Are cancer risks associated with exposures to ionising radiation from internal emitters greater than those in the Japanese A-bomb survivors?

Mark P. Little, Per Hall, Monty W. Charles

After ingestion or inhalation of radionuclides, internal organs of the human body will be exposed to ionising radiation. Current risk estimates of radiation-associated cancer from internal emitters are largely based on extrapolation of risk from high-dose externally exposed groups. Concerns have been expressed that extrapolated risk estimates from internal emitters are greatly underestimated, by factors of ten or more, thus implying a severe underestimation of the true risks. Therefore, data on cancer mortality and incidence in a number of groups who received exposure predominantly from internal emitters are examined and excess relative risks per Sv are compared with comparable (age at exposure, time since exposure, gender) matched subsets of the Japanese atomic bomb survivor cohort. Risks are examined separately for low LET and high LET internal emitters. There are eight studies informative for the effects of internal low LET radiation exposure and 12 studies informative for the effects of internal high LET radiation. For 11 of the 20 cancer endpoints (subgroups of particular study cohorts) examined in the low LET internal emitter studies, the best estimate of the excess relative risk is greater than the corresponding estimate in the Japanese atomic bomb survivors and for the other nine it is less. For four of these 20 studies, the relative risk is significantly (2-sided P < 0.05) different from that in the Japanese atomic bomb survivors, in three cases greater than the atomic bomb survivor relative risk and in one case less. Considering only those six low LET studies/endpoints with 100 or more deaths or cases, for four out of six studies/endpoints the internal emitter risk is greater than that in the Japanese atomic bomb survivors. For seven of the 24 cancer endpoints examined in the high LET internal emitter studies the best estimate of the ERR in the internal emitter study is greater than the corresponding estimate in the Japanese atomic bomb survivors and for the other 17 it is less. For six studies, the relative risk is significantly (2-sided P < 0.05) different from that in the Japanese atomic bomb survivors, in one case greater than the atomic bomb survivor relative risk and in five cases less. Considering only those eight high LET studies/endpoints with 100 or more deaths or cases, for five out of eight studies/endpoints the internal emitter risk is greater than that in the Japanese atomic bomb survivors. These results suggest that excess relative risks in the internal emitter studies do not appreciably differ from those in the Japanese atomic bomb survivors. However, there are substantial uncertainties in estimates of risks in the internal emitter studies, particularly in relation to lung cancer associated with radon daughter (alpha particle) exposure, so a measure of caution should be exercised in these conclusions.