Comparisons of lung tumour mortality risk in the Japanese A-bomb survivors and in the Colorado Plateau uranium miners: support for the ICRP lung model

Abstract

Purposes : To estimate the ratio of risks for exposure to radon progeny relative to low-LET radiation based on human lung cancer data, taking account of possible time and age variations in radiation-induced lung cancer risk. Materials and methods : Fitting two sorts of time- and age-adjusted relative risk models to a case-control dataset nested within the Colorado Plateau uranium miner cohort and to the Japanese atomic (A)-bomb survivor mortality data. Results : If all A-bomb survivors are compared with the Colorado data, there are statistically significant (two-sided p < 0.05) differences between the two datasets in the pattern of the variation of relative risk with time after exposure, age at exposure and attained age. The excess relative risk decreases much faster with time, age at exposure and attained age in the Colorado uranium miners than in the Japanese A-bomb survivors. If only male A-bomb survivors are compared with the Colorado data, there are no longer statistically significant differences between the two datasets in the pattern of variation of relative risk with time after exposure, age at exposure or attained age. There are no statistically significant differences between the male and female A-bomb survivors in the speed of reduction of relative risk with time after exposure, age at exposure or attained age, although there are indications of rather faster reduction of relative risk with time and age among male survivors than among female survivors. The implicit risk conversion factor for exposure to radon progeny relative to the A-bomb radiation in the male survivors is 1.8 ×10-2 Sv WLM-1 (95% CI 6.1 ×10-3, 1.1 ×10-1) using a model with exponential adjustments for the effects of radiation for time since exposure and age at exposure, and 1.9 ×10-2 Sv WLM-1 (95% CI 6.2 ×10-3, 1.6 ×10-1) using a model with adjustments for the effects of radiation proportional to powers of time since exposure and attained age. Estimates of the risk conversion factor calculated using variant assumptions as to the definition of lung cancer in the Colorado data, or by excluding miners for whom exposure estimates may be less reliable, are very similar. The absence of information on cigarette smoking in the Japanese A-bomb survivors, and the possibility that this may confound the time trends in radiation-induced lung cancer risk in that cohort, imply that these findings should be interpreted with caution. Conclusions : There are no statistically significant differences between the male A-bomb survivors data and the Colorado miner data in the pattern of variation of relative risk with time after exposure and age at exposure. The risk conversion factor is very close to the value suggested by the latest ICRP lung model, albeit with substantial uncertainties.