The correction for the gamma‐ray component in neutron therapy

Neutron beams for therapy always contain some γ ray contamination that varies with depth and with distance from the beam axis. The problem therefore arises how the varying γ ray contribution should be accounted for in dose specification. Not infrequently a “total effective dose” D E is quoted that is equal to the neutron dose plus the γ ray dose divided by a constant weight factor τ. On general biophysical considerations this appears to be not a valid approach since it must be assumed that τ decreases with increasing dose. The nature and the magnitude of this dose dependence is derived in the present article. Application of the results to actual doses per fraction and to factual γ ray to neutron ratios demonstrates that the dose dependence of τ has, in fact, very minor influence on the numerical values of D E . Utilization of a constant value τ is therefore satisfactory in practice.