An Operational Approach for Accurate 177lu and 177mlu Activity Quantifications to Comply with the Environmental Release Criteria: the Role of GEANT4 for Efficiency Curve and True Coincidence Summing Effect Estimation

The increasing use of 177 Lu-labelled agents for targeted radionuclide cancer therapy highlights the radiation protection challenges in the management of radioactive waste due to 177m Lu impurities. To ensure safe disposal, compliance with clearance criteria must be verified using calibrated systems, such as Hyper Pure Germanium (HPGe) spectrometers. This work aims to validate a customized GEANT4 model of our HPGe system in Marinelli beaker geometry to propose an operational approach properly quantifying 177 Lu and 177m Lu in waste samples. The system's efficiency curve was calculated by modelling gamma sources in the energy range of interest and validated by spectra measurements of 177 Lu and 99m Tc sources. Correction factors accounting for true coincidence summing (TCS) effect were simulated for 177 Lu, 177m Lu and 99m Tc and they were applied to the spectrum measurement of a waste sample model with known activities of 177 Lu and 177m Lu. Thus, an operational approach for activities quantification was tested comparing the results with the nominal values. No significant differences were observed between simulated and measured efficiency values. TCS correction factors are significant only for 177m Lu (1.6 at 112.95 keV and 204.11 keV). Eventually, the proposed framework to quantify 177 Lu in a waste sample allowed to estimate the 177 Lu and 177m Lu component activities within a maximum 16% uncertainty. Results show that the HPGe model could be a powerful resource for a wide range of applications in daily clinical routine and it could be used to build a simple quality assurance program to monitor the detector response constancy in time.