177Lutetium-PSMA-617 is an established treatment for patients with metastatic castration-resistant prostate cancer (mCRPC). Early response assessment is critical for optimizing patient management. Response Evaluation Criteria in Prostate Imaging (RECIP) 1.0, initially developed for Positon Emission Tomography (PET) with 68Ga-PSMA, have demonstrated prognostic value, particularly after two treatment cycles. Recent studies suggest that SPECT at cycle 2 provides prognostic information, although it reflects only the first treatment cycle, whereas PET-based RECIP, assessed after cycle 2, captures the effect of two cycles. This study aims to evaluate the prognostic value of quantitative 177Lu-PSMA-SPECT biomarkers and determine the optimal timing for RECIP-based assessment.