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Stereotactic Radiation Therapy (SRT) for Treatment of Limb Osteosarcoma

Osteosarcoma (OSA) is the most common primary bone cancer of dogs (and people). It is an aggressive cancer that destroys normal bone, causing the clinical signs of lameness and pain and the characteristic changes seen on X-Rays (radiographs). It also spreads rapidly to other parts of the body, most commonly to the lungs or to other bones. Osteosarcoma can affect any bone but most commonly occurs in the leg bones: the radius bone (near the wrist joint), the humerus bone (near the shoulder joint) and the tibia or femur bones in the back leg, often near the knee joint.

The current standard of care treatment for dogs with OSA is amputation of the affected limb followed by 4-6 cycles of chemotherapy. The majority of dogs, even the large and giant breeds, do extremely well after amputation surgery. The median survival time for dogs with this treatment is 12 months. Some dogs may not be good candidates for amputation due to concurrent orthopedic disease (e.g. cruciate disease, hip or elbow arthritis) or neurologic disease. Surgical limb sparing surgery is a treatment option for dogs that have OSA in selected sites and situations. In this surgery, the affected bone is removed and the defect is replaced by either a bone graft or a metal spacer. Dogs generally function well with this treatment, but there can be a high complication rate with implant loosening or breakage, infection or local tumor recurrence in the leg.

Stereotactic Radiation Therapy (SRT) combined with chemotherapy is a new curative intent treatment that precisely delivers a high dose of radiation to the affected bone to kill the cancer cells. First experiences with this treatment in dogs with OSA were reported by veterinarians at the University of Florida. Since the recent installation of the state of the art Varian Trilogy linear accelerator at the CSU Veterinary Medical Center, we have been treating dogs with limb OSA with this treatment. SRT has the advantage of being able to treat OSA in any anatomic location and, despite the name, does not usually require a surgical procedure and placement of bone allograft or metal implants.

Our treatment protocol involves diagnostic testing to stage the disease (blood tests, chest X-Rays +/- whole body nuclear scintigraphy bone scan) followed by a CT scan of the leg to develop an individualized radiation treatment plan. The SRT radiation treatment is delivered in 3 fractions over 3-4 days. Patients must be positioned very precisely for each radiation fraction in exactly the same position as they were for the planning CT. Patients must be healthy enough to have general anesthesia for the CT scan and each of the radiation fractions. Treatment with a bisphosphonate drug at least 24 hours before the first SRT therapy is recommended to help strengthen the bone. The first dose of chemotherapy is normally given along with one of the radiation fractions to increase the killing effect of the radiation on the cancer cells.

Most dogs experience a significant decrease in pain and increased limb usage within 1-2 weeks after SRT treatment. Short term radiation skin effects can be seen and if they occur, will generally develop 2-4 weeks after radiation treatment. They are most commonly mild, localized to the leg and improve over 2-3 weeks. Longer term changes in the skin can occur and include regions where the hair does not regrow, the hair coat or skin changes color, or the skin becomes thickened or thin.

Complications associated with SRT therapy for bone cancer that we have observed include acute radiation skin effects and fracture through weakened tumor bone (in up to 30% cases). The radiation that kills the cancer cells also affects the normal bone in the treatment area making the bone weaker after treatment. Pathologic fracture can be managed by amputation, surgical stabilization of the fracture or use of a custom orthotic.

We continue to refine the SRT protocol based on our clinical and research experience to determine the optimal radiation dose to kill all of the tumor cells in the bone and have the minimal effect on the normal tissues (skin, bone, muscle and cartilage) and develop bone preservation/ regenerative therapies to decrease the risk of fracture after SRT.

We respectfully request and appreciate regular follow up information from clients and referring veterinarians on patients treated with SRT including how well your dog is using their leg, any complications associated with therapy, development and date of any secondary cancer in the lungs or other bones and date of eventual death. Valuable information is gained from the analysis of SRT treated limbs in the event of death or amputation. We will cover the costs of shipping back to CSU and testing in this circumstance. All of this information helps us to provide the best possible standard of care with this exciting new treatment.

If you have any questions regarding SRT and whether it is an appropriate treatment option for your dog, please do not hesitate to discuss these with your attending Oncology clinician. They will also be able to put you in direct contact with the radiation oncologist or other clinician researchers if appropriate.