Assessing the clinical utility of rapid post-therapy whole-body digital SPECT/CT in evaluating early treatment response of <sup>177</sup>Lu-PSMA-617 treatment.
Authors: Hong Song, Maria Isabel Leonio, Valentina Ferri, Heying Duan, Jagruti Shah, Farshad Moradi, Carina Mari Aparici, Benjamin Lewis Franc, Guido Davidzon, Colin P Bergstrom, Alice C. Fan, Sumit Shah, Ali Raza Khaki, Sandy Srinivas, Andrei Iagaru
Journal: Journal of Clinical Oncology (2024)

32 Background: Multi-detector cadmium-zinc-telluride (CZT) based whole-body digital SPECT/CT is a new generation of imaging system. We aim to assess the clinical utility of rapid post-therapy whole-body SPECT/CT scan in evaluating early treatment response of 177 Lu-PSMA-617 treatment. Methods: We retrospectively reviewed patients with progressive metastatic castration resistant prostate cancer (mCRPC) who were treated with at least two cycles of 177 Lu-PSMA-617 at our institution from June 2022 to June 2023. Post-therapy whole-body digital SPECT/CT (GE StarGuide) was performed from vertex to mid-thigh at 1-2 hours after 177 Lu-PSMA-617 infusion. Post-therapy SPECT/CT images were quantified with MIM software. Lu177-PSMA positive lesions were delineated with liver parenchyma uptake as cut-off. Lu-PSMA positive total tumor volume (Lu-TTV), tumor SUVmax and SUVmean were obtained. Post-therapy SPECT/CT image after cycle 1 was used as baseline for comparison with SPECT/CT after cycle 2 and 3. Overall survival, PSA progression free survival (PSA-PFS) as defined by PCWG3 and PSA decline &gt; 50% from baseline (PSA50) at any time after treatment were measured. Changes in post-therapy SPECT/CT were correlated with clinical outcomes to assess quantitative SPECT/CT as a tool for early treatment response. Analyses were performed with SPSS. Results: A total of 56 patients (76±8, mean ± SD, range 60-93 years old) who were imaged with at least 2 post-therapy SPECT/CT were included in the analysis. Post-therapy whole body SPECT/CT was acquired in ~12 mins. All scanned patients tolerated the rapid whole-body SPECT/CT very well. With median follow up of 10 months, median PSA-PFS was 5.0 months (range 1.0-15 months), 33 of 56 patients (58.9%) achieved PSA50 at any time after treatment, and 42 of 56 patients (75%) were alive at data cutoff. Quantitative analysis of SPECT/CT images showed that 36 of 56 patients (64%) had a &gt;30% decrease in Lu-TTV on early follow-up post-therapy SPECT/CT after cycle 2 or 3. Kaplan-Meier survival analysis showed that a &gt;30% decrease in Lu-TTV was associated with longer overall survival (median not reached vs 6 months, P = 0.008) and longer PSA-PFS (6 months vs 1 months, P &lt; 0.001). Decrease in SUVmax or SUVmean, however, was not associated with PSA-PFS or overall survival. Conclusions: Rapid post-therapy whole-body digital SPECT/CT was well tolerated in post-therapy SPECT/CT imaging following 177 Lu-PSMA-617 treatment. Quantitation of post-therapy SPECT/CT may be useful in evaluating early treatment response of 177 Lu-PSMA-617. Prospective validation and larger patient cohorts are needed to further elucidate the role of post-therapy SPECT/CT imaging in guiding patient management in 177 Lu-PSMA-617 therapy.