Monte Carlo simulations to determine counting efficiencies in whole-body counting measurement considering age, gender, and radionuclide biokinetics

A large radiological release can result in internal contamination of the affected population, necessitating assessment through population monitoring methods such as whole-body counting measurements. Accurate calibration of whole-body counting systems is essential for reliable contamination assessment. However, conventional calibration methods using physical phantoms do not account for age- or gender-dependent anatomical variability. Moreover, inhaled or ingested radionuclides present radionuclide-specific biokinetics, resulting in heterogeneous activity distributions within the body that can substantially affect counting efficiency. To address these limitations, this study aims to determine time-dependent counting efficiencies for whole-body counting measurement using Monte Carlo-based calibration methods with age- and gender-specific computational human phantoms. Biodistribution data for radionuclides, including 137 Cs, 134 Cs, and 131 I, following intake are applied to the phantoms to estimate counting efficiencies. The results demonstrate that counting efficiencies are influenced by age- and gender-related anatomical differences and vary with post-intake time because of changes in biodistribution. This effect is particularly pronounced for 131 I, for which heterogeneous distribution results in significant variations in counting efficiency. By incorporating both biokinetic behavior and anatomical characteristics, the proposed approach provides more realistic estimates of counting efficiency than conventional calibration methods, thereby reducing systematic errors and supporting more accurate internal contamination assessments during radiological emergencies.