The Physics of Hypofractionationand SRS/SBRT

The practice of SRS/SBRT and other hypofractionated RT relies on the accurate delivery of large doses in a limited number of fractions. To minimize normal tissue toxicities, RT is typically required to be highly conformal with rapid dose falloff immediately outside of the target volume. This combination of high target dose, shortened fractionation and steep dose gradient, permits a very small tolerance for error. Therefore, establishing and following guidelines for rigorous quality assurance (QA) and quality control is extremely important. This chapter outlines the physics of hypofractionation by starting with definitions and basic premise, reviewing some currently available delivery systems, and finally discussing safety measures and important QA procedures at each step of the treatment process, from simulation to delivery.