New dosimetry system (DS86) for Hiroshima-Nagasaki A-bomb survivors and its effects on risk estimates

Radiation Effects Research Foundation (RERF) at Hiroshima has recently published reports on the ongoing revision of the dose estimates to A-bomb survivors and the consequent changing trends in the risk estimates. A brief summary is presented here on the physics of the new dosimetry system calle d DS86 and the revised risk estimates from the presently available RERF reports. The important findings are: (i) The tissue free-in-air gamma doses are higher for Hiroshima and lower for Nagasaki compared to the earlier estimates; (ii) The free-in-air kerma values for neutrons are lower for both the cities and the reduction is nearly by an order of magnitude for Hiroshima; (iii) The excess cancer mortalities per 104 PYGy (combined shielded kerma) are increased from 1.38 to 1.61 fold for the various cancer types in the revised estimates; (iv) The difference in cancer mortality risk rates between Hiroshima and Nagasaki is smaller and no longer statistically significant; (v) Assumption of various values of RBE for the neutrons do not change significantly the risk estimates under the new system; (vi) The excess cancer mortality per 104 PYSv attributable to γ-rays (RBE=1) turns out to be 4 per cent lower for leukaemia and 27 per cent lower for all other cancer together, as compared to T65D estimates at RBE=1; (vii) At RBE=10 for neutrons and assuming the additive risk model for leukaemia and the multiplicative risk model for other cancers, the revised risk factor for the general Japanese population works out to be 11.5 per cent Sv-1 as against the ICRP-26 risk factor of 1.25 per cent Sv-1. (author). 6 refs., 8 figs