Baseline [Ga]Ga-PSMA-11 PET/CT before [Lu]Lu-PSMA-617 Radioligand Therapy: Value of PSMA-Uptake Thresholds in Predicting Targetable Lesions.

Baseline uptake on prostate-specific membrane antigen (PSMA)-targeted imaging is a prerequisite for radioligand therapy (RLT) with [177Lu]Lu-PSMA-617. This study aims to quantify lesion-based response to RLT in relation to pretreatment standard molecular imaging metrics derived from [68Ga]Ga-PSMA-11 PET/CT. Sixty-one patients with mCRPC underwent [68Ga]Ga-PSMA-11 PET/CT imaging before and after a median of 4 (IQR 2−6) RLT cycles. Maximum and mean standardized uptake values (SUVmax, SUVmean), as well as tumor-to-liver ratio (TLR), were assessed. A median of 12 (IQR 7−17) lesions was analyzed per patient, resulting in a total of 718 lesions. Lesions with ≥30% SUVmax decline or falling below the blood pool uptake were considered responsive; ≥30% SUVmax increase marked lesion progression. Additionally, 4-point visual scoring was performed according to E-PSMA consensus. In total, 550/718 (76.6%) lesions responded to RLT, including 389/507 (76.7%) bone metastases and 143/181 (79.0%) lymph node metastases. Baseline SUVmax, SUVmean, and TLR values were associated with lesion response by a moderate but significant correlation (rs = 0.33, p < 0.001, rs = 0.32, p < 0.001, and rs = 0.31, p < 0.001, respectively). For the classification of lesion progression based on baseline PSMA uptake, receiver operating characteristics (ROC) found SUVmax, SUVmean, and TLR to have comparable discriminatory value (AUC 0.85, 0.87, and 0.83). Of 42 tumor sites with baseline uptake below the liver (V-score < 2), 19/42 (45.2%) were responsive, 9/42 (21.4%) were stable, and 14/42 (33.3%) showed progression, leaving liver uptake a threshold with low prognostic value for the identification of RLT-refractory lesions (PPV 33%). This was observed accordingly for various liver uptake-based thresholds, including TLR < 1.5, <2.0 with a PPV at 24%, 20%, respectively. Standard uptake parameters quantified by routine baseline [68Ga]Ga-PSMA-11 PET/CT are moderately associated with post-treatment lesion response to [177Lu]Lu-PSMA-617. Commonly applied liver-based uptake thresholds have limited value in predicting refractory lesions at individual tumor sites.