Fetal specific absorbed fractions of ICRP pregnant-female mesh phantoms: methodology and dosimetric trends.

OBJECTIVE: Recently, the ICRP developed the pregnant-female mesh-type reference computational phantoms (pfMRCPs) to assess radiation doses to the fetus. To support the generation of reference internal fetal dose coefficients, a fetal specific absorbed fraction (SAF) dataset for photons, electrons, neutrons, and alpha particles will be provided in a forthcoming ICRP publication. Prior to their release, in this study, the computed SAFs were validated and their source- and age-dependent dosimetric trends were systematically analyzed for both maternal and fetal source regions. APPROACH: Photon and electron SAFs were computed using the Geant4 Monte Carlo code, while neutron SAFs were produced using PHITS, following the methodologies adopted in ICRP Publication 155 for pediatric SAF calculations. Alpha SAFs were defined analytically based on an assumption of complete local energy deposition due to the limited range of alpha particles. MAIN RESULTS: Cross-code validation was performed for photon and neutron SAFs using PHITS and Geant4, demonstrating general agreement across all compared cases. The results demonstrate clear dependencies of SAFs on fetal mass, source-target geometry, and fetal age. For radiations emitted from maternal source regions, SAFs are generally higher for younger fetuses when the source regions are located in close proximity to the fetus, whereas SAFs for older fetuses become larger as the source-target geometry changes with increasing fetal growth. For fetal source regions, SAFs consistently decrease with increasing fetal age due to the increasing mass of fetal target regions with age. SIGNIFICANCE: The established dataset provides physically consistent fetal SAFs across multiple radiation types and will be used to support the development of standardized fetal dose coefficients in future ICRP publications.