Organs at Risk (OAR) Tolerance in Hypofractionated Radiosurgery

For CyberKnife radiosurgery, the precise interpretation of the normal probability of tissue complications (NTCP) is extremely important due to the reverse planning algorithm and the non-isocentric irradiation geometry adopted by the system, which requires the setting of dose constraints for any organ at risk (OAR). Despite eight decades of practice in radiation therapy, the current understanding of radiobiology remains fairly imprecise, especially the tolerance limits of OAR at hypofractioned schedules. Here, we provide an overview of the radiation tolerance limits of the optic pathway, spinal cord, brain and other central nervous system OARs. Above all, we summarize the basic principles of radiobiology and describe how these can be used to aid decision making for hypofractionated treatments. Basically, we provide radiobiological bases to build models that can be used to safely and effectively extrapolate the doses to be delivered in hypofractioned schedules starting from single fraction clinical data and conventionally fractionated radiotherapy. Although this approach has several limitations, it can provide some practical suggestions and help users to increase confidence with hypofractionated approaches