Inverse dose rate effects for neutrons: general features and biophysical consequences

Evidence has accumulated to suggest that when a neutron dose is protracted, its effectiveness is enhanced for transformational endpoints. A pattern has emerged as to the dependence of the effect on dose, dose rate and radiation type. An explanation of the effect is that cells in part of their cycle are more sensitive to radiation than in the rest of the cycle. An acute high LET exposure of cycling cells will thus results in some fraction of the sensitive cells receiving large depositions of energy -greater than required for the effect. In a protracted exposure a larger proportion of sensitive cells will be exposed to smaller numbers of energy depositions - though still adequate to induce the effect. The model produces results consistent with all available data. Upper limit predictions for the field of radiation protection are made: at the non-occupational annual effective dose limit the enhancement would be insignificant; at the annual occupational limit an enhancement of ≤1.5 might be expected. For radon, environmentally, there should be little dose rate effect; for miners on whom risk estimates are primarily based, there might be an effect, resulting in misleading extrapolations to environnmental risk. (author)