Dosimetric comparability validation of small animal photon and neutron irradiations

Crew members on missions beyond low-earth orbit receive considerable radiation doses, but the effects and relative biological effectiveness of many relevant types of irradiation, including neutrons with energies of hundreds of MeV, largely remain under-investigated. Such small animal irradiations can only be compared to respective photon irradiations if comparable doses can simultaneously be delivered to a variety of organs during both irradiations, despite the different underlying dose deposition patterns. To evaluate the dosimetric comparability of upcoming small animal neutron and photon irradiations, experimental depth-dose measurements were performed at the TRIUMF neutron facility and the British Columbia Cancer Research Centre, using a neutron beam with energies of up to 450 MeV and a Cs-137 irradiator. The MOBY digital mouse phantom was used to perform Monte Carlo simulations of neutron and photon animal irradiations. Evaluated metrics included the ratio between the dose delivered to a variety of different organs (including lungs, brain, and heart) during neutron and photon irradiation. A sensitivity analysis including a variety of animal parameters (tissue elemental compositions and mass densities, animal size, and animal orientation) was performed, and the statistical significance (p< 0.05) of the dosimetric impact of uncertainties in simulation parameters was analyzed. During nominal simulations, differences in organ doses during neutron and photon irradiation were <9% in all organs except the lungs (13%), in agreement with the dosimetric measurements performed, which exhibited differences of up to ≈20% depending on depth. During sensitivity analysis, no investigated source of uncertainty had a statistically significant dosimetric impact. Organ doses during simulated neutron and photon irradiations were found to be comparable for various organs. Investigated sources of uncertainties had no statistically significant impact. These findings are therefore expected to be robust to realistic variations in animal parameters during upcoming small animal irradiations.