VARIATIONS WITH TIME AND AGE IN THE RISKS OF SOLID CANCER INCIDENCE AFTER RADIATION EXPOSURE IN CHILDHOOD

The Japanese atomic bomb survivor incidence data set and data on five other groups exposed to ionizing radiation in childhood are analysed a nd evidence found for a reduction in the radiation-induced relative ri sk of cancers other than leukaemia with increasing time since exposure . Overall, reductions of 5.7-6.1 per cent per year of time since expos ure are indicated, depending on the time at which the reduction is pre sumed to start, and all the reductions are statistically significant a t the 5 per cent level, There is no significant heterogeneity in the s peed of the reductions in relative risk with time by cohort, by cancer type, sex, or age at exposure group. There is a significant reduction of relative risk with increasing age at exposure, but adjustment for age at exposure does not markedly affect the time trends of relative r isk. For all of the groups considered, there is a statistically signif icant increase in the excess absolute risk with increasing time since exposure. However, by contrast with the relative homogeneity of the ti me trends of relative risk, there is statistically significant heterog eneity by cancer type within the Japanese cohort (P = 0.05) and betwee n the cohorts (P < 0.0001) in the speed of increase of the excess abso lute risk with time since exposure. (C) 1998 National Radiological Pro tection Board.