Environmental dose evaluation following the installation of a SPECT/CT scanner in a major public hospital using Monte Carlo simulation

Abstract

Background: Accurate shielding design in nuclear medicine requires reliable dose estimates to maintain occupational and public exposures as low as reasonably achievable. Shielding calculations are conventionally performed analytically. A Monte Carlo (MC) approach offers more detailed, site-specific insight but has not yet been systematically applied locally. Objectives: Acquire proficiency in FLUKA; develop a detailed model of a site-specific SPECT/CT facility; simulate a typical 99mTc imaging protocol; benchmark the model against experimental data and national dose regulations; extend the framework to 177Lu. Research Methodology: A hybrid approach combining simulation, experimental measurements, and analytical calculations was adopted. The facility and scanner were modelled based on technical drawings and material data. Ambient dose equivalents were physically measured at multiple locations using thermoluminescent dosimeters exposed to a 99mTc-injected phantom for model validation. Quantities scored in FLUKA included ambient dose equivalents, fluence distributions, regional energy deposition, and fluence-energy spectra. Dose equivalents were normalised to clinical workloads to derive annual dose estimates. Results: A high-fidelity MC model of the SPECT/CT facility was successfully developed, simulated and validated. Weighted mean ratios and two-tailed paired t-tests (n = 11) showed values of 1.17 ± 0.02 (analytical vs experiment, significant, p < 0.03) and 1.20 ± 0.01 (simulation vs experiment, not significant, p = 0.10). The ratio findings indicated that the MC method performed at least as well as the analytical approach, while the paired t-test suggested superior performance. Workload-adjusted annual dose equivalents verified compliance of installed shielding with national dose limits and regional energy deposition revealed strong phantom self-absorption (62% for 99mTc, 87% for 177Lu). Conclusions and Recommendations: This study introduced and validated an MC facility model for nuclear medicine shielding assessment. Future planning should combine analytical, experimental, and MC methods for accurate, yet time-efficient assessments. Further work should address validation against alternative MC codes, CT source modelling, and dynamic patient and equipment motion.