Figure 4. Absolute risk coefficients for mortality obtained from solid-cancer data in Hiroshima in terms of a tentatively modified DS86 and the linear-quadratic relation of equation 1. Light and heavily shaded areas show 95% confidence and standard error regions of combinations of photon and neutron risk estimates. The calculations illustrate that the estimates of risk coefficients for protons and neutrons are inversely related; high RBE of neutrons cannot be excluded on the basis of the data but imply reduced risk estimates for photons.

The results show that a modified dosimetry could exclude risk coefficients for neutrons that are substantially larger than now assumed. The broken line corresponds to the results shown in figure 2. See legend to figure 2 with regard to the difference between the parameter R and the radiation weighting factor wR, that is used in the definition of the effective dose of neutrons (in Sv).

With the dosimetry as modified here, roughly the same risk estimates are obtained for both photons and neutrons if a low neutron RBE, between 10 and 20, is assumed. Nevertheless, there are potentially major implications of any modified neutron dosimetry. The inverse relation between the risk coefficients for gamma rays and neutrons shows that very high values of neutron RBE would be consistent with the data. But they would correspond primarily to a largely reduced linear component in the dependence of dose due to photons, rather than an enhanced effectiveness of the neutrons. In fact, with this tentative modification, the solid-cancer data from Hiroshima would be consistent with a vanishing linear component for photons but a substantial quadratic component, that is, an effectiveness of gamma rays at larger doses in line with the Nagasaki observations. The maximal risk coefficient for neutrons, however, would differ little from the one now assumed.

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