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 RBE_M, was found to be significantly different for ⁶⁰Co (0.0268 ± 0.0075 Gy^-1) compared to ¹³⁷Cs (0.0730 ± 0.0135, <i>P</i> < 0.01) and 250 kVp X-ray (0.1063 ± 0.0248, <i>P</i> < 0.01). Applying this finding to our previous thermal and fast neutron DCA evaluations, the RBE_M 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 RBE_M literature affirmed the finding that reference radiation selection can influence RBE_M 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, <i>w</i>_R, 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.