ABSTRACT
From a radiobiological perspective, the rationale for hypofractionation does not appear to follow the basic tenets of radiobiology that are traditionally invoked to explain the tumor responses achieved by conventional fractionation schemes (Withers 1975). During the 20th century, the radiation treatment schedule for most tumors evolved into a regimen of multiple, small fractions of irradiation. The governing scientific principles involved in this approach are widely accepted to be the classic four “Rs” of radiobiology (Withers 1975): • Repair. Multiple small fractions of radiation allow for the repair of sublethal DNA damage in slowly
dividing normal tissues, but less repair occurs in more rapidly cycling tumor cells (Time scale: hours-days).
