The impact of possible modifications to the DS86 dosimetry on neutron risk andrelative biological effectiveness

Abstract

The current DS86 dosimetry system for the Japanese bomb survivors indicatesthat neutron doses were so low that they prevent the direct derivation of anyuseful estimates of neutron risk. However, the large body of thermal neutronactivation measurements carried out over many years in Hiroshima and Nagasakiappear to indicate that current DS86 neutron doses may have been significantlyunderestimated in Hiroshima. An earlier companion paper has provided an updateof neutron activation measurements. While a large body of data appears tosupport a significant increase, there is ongoing debate and review regarding itsvalidity. However, as yet, there are no detailed, peer-reviewed, publishedrefutations of the neutron activation data which appear to support an increase inneutron doses. In this paper, we consider the impact of possible futurerevisions in the DS86 dosimetry on radiation risk estimates. We considerthe extreme range of possibilities from maintaining the existing DS86values, to changes in neutron doses in accord with the majority of existingneutron activation data. We have used the latest cancer incidence dataand cancer mortality data for the A-bomb survivors, and neutron doseshave been modified using a neutron revision factor (NRF) in line withthe latest thermal neutron activation measurements in Hiroshima. Incontrast to previous analyses, a nonlinear relationship between log(NRF)and slant range has been used which better represents the data beyondslant ranges of ∼1 km. The impact on the evaluation of neutron relativebiological effectiveness (RBE) and gamma radiation risk estimates has beenassessed. While DS86 neutron doses are too low to allow any useful directevaluation of neutron risk or neutron RBE, it becomes possible to derive moremeaningful values if neutron doses are increased in Hiroshima in linewith the broad range of thermal neutron activation measurements. Theuncertainties are smallest for the cancer incidence data. The best estimates ofneutron RBE give upper 95% confidence limits of about 6 for all solidtumours for the incidence data and about 28 for the mortality data. Theuncertainties in neutron RBE for leukaemia incidence are larger, andestimation at doses below about 0.1 Gy is not possible. There is no significantchange in the excess relative risk for gamma radiation for all solid tumourstaken together, compared with the current DS86 dosimetry. The resultspreclude neutron RBE values significantly greater than the current ICRPradiation weighting factors, which range between 5 and 20, depending onenergy. Whether or not the Japanese bomb survivors can indeed form thebasis for useful, directly determined neutron risks clearly depends on theveracity of existing neutron activation data. This is currently the subjectof careful international scrutiny and the outcome is eagerly awaited.