Radiobiological considerations for the effect by dose-rate change on radiotherapy; summary of reports in recent five years

For many years, a wide range of dose-rates have been used for brachytherapy, but there has been a move towards high dose-rate (HDR) treatment in Japan. It has been generally believed that late effects should be worse for HDR compared with low dose-rate (LDR) protocol. We report the changing situations of HDR treatment with the clinical and radiobiological considerations. When the dose of HDR treatment is matched to produce early effects equal to those of LDR treatment, if the dose responsible for late effects is less than the tumor dose by a factor more than the ratio of α/β for late effects to α/β for early effects (this ratio can be assumed about 1/2 from experimental data), then the HDR late effects will not be worse than those at LDR. This implies that if the normal tissue dose were about half that of the tumor dose, HDR would produce late effects similar to those of LDR.