Validation of Simplified Renal Dosimetry Protocols for ¹⁷⁷Lu-DOTATATE Therapy using the IDAC-Dose software

Objective: The multi-time-point imaging method for renal dosimetry in ¹⁷⁷Lu-DOTATATE peptide receptor radionuclide therapy (PRRT) imposes a significant burden on patients. While simplified protocols have been explored, their validation has predominantly relied on legacy MIRD-based software, hindering inter-institutional standardization. This study aimed to provide the first systematic validation of simplified two-time-point (2TP) and three-time-point (3TP) protocols using a modern, open-source, ICRP-compliant platform to establish a foundation for a standardized and accessible methodology. Methods: This retrospective study analyzed data from 17 neuroendocrine neoplasm patients (28 kidneys) who underwent PRRT. Renal absorbed doses were calculated using various 2TP and 3TP combinations derived from a reference four-time-point (4TP) imaging schedule (at 4-, 24-, 72-, and 120-hours post-administration) with the open-source, ICRP-compliant software IDAC-Dose 2.1. The accuracy of each simplified protocol was evaluated against the 4TP reference using mean percent error (MPE), mean absolute percent error (MAPE), root mean square error (RMSE), and Bland-Altman analysis. Results: Protocols including the 120-hour late-phase time point demonstrated significantly higher accuracy. The 2TP (4, 24) protocol was the least accurate, showing a profound underestimation (MPE: -14.3%), high absolute error (MAPE: 29.6%), and the largest overall error (RMSE: 1271.9 mGy). In contrast, the 2TP (24, 120) protocol showed excellent agreement with the reference (MPE: 0.57%, MAPE: 5.5%) with no significant systematic bias. Among the 3TP methods, the 3TP (24, 72, 120) protocol yielded the highest accuracy and precision (MAPE: 1.97%; RMSE: 58.1 mGy). A well-chosen 2TP protocol was more accurate than a poorly chosen 3TP protocol. Conclusions: Accurate and reproducible renal dosimetry in ¹⁷⁷Lu-DOTATATE therapy can be achieved with simplified protocols. The key principle is the strategic inclusion of a late-phase time point (around 120 hours). The 2TP (24, 120) and 3TP (24, 72, 120) combinations are recommended as practical, high-accuracy alternatives. This work provides the foundational evidence for adopting these protocols within a modern dosimetric framework, representing a critical step toward developing internationally standardized and clinically feasible dosimetry for PRRT.