Establishing a representative dose rate constant for ambient dose equivalent of Iodine-131: A first step toward patient body attenuation studies

This work establishes a representative ambient dose equivalent rate constant (ΓH(*10)) for I-131 as a first step toward patient body-attenuation studies. Monte Carlo simulations with the EGSnrc code were used to evaluate ΓH(*10) for point sources and the ICRU sphere, employing two photon-spectra datasets (RADAR and ICRP 107) and two scoring methods (fluence and air-kerma scoring). Simulations considered only photons above 20 keV, omitted electron transport, and achieved statistical uncertainties ≤3%. Our results were consistent across spectra and geometries, yielding a representative mean ΓH(*10) value of 63.6 ± 0.2% µSv.m².GBq⁻¹.h⁻¹. Comparison with published ΓH(*10) values for I-131 showed differences ranging from -17% to +22% and close agreement with recent values and the literature average value. An adult male voxel phantom with biodistributed I-131 was then simulated to obtain the body ambient dose equivalent rate constant (ΓH*(10),body) and a corresponding body attenuation factor (BAF) of 0.729 ± 0.002 at 1 m was estimated. As complementary validation, F-18 (511 keV photons) with FDG biodistribution produced a BAF of 0.628 ± 0.002 in the model used in this work. This value agrees with the literature. The proposed representative ΓH(*10) reduces uncertainty in BAF estimates and can improve shielding calculations and dose assessments for caregivers and family members of patients undergoing I-131 therapy.