RECIP 1.0 using post-treatment SPECT/CT vs. PSMA PET/CT in patients with extended [177Lu]Lu-PSMA-I&T radiopharmaceutical therapy beyond 6 cycles

Ziel/Aim: Response Evaluation Criteria in Prostate-Specific Membrane Antigen Imaging (RECIP) 1.0 is an evidence-based framework that is used to assess the response to therapy in PSMA-PET/CT. Innovative approaches of 177 Lu-PSMA radiopharmaceutical therapy (RPT) extend treatment beyond the currently approved 6 cycles. So far, no data are available on the use of RECIP 1.0 in post-therapeutic SPECT under extended 177 Lu-PSMA RPT. We aimed at comparing RECIP 1.0 results of PET and SPECT with biochemical response in patients with extended 177 Lu-PSMA RPT. Methodik/Methods: Patients with metastatic castration-resistant prostate cancer (mCRPC) who completed more than 6 cycles of [ 177 Lu]Lu-PSMA-I&T RPT were included in the retrospective analysis. Total tumor volume (TTV) was measured using 24h post-therapeutic SPECT (SUV>3) after the 6 th and 8 th cycles, and PSMA-PET after the 6 th , and after the 8 th cycles. TTVs and PSA levels were compared, with RECIP 1.0 criteria used to assess response to PSMA therapy in both SPECT and PET. Ergebnisse/Results: In total, 9 patients were included in the analysis. The PSA level significantly increased between the 6 th and 8 th cycle (6 th : median 18.9 ng/mL; Q1: 8.01 ng/mL; Q3: 35.6 ng/mL; 8 th : median 33.2 ng/mL; Q1: 7.4 ng/mL; Q3: 68.7 ng/mL; p=0.039). In contrast to the PET-derived total tumor volume (TTV; p=0.039), the SPECT-derived TTV did not increase significantly (p=0.164). According to RECIP 1.0 criteria progressive disease (PD) was seen in SPECT in 8/9 cases and in PET in 7/9 cases. Stable disease (SD) was seen in PET in 2/9 and partial response (PR) in SPECT in 1/9 cases. A PSA increase of>10% was detected in 6/9 cases; out of those, 1/6 showed PD in PET and SPECT, 1/6 showed SD in PET and PR in SPECT and 1/6 showed SD in PET and PD in SPECT. Schlussfolgerungen/Conclusions: These preliminary data suggest that post-treatment SPECT/CT may be suited for response assessment of extended 177 Lu-PSMA RPT beyond 6 cycles, even if it may capture total PSMA-positive tumor burden less accurately than PSMA PET. A comparison of SPECT-based response across RPT cycles with PSMA PET at different time points of RPT is ongoing. Whether post-treatment SPECT-based response assessment is prognostic has to be shown in larger studies including outcome-correlated prospective trials. Publication History Article published online: 12 March 2025 © 2025. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany