Pitfalls of Hyperfractionation: Theoretical considerations of effect of repair time on late radiation damage

The need of radiation therapy in cancer treatment is increasing, however, the dose of radiation is limited by its late effects on normal tissues. Hyperfractionated radiotherapy was applied to patients with various kinds of carcinomas in the 1980s, expecting to improve tumor control rates. Some reports showed that the total radiation dose could be increased without increasing late radiation damage by using hyperfractionated radiotherapy. We calculated the early and late biologically effective doses (BED)for conventional and hyperfractionated radiotherapy to predict the early and late radiation damage based on the linear-quadratic model (L-Q model). Setting the values of the α/β ratio and t_1/2 (half time repaire) in the L-Q model is still controversial, and few reports deal with t_1/2. Assuming t_1/2 = 0.5hour, the total radiation dose can be up to 71Gy by hyperfractionated schedule (1.2Gy/fr. 2 times/day) without increasing the late BED of conventional schedule (2Gy/day, total 60Gy). However, the late BED should be increased in tissues with a larger t_1/2 value, even if applying the same hyperfractionated schedule. Thus, it is not recommended to apply the same hyperfractionated protocol to all organs.