The 4Rs in radiation therapy

Abstract

It has been realized that the 4Rs (repair, repopulation, redistribution, and reoxygenation) would affect the result of cell irradiation, and thus radiation treatment. The 4Rs each occurs at different dose rates, usually very low dose rates. Depending on the dose rate used for treatment, the corresponding R should be included in the linear－quadratic equation (LQ) and biological effective dose (BED) calculation. For low dose rate brachytherapy(LDR) especially permanent implant, all the 4Rs should be included in LQ for BED calculation. The 4Rs, especially repair and repopulation, play a critical role in dose fractionation. Various dose fractionation schemes such as hyperfractionation and hypofractionation are determined in consideration of the 4Rs. Stereotactic radiation therapy uses hypofractionation with high fractional doses and combine with high accuracy target localization techniques to achieve high local control rates compared to conventional dose fractionation schemes. The 4Rs have been taken into account for LDR and permanent implant. Recently, LQ for permanent implant brachytherapy has been modified to include all the 4Rs for gynecological malignancy 131Cs permanent implants. Including the 4Rs in radiation therapy has significantly improved the effectiveness and efficiency of radiation therapy for cancer treatment. Key words: Repair; Repopulation; Redistribution; Reoxygenation; Linear quadratic equation