Figure 1 exemplifies the approach in terms of exploratory computations for solid-cancer and leukemia mortality in Hiroshima (RERF 1994, 1997). A linear-quadratic dependence on dose is used that has the form , where Dγ is the gamma ray organ dose and Dn the neutron organ dose. R corresponds to the limiting relative biologic effectiveness (RBE) of neutrons at low doses and in these computations is assumed to equal 15, which is close to the current radiation weighting factor assumed for fission neutrons. There would be little difference in the results if the quadratic term were taken to be of the form b(Dγ. +Dn)2.

Figure 1. The gray areas give acceptable values (for the 95% confidence level) of parameter combinations aγ and Δ in linear-quadratic fit for all solid-cancer and all leukemia deaths in Hiroshima. ag: linear coefficient, that is, excess relative risk (ERR) per gray at low doses; Δ: 'crossover dose,' which equals ratio of linear and quadratic coefficient in the dose dependence for γ-rays. Diagrams are given for an assumed maximal RBE of neutrons R = 15. Data on solid cancers fit best to large values of Δ, that is, to linear dependence; but Δ = 1Gy cannot be rejected at the 95% confidence level, and this value corresponds to the DDREF = 2 assumed by ICRP. For leukemias, best estimate is obtained for Δ = 0.7 Gy, and this is fully consistent with DDREF =2.

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