Establishing a representative dose rate constant for ambient dose equivalent of Iodine-131: a first step toward...

This work establishes a representative ambient dose equivalent rate constant (Γ_H*(10)) for I-131 as a first step toward patient body-attenuation studies. Monte Carlo simulations with the EGSnrc code were used to evaluate Γ_H(*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 ⩽5%. Our results were consistent across spectra and geometries, yielding a representative mean Γ_H*(10)value of 63.6 ± 0.1µSv m² GBq^-¹ h^-¹. Comparison with published Γ_H*(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 (Γ_H*(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 Γ_H*(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.