Estimates of neutron relative biological effectiveness derived from the Japanese atomic bomb survivors

Abstract

Purpose: To investigate neutron relative biological effectiveness. Materials and methods: The latest Japanese atomic bomb survivor cancer incidence and mortality datasets with the current (DS86) dosimetry system are analysed using generalized relative risk models and generalized absolute risk models, both with and without recently indicated adjustments to the Hiroshima DS86 neutron dose estimates. Results: Without adjustments to the Hiroshima neutron doses, the best estimate of neutron relative biological effectiveness for all tumours in the incidence data is 63.3 (95% CI 0-275.3) when a generalized relative risk model is used; when a generalized absolute risk model is used in the incidence data the best estimate is 53.5 (95% CI 0-201.0); when a generalized relative risk model is used in the mortality data, the best estimate is 287.7 (95% CI 38.0- 103). When likely adjustments are made to the Hiroshima neutron doses the best estimate of neutron relative biological effectiveness in the incidence data using a generalized relative risk model is 15.1 (95% CI 0-51.4); when a generalized absolute risk model is used in the incidence data the best estimate is 9.0 (95% CI 0-32.9); when a generalized relative risk model is used in the mortality data the best estimate is 55.1 (95% CI 9.5-280.3). Although there are no significant differences between groupings of the solid tumour sites in their estimated neutron relative biological effectiveness, there are indications that the neutron relative biological effectiveness of solid tumours is lower than that of leukaemia, whether or not adjustments are made to the Hiroshima neutron dose estimates. Uncertainties in the likely adjustments to the DS86 Hiroshima neutron and gamma dose estimates as well as uncertainties in the modelling of excess risk in the two cities (Hiroshima and Nagasaki) imply that these findings should be treated with caution. Conclusions: Likely adjustments to the Hiroshima neutron dose estimates imply a substantial increase in information on neutron relative biological effectiveness. Whether or not adjustments are made to the Hiroshima neutron doses, there are indications of inconsistency between the estimates of neutron relative biological effectiveness for solid tumours and leukaemia. Dosimetric and modelling uncertainties mean that these findings should be treated with caution.