The apparent increase in the {beta}-parameter of the linear quadratic model with increased linear energy transfer during fast neutron irradiation.

The issue of whether the beta-parameter of the linear quadratic model changes with linear energy transfer (LET) remains controversial. Retrospective analysis of UK fast neutron experimental data using human cell lines at Clatterbridge shows that the beta-parameter of the linear quadratic model probably does increase with LET during neutron irradiation. For cells without a deficiency in DNA damage repair and for experiments in which beta-parameter estimates were considered to be unreliably low, a provisional relationship of beta(H) = 1.82 beta(L) was found (where the suffixes refer to high and low LET exposures, respectively). This implies that radicalbeta increases by around 1.35 in the specific case of 62.5 MeV neutrons relative to 4 MeV X-rays. Increments in the beta-parameter with LET influence the relative biological effect (RBE), especially at high doses per fraction. Large fractions are being used in experimental carbon ion therapy, in which broadly similar RBE values to fast neutrons are found. These interesting findings after fast neutron exposure need to be studied further for applications in charged particle beam therapy using light ions, which is presently undergoing a worldwide expansion.