There is a need for a radiation treatment for osteosarcoma that provides similar pain relief without the high risk of skin side effects and fracture. FLASH radiation therapy is a type of ultra-high dose rate (UHDR) delivery that has shown to reduce damage to normal tissues for a given total dose delivered, while simultaneously preserving an equivalent tumor response compared with conventional dose-rate irradiation in several experimental models. In this dose finding study, we will determine the maximum tolerable dosages of FLASH and conventional dose radiation in dogs.