How to incorporate the dose-rate effect into evaluation of cancer risk for radiation protection

The current radiation rules, based on epidemiological data obtained from the cancer risk of A-bomb survivors, were formulated in the context of extremely high dose rates. The International Commission for Radiological Protection (ICRP) proposed a dose and dose-rate effectiveness factor (DDREF) of 2 based on two high and low dose-rate categories of A-bomb data for an extremely high dose-rate range (ICRP Publication 60, 1991). From a scientific point of view, this value is valid only for the A-bomb dose-rate range, i.e. an extremely high dose rate, and cannot be applied at an environmental level. Nevertheless, this dose-rate factor is currently widely applied in estimating the cancer risk of low dose and low dose-rate radiation and creates an overestimation of cancer risk from radiation at the environmental level, such as in the Fukushima region following the accident at the nuclear power plant. Overestimation of radiation risk results in unnecessary psychological and economical burden on our society. There is a need to reassess the validity of the currently adopted understanding of the dose-rate effect of radiation on cancer risk.