Promoting action of radiation in the atomic bomb survivor carcinogenesis data?

The age-time patterns of risk in the atomic bomb survivor data on incidence of solid cancers suggest an action of low-LET radiation not only on the initiating event but also on promotion in a biologically motivated model that allows for both actions. The favored model indicates a decrease of radiation risks with age at exposure due to the initiating effect and with time since exposure due to the promoting effect. These result in a relative risk that depends mostly on attained age for ages at exposure above 20 years. According to the model, a dose of 100 mGy is inducing about the same number of initiating events that occur spontaneously in 1 year. Assuming that several mutations are needed to obtain intermediate cells with growth advantage does not improve the quality of fit. The estimated promoting effect could be explained if the number of intermediate cells increases by 80% at 1 Gy, e.g. due to stimulated cell repopulation.