Multicenter Comparison of Radionuclide Calibrators and SPECT/CT Protocols for Quantitative 177Lu Imaging in Clinical Practice

Abstract

Purpose: Following the clinical success of 177Lu-based therapies, accurate quantification of 177Lu using radionuclide calibrators (RNCs) and SPECT/CT is gaining importance as prerequisite for accurate treatment delivery and dosimetry. However, the lack of standardization can introduce inter-system variability, compromising multi-center clinical trials. This study aimed to assess the accuracy and variability of 177Lu measurements using RNCs and SPECT/CT across different systems and hospitals. Methods: A uniform cylindrical phantom and a NEMA phantom with hot spheres were prepared using traceable activities and imaged at 8 different hospitals using 13 SPECT/CT systems (9 conventional and 4 3D CZT). Acquisitions and reconstructions were performed using both site-specific and standardized protocols. The cylindrical phantom images were used to evaluate the system calibration and establish image calibration factors (ICFs), the NEMA images to evaluate effective resolution by calculating recovery coefficients (RCs). In parallel, two vials were measured to test RNC accuracy. Results: RNC measurements differed up to 11% between centers, while SPECT quantification of the cylindrical phantom differed up to 20%. While ICFs were consistent for systems of the same type, image quality varied strongly when using clinical protocols (36% difference in RCs in the largest sphere). Standardized reconstruction reduced variability in RCs for each system type (maximum 12% difference), regardless of acquisition protocol, but differences between system types persisted when standardizing acquisition and reconstruction (33% difference). Conclusion: Current 177Lu measurement practices yield significant variability in quantification and image quality. Harmonization efforts should prioritize standardized calibration and reconstruction protocols to improve multicenter reproducibility of quantitative 177Lu-SPECT/CT.