The biology rationale for radiotherapy in the treatment of malignant disease is based on repair, repopulation, reoxygenation and cell cycle redistribution. Various aspects of the roles of the 4R' are discussed, including in determining the sensitivity of tumors and normal tissue tolerances. An improvement in therapeutic ratio may derive from reducing the dose per fraction and minimizing the overall treatmemt duration. Some methods have developed to predict the response of normal and tumor tissues before radiotherapy. The parameters of cell survival at 2Gy (SF2) was correlated with clinical outcome. There is reasons to suppose that the pretreatment tumor LI and Tpot mat be good predictors for tumor repopulation kinetics. This review also discussed the rationale for the use of LQ model in fractionated radiotherapy.
