Optimization of SPECT/CT-based Dosimetry in Lutetium-177 Radioligand Therapy

Internal dosimetry in Lutetium-177 radioligand therapy is employed to quantify the absorbed radiation dose in tumor targets and organs at risk through multiple SPECT/CT acquisitions per treatment cycle. The current clinical protocol, based on two acquisitions at 24 and 96 hours post-administration, requires patient hospitalization. The transition to a day-hospital regimen necessitates a revision of the acquisition protocol, introducing an earlier first time point. This thesis aims to optimize SPECT/CT-based dosimetric protocols by evaluating the feasibility of a day-hospital protocol with an early first time point at 4 hours post-administration, comparing it with the standard protocol (24–96 hours). To this end, SPECT/CT acquisitions at 4, 24, and 96 hours were performed in a selected patient cohort, defining a three-time-point (3TP) reference dosimetric protocol. Images were processed using MIM software to extract dosimetric and kinetic parameters. Reference kinetic curves were derived from the three-time-point data, and simplified kinetic models (24–96 h vs 4-96 h) were subsequently simulated via Monte Carlo methods to assess bias and variability across different regions of interest. The results demonstrate that early imaging introduces significant variability in tumor kinetics, leading to systematic over- or underestimation of the absorbed dose. While more reliable estimates were obtained for organs at risk, the analysis confirms that the current standard protocol remains the most accurate simplified dosimetric approach for clinical use.