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Abstract
Radiohybrid PSMA (rhPSMA) ligands, a new class of theranostic prostate-specific membrane antigen (PSMA)-targeting agents, feature fast 18F synthesis and utility for labeling with radiometals. Here, we assessed the biodistribution and image quality of 18F-rhPSMA-7 to determine the best imaging time point for patients with prostate cancer. Methods: In total, 202 prostate cancer patients who underwent a clinically indicated 18F-rhPSMA-7 PET/CT were retrospectively analyzed, and 12 groups based on the administered activity and uptake time of PET scanning were created: 3 administered activities (low, 222-296 MBq; moderate, 297-370 MBq; and high, 371-444 MBq) and 4 uptake time points (short, 50-70 min; intermediate, 71-90 min; long, 91-110 min; and extra long, ≥111 min). For quantitative analyses, SUVmean and organ- or tumor-to-background ratio were determined for background, healthy organs, and 3 representative tumor lesions. Qualitative analyses assessed overall image quality, nonspecific blood-pool activity, and background uptake in bone or marrow using 3- or 4-point scales. Results: In quantitative analyses, SUVmean showed a significant decrease in the blood pool and lungs and an increase in the kidneys, bladder, and bones as the uptake time increased. SUVmean showed a trend to increase in the blood pool and bones as the administered activity increased. However, no significant differences were found in 377 tumor lesions with respect to the administered activity or uptake time. In qualitative analyses, the overall image quality was stable along with the uptake time, but the proportion rated to have good image quality decreased as the administered activity increased. All other qualitative image parameters showed no significant differences for the administered activities, but they showed significant trends with increasing uptake time: less nonspecific blood activity, more frequent background uptake in the bone marrow, and increased negative impact on clinical decision making. Conclusion: The biodistribution of 18F-rhPSMA-7 was similar to that of established PSMA ligands, and tumor uptake of 18F-rhPSMA-7 was stable across the administered activities and uptake times. An early imaging time point (50-70 min) is recommended for 18F-rhPSMA-7 PET/CT to achieve the highest overall image quality.

Affiliations

So Won Oh  1   2 , Alexander Wurzer  3 , Eugene J Teoh  4 , Sohee Oh  5 , Thomas Langbein  1 , Markus Krönke  1 , Michael Herz  1 , Saskia Kropf  6 , Hans-Jürgen Wester  3 , Wolfgang A Weber  1 , Matthias Eiber  7
1 Department of Nuclear Medicine, School of Medicine, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany.
2 Department of Nuclear Medicine, Seoul National University Boramae Medical Center, Seoul, Korea.
3 Technical University of Munich, Garching, Germany.
4 Department of Radiology and Nuclear Medicine, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom.
5 Department of Biostatistics, Seoul National University Boramae Medical Center, Seoul, Korea; and.
6 Scintomics GmbH, Fuenstenfeldbruck, Germany.
7 Department of Nuclear Medicine, School of Medicine, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany matthias.eiber@tum.de.