ANALYSIS OF THE INCIDENCE OF SOLID CANCER AMONG ATOMIC-BOMB SURVIVORSUSING A 2-STAGE MODEL OF CARCINOGENESIS

A two-stage stochastic model for carcinogenesis was used to analyze th e incidence of cancer of the lung, stomach and colon in the cohort of atomic bomb survivors. We fitted the model assuming that acute exposur e to radiation results in the creation of initiated cells that are add ed to the pool of spontaneously initiated cells. In the cancers analyz ed, with the exception of lung cancer in females, we found no evidence that radiation-induced initiation was dependent upon age at exposure. In contrast, we found that spontaneous initiation was dependent upon age at exposure in the cancers analyzed except stomach cancer among ma les, Because exposure to radiation in this cohort occurred at the same time for all members of the cohort, age at exposure is exactly correl ated with birth cohort, and the dependence of spontaneous initiation o n age at exposure is a reflection of the cohort effects seen in these cancers in Japan. Even without a dependence of radiation-induced initi ation on age at exposure, the two-stage model can explain the temporal behavior of the excess relative risk with age at exposure and time si nce exposure, In particular, the model predicts that excess relative r isk is highest among those exposed as children. Moreover, since radiat ion-induced initiation is not higher among those exposed as children, the excess relative risk in this group is not due to an inherently hig her sensitivity to radiation, Our biologically based approach provides another perspective on the temporal behavior of risk after acute expo sure to ionizing radiation. (C) 1997 by Radiation Research Society.