Evaluation of 9-Minute Dual-Energy Peak Time-Reduced 360° CZT Total-Body [<sup>177</sup>Lu]Lu-PSMA SPECT/CT Imaging with Comparisons to [<sup>68</sup>Ga]Ga-PSMA PET.

Abstract

Eighteen-minute 360° cadmium zinc telluride (CZT) total-body 177Lu-labeled SPECT acquisitions on the single 113-keV 177Lu photopeak (SP-18) have been reported to be suitable for monitoring [177Lu]Lu-PSMA therapy. This study evaluated dual-energy 113- and 208-keV acquisitions with a more than 2-fold increase in acquired counts, allowing acquisition time to be reduced to 9 min (DP-9). Methods: In 20 patients with prostate cancer, 360° CZT total-body 177Lu-labeled SPECT acquisitions performed after the first [177Lu]Lu-PSMA therapeutic injection were reconstructed as SP-18, DP-9, and dual-energy 18-min reconstructions (DP-18), all from the same list-mode dataset, and tumor volume and SUVmean were compared. A standard pretherapy [68Ga]Ga-PSMA PET was also used as an external imaging reference. Results: DP-9 yielded tumor volume and SUVmean that were comparable to those of DP-18. Compared with SP-18, DP-9 produced lower tumor volumes (median percentage difference, -15% [interquartile range, -25% to -12%]) with similar SUVmean Compared with PET, both SP-18 and DP-9 showed higher SUVmean (+14% [+7% to + 36%] and +19% [-3% to +38%], respectively), likely reflecting the longer injection-to-imaging delay for SPECT, and higher tumor volumes (+68% [+5% to + 122%] and +47% [-15% to +95%], respectively), consistent with the lower spatial resolution of SPECT. Phantom results were similar for SPECT reconstructions, with PET showing better recovery coefficients for SUV and volume. Conclusion: Dual-energy 360° CZT total-body [177Lu]Lu-PSMA SPECT/CT enables 9-min acquisitions while providing tumor volume and SUVmean measurements comparable to those of the previously validated 18-min single-energy protocol. Tumor volumes are lower than those with SP-18 but are closer to PET-derived values. Differences between SPECT and PET mainly reflect the injection-to-imaging time for SUVmean and the spatial resolution for tumor volume.