It is an internationally accepted assumption that genetic or tumor-causing risk of exposure to radiation can never be zero at any low dose. Essential data supporting this assumption are reviewed and used for discussion on no increase in genetic diseases in F1 offspring of A-bomb survivors and on tumor incidence data of A-bomb survivors. A working hypothesis is proposed that radiation initiates carcinogenesis followed by “spontaneous” tumor promotion or that spontaneous tumor-initiation is followed by radiation-induced promotion, which occurs only after exposure with doses above threshold values. Supporting evidence for this assumption is given by reviewing experimental data of mice. The data clearly show that initiation is caused by somatic mutation, an indication that there should be no threshold effect in radiation-induced tumor-initiation, i. e., in some types of radiation-caused tumors. However, in other types of radiation-caused tumors that occur only when radiation works as tumorpromoter, there could be threshold effects.
