Reference radiation selection is confirmed as a significant source of relative biological effectiveness variation for neutrons

The linear slope coefficient of the linear-quadratic dose-response curve, used to evaluate RBEM, was found to be significantly different for ⁶⁰Co (0.0268 ± 0.0075 Gy⁻¹) compared to ¹³⁷Cs (0.0730 ± 0.0135, P < 0.01) and 250 kVp X-ray (0.1063 ± 0.0248, P < 0.01). Applying this finding to our previous thermal and fast neutron DCA evaluations, the RBEM varied by a factor of 2.7 for ⁶⁰Co versus ¹³⁷Cs, and by a factor of four for ⁶⁰Co versus 250 kVp X-ray. A review of prior reported neutron RBEM literature affirmed the finding that reference radiation selection can influence RBEM magnitude. The selection of the reference radiation has implications for RBE evaluations of neutrons and other radiation qualities, as these RBE values underpin the radiation weighting factor, wR, which informs radiation protection measures both terrestrially and in space. These experiments and reanalysis reconfirm and strongly demonstrate that reference radiation selection is a significant determinant of RBE variability, especially as applied to neutrons.