Pretherapeutic 68Ga-PSMA-617 PET May Indicate the Dosimetry of 177Lu-PSMA-617 and 177Lu-EB-PSMA-617 in Main Organs and Tumor Lesions

Aim Combined Ga-68-PSMA-617 PET imaging and Lu-177-PSMA-617 therapy is a precise targeted theranostic approach for patients with metastatic castration-resistant prostate cancer (mCRPC). The purpose of this study was to determine whether pretherapeutic standard uptake value (SUV) in Ga-68-PSMA-617 PET could indicate the effective dose in the main organs and absorbed dose in tumor lesions. Methods After institutional review board approval and informed consent, 9 patients with mCRPC were recruited and underwent Ga-68-PSMA-617 PET/CT scans. Five patients received Lu-177-PSMA-617 (1.30-1.42 GBq, 35-38.4 mCi) and then underwent serial whole-body planar imaging and SPECT/CT imaging of both thoracic and abdominal regions at 0.5-, 2-, 24-, 48-, and 72-hour time points. The other 4 patients received Lu-177-EB-PSMA-617 (0.80-1.1 GBq, 21.5-30 mCi) and then underwent the same imaging procedures at 2-, 24-, 72-, 120-, and 168-hour time points. The effective dose in the main organs and the absorbed dose in tumor lesions were calculated. Detailed correlations between the pretherapeutic SUV in Ga-68-PSMA-617 PET and effective dose in the main organs as well as absorbed dose in the tumor lesions were analyzed. Results SUV of Ga-68-PSMA-617 PET was moderately correlated with effective dose in main organs (r = 0.610 for Lu-177-PSMA-617, r = 0.743 for Lu-177-EB-PSMA-617, both P < 0.001). SUV of tumor lesions in Ga-68-PSMA-617 PET had high correlation with those in Lu-177-PSMA-617 (r = 0.915, P < 0.001) and moderate correlation with those in Lu-177-EB-PSMA-617 (r = 0.611, P = 0.002). Conclusions Pretherapeutic Ga-68-PSMA-617 PET may indicate the dosimetry of Lu-177-PSMA-617 and Lu-177-EB-PSMA-617. Both the effective dose in main organs and absorbed dose in tumor lesions correlate with SUV of Ga-68-PSMA-617 PET. This relationship may help select appropriate candidates for peptide receptor radionuclide therapy. Further investigations of larger cohorts are needed to confirm these initial findings.