Occupational exposure during radiopharmaceutical production and handling depends on the radionuclide, activity, working distance, task duration, and shielding. Although personal dosimetry remains essential, it cannot independently resolve the contribution of each operational variable. This study developed a reproducible Monte Carlo framework to compare local absorbed dose under controlled combinations of distance and shielding and to derive model-based exposure-time indicators. A point gamma source, an air-filled world, an optional lead or tungsten barrier, and a cm³ water detector were implemented in OpenGATE/GATE on Geant4 and automated in Python. F-18 (511 keV), Tc-99m (140 keV), and I-131 (364 keV) were evaluated at distances of 0.5, 1.0, and 2.0 m under seven shielding conditions: no shielding and 1–3 cm of either lead or tungsten. The resulting 63 physical scenarios were simulated in five or eight independent runs using –  photons per run. The primary endpoint was the mean absorbed dose within a central -voxel region of interest, with cross-scenario comparisons normalized to simulated photons. Statistical usability was classified according to the coefficient of variation (CV), and an explicitly theoretical time to a 20-mGy local absorbed-dose benchmark was calculated for F-18. Thirty-nine scenarios met the CV ≤ 5% criterion, one was classified for cautious use (5% < CV ≤ 10%), and 23 were considered unusable. All 21 F-18 scenarios were acceptable. Tc-99m was acceptable only without shielding, whereas 15 I-131 scenarios were acceptable and one was conditionally usable. Under unshielded conditions, absorbed dose followed an inverse-distance power law, with exponents of 2.004 for F-18, 2.016 for Tc-99m, and 2.012 for I-131 (log–log ). For F-18, mean attenuation relative to the unshielded condition was 80.94%, 96.36%, and 99.31% with 1, 2, and 3 cm of lead, respectively, compared with 91.16%, 99.20%, and 99.929% for the corresponding tungsten thicknesses. At 200 GBq, the model-derived time to the benchmark for unshielded F-18 increased from 16.3 min at 0.5 m to 1.09 h at 1.0 m and 4.36 h at 2.0 m. At 0.5 m, it increased to 1.42 h with 1 cm of lead and 3.05 h with 1 cm of tungsten. The framework reproduced the expected inverse-square distance response and quantified internally consistent shielding effects for F-18, providing a transparent platform for comparative optimization and simulation planning. However, the calculated times should not be interpreted as occupational clearance values because the endpoint represents local absorbed dose in a simplified water detector rather than personal or whole-body effective dose. Experimental validation, realistic source and worker geometries, complete emission spectra, and verified angular scaling remain necessary before operational application.

Keywords

Monte Carlo simulation; OpenGATE; Geant4; occupational radiation protection; radiopharmaceuticals; shielding; F-18; computational dosimetry