantry would stop at each angle and deliver a set amount of dose. Now with RapidArc®, our “beam on” to “beam off” time is less than five minutes for most stereotactic body radiation plans making it ideal for treating our veterinary patients.
Radiation therapy at the Flint Animal Cancer Center
The Flint Animal Cancer Center oncology service includes three radiation oncology faculty, a medical physicist, four radiation oncology residents, and four dedicated radiation oncology technicians. We treat over 500 cases a year with radiation therapy, and approximately 50 of those cases are diagnosed with osteosarcoma. For more information about stereotactic radiation therapy to treat osteosarcoma, please email [email protected].