Choice of model and uncertainties of the gamma-ray and neutron dosimetry in relation to the chromosome aberrations data in Hiroshima and Nagasaki

Chromosome data pertaining to blood samples from 1,703 survivors of the Hiroshima and Nagasaki A-bombs, were utilized and different models for chromosome aberration dose response investigated. Models applied included those linear or linear-quadratic in equivalent dose. Models in which neutron and gamma doses were treated separately (LQ-L model) were also used, which included either the use of a low-dose limiting value for the relative biological effectiveness (RBE) of neutrons of R(0)=70+/-10 or an RBE value of R(1)=15+/-5 at 1 Gy. The use of R(1) incorporates the assumption that it is much better known than R(0), with much less associated uncertainty. In addition, error-reducing transformations were included which were found to result in a 50% reduction of the standard error associated with one of the model fit parameters which is associated with the proportion of cells with at least one aberration, at 1 Gy gamma dose. Several justifiable modifications to the DS86 doses according to recent nuclear retrospective dosimetry measurements were also investigated. Gamma-dose modifications were based on published thermoluminescence measurements of quartz samples from Hiroshima and on a tentative reduction for Nagasaki factory worker candidates by a factor of 0.6. Neutron doses in Hiroshima were modified to become consistent with recent fast neutron activation data based on copper samples. The applied dose modifications result in an increase in non-linearity of the dose-response curve for Hiroshima, and a corresponding decrease in that for Nagasaki, an effect found to be most pronounced for the LQ-L models investigated. As a result the difference in the dose-response curves observed for both cities based on DS86 doses, is somewhat reduced but cannot be entirely explained by the dose modifications applied. The extent to which the neutrons contribute to chromosome aberration induction in Hiroshima depends significantly on the model used. The LQ-L model including an R(1) value of 15 at 1 Gy which is recommended here, would predict between 10% and 20% of the observed chromosome aberrations to be due to neutrons, at all doses. Because of the good agreement between DS86 predictions and the results of retrospective gamma and neutron dosimetry, the modifications applied here to DS86 doses are relatively small. Consequently, the choices of model and RBE values were found to be the major factors dominating the interpretation of the chromosome data for Hiroshima and Nagasaki, with the dose modifications resulting in a smaller influence.