From Point Sources to Phantoms: Refining Dose Rate Models in Nuclear Medicine with Tc-99m, F-18, I-131 and Lu- 177

Healthcare workers in nuclear medicine (NM) are often exposed to external radiation from patients who have been administered radiopharmaceuticals, particularly during procedures requiring close contact. Traditional dose rate estimates commonly assume patients to be unattenuated point sources and apply inverse square law calculations, which tend to overestimate exposures at clinical distances. This study presents an experimental approach to quantify dose rates more accurately from NM patients using point, line, and phantom source models of Tc-99m, F-18, I-131, and Lu-177.Measurements were performed with calibrated ionisation chambers under controlled low-background conditions. Radioisotope activities were selected to achieve detectable dose rates while minimising staff exposure. Point and line source measurements provided baseline external exposure data across 10–100 cm. Phantom models, including a homogeneous water-filled NEMA body phantom and a PMMA neck phantom, were used to replicate clinical patient geometries. Dose rates were decay-corrected, normalised to activity, and fitted with biexponential models to generate exposure curves. Gamma factors (GFs) were evaluated at 0.3 m and 1.0 m and compared with internationally recognised literature values. Self-absorption factors (SFs) were derived by comparing phantom and point source curves across clinical distances.Results showed that measured GFs agreed within 10% of published values for most isotopes, with discrepancies largely explained by methodological differences. SFs varied by isotope and geometry, with lateral phantom orientations exhibiting the highest attenuation. For F-18 and Tc-99m, phantom-derived SFs were consistent with patient data and Monte Carlo simulations. I-131 neck phantom results demonstrated orientation-dependent shielding, with posterior measurements showing the greatest attenuation.This work provides a validated dataset and modelling framework for dose rate estimation in NM. These results support evidence-based clinical decisions regarding shielding, procedure scheduling, and occupational safety, reducing reliance on overly conservative assumptions and enabling accurate, practical radiation protection strategies.