From point sources to phantoms: refining dose rate models in nuclear medicine with Tc-99m, F-18, I-131 and Lu-177.

Healthcare workers are often exposed to external radiation from patients who have been administered radionuclides for nuclear medicine (NM) studies, particularly during subsequent 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. Radionuclide 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 from the centre of the radioactive source and compared with internationally recognised literature values. Self-absorption factors (SFs) were derived by comparing phantom and point source curves across clinical distances. Measured GFs showed excellent agreement with published data and internationally recognised standards, differing by only 2% for F-18 at both 30 cm (1.84 μSv/h/MBq) and 100 cm (0.161 μSv/h/MBq), and by 1% for I-131 at 30 cm (0.721 μSv/h/MBq). The average SF for F-18 (0.71 ± 0.04), representing patient self-attenuation for an unknown body orientation, was consistent with published Monte Carlo estimates. Differences reported in the literature were attributed to methodological variations, including the use of computational modelling rather than direct experimental measurements, along with variations in phantom design, source geometry, and detector configuration. This work presents an experimentally measured dataset with corresponding dose rate models for NM applications. The results contribute additional empirical evidence that may be used alongside established radiation protection methodologies to support assessments of shielding, procedure scheduling, and occupational and public radiation risk, potentially reducing reliance on overly conservative assumptions when evaluating dose rates at clinical distances.