Organ Dose Conversion Factors for Murine Galactic Cosmic Ray Irradiation

Galactic cosmic rays (GCR) are a principal source of ionizing radiation exposure for astronauts during deep space missions. Given the ambition to expand manned space exploration to distant destinations like Mars, it is essential to accurately predict the radiation doses astronauts are likely to encounter and the consequent biological impacts. Accurate dose predictions are important for operational radiation safety, ensuring risk assessments and protective measures are properly calibrated to challenges of deep space travel. The GCRsim facility at the NASA Space Radiation Laboratory enables small animal radiobiology studies of GCR exposure, offering a controlled setting to mimic the complex radiation conditions in deep space. This manuscript introduces Dose Conversion Factors (DCFs) enabling rigorous absorbed dose calculations for mice irradiated at the GCRsim. A formalism was introduced for calculating organ-level and voxel-level radiation dose to a representative mouse phantom, based on DCFs quantifying absorbed dose per unit fluence of different GCRsim species for various irradiation orientations. The PHITS Monte Carlo code was employed to compute the DCFs in units of Gy m^2 ion^-1. A library of murine DCFs was derived using the PHITS Monte Carlo code for six irradiation orientations: right-left, anterior-posterior, superior-inferior, and their opposed variations. Absorbed doses to the murine total body were calculated and compared with ion chamber measurements, which agreed within 10%. A library of dose conversion factors for mouse irradiation at GCRsim was developed and validated against measurements. These DCFs account for organ-specific variations in radiation dose from different GCR species, enabling improved assessments of potential radiogenic effects and enhancing astronaut safety for future deep space missions.