The Impact of Peptide Amount on Tumor Uptake to Assess PSMA Receptor Saturation on 68Ga-PSMA-11 PET/CT in Patients with Primary Prostate Cancer

Ga-68-labeled prostate-specific membrane antigen (PSMA) is often produced on-site, where usually a fixed amount of peptide is conjugated to the generator eluate. However, fluctuations in specific activity might influence tracer distribution and tumor accumulation. Therefore, our aim was to investigate the potential effect of varying the administered peptide amount on Ga-68-PSMA-11 uptake in tumors using PET/CT in patients with primary prostate cancer (PCa). Additionally, the impact of tumor volume on this potential effect and on accumulation in reference organs was assessed. Methods: The imaging data of 362 men with primary PCa who underwent Ga-68-PSMA-11 PET/CT were retrospectively included. Scans were quantified for normal tissue and primary tumors. Patients were divided into 3 groups based on their tumor volume. Correlation and multivariable linear regression analyses were performed. Results: The median index lesion volume was 9.50 cm(3) (range, 0.064-174 cm(3)). Groups were based on quartiles of prostatic lesion volume: #4.11 cm(3) (group 1), 4.11-20.6 cm(3) (group 2), and $20.6 cm(3) (group 3). No correlation was found between administered peptide amount and tumor uptake (SUVmean or SUVpeak) for any group, except for a significant correlation for SUVmean in the first group (P 5 0.008). Linear regression analysis supported these findings. Conclusion: The amount of administered peptide had no evident effect on Ga-68-PSMA-11 uptake in tumors, except for a significant positive correlation between administered peptide amount and tumor SUVmean for group 1. The findings imply that no receptor saturation occurs in men with primary PCa at peptide levels of about 2.5 mu g.