To exploit the 5 'R' of radiobiology and unleash the 3 'E' of immunoediting: 'RE'-inventing the radiotherapy-immunotherapy combination

How radiotherapy counters immune evasion In conventional radiotherapy (RT), the relative biologic effectiveness of radiation is influenced by radiobiological determinants, the so-called '5Rs': Repair, Repopulation, Redistribution, Reoxygenation, and Radiosensitivity.1 A linear-quadratic model prevails to describe the radiation response of the tumor, in which the α/β ratio is used to characterize the sensitivity of a particular tissue type to fractionation. Fractionation serves to decrease acute, and especially late, toxicity to surrounding normal tissue exposed to RT. Commonly, curative RT is delivered in daily doses of 1.8–2.2 Gy for 5–8 weeks, whereas hypofractionation is defined as a delivery of greater than 2.2 Gy per fraction. With sophisticated advances in RT technologies, delivering higher doses of RT per fraction [i.e. increased biologically effective dose (BED)] in a shorter timeframe appears a safe option. Indeed, increased BED could be achieved with larger fraction sizes relative to conventionally fractionated RT. While early radiobiological studies had found that the major mechanisms of action of radiation were related to DNA damage and subsequent cell death of dividing cells, novel insights on radiation effects have uncovered the immunomodulatory properties of ionizing radiation.2