Intra-Individual Comparison of Physiologic [(68)Ga]Ga-PSMA-11 and [(18)F]PSMA-1007 Uptake in Ganglia in Patients with Prostate Cancer: A Retrospective, Monocentric Analysis

SIMPLE SUMMARY: Since the introduction of PSMA imaging around a decade ago, the faint tracer uptake of ganglia in the neck, abdomen, and presacral region has been an imaging challenge due to difficult discrimination from suspicious lymph nodes. Moreover, PSMA tracers labelled with different radionuclides demonstrate varying levels of ganglion uptake despite having the same target structure on the cell membrane, resulting in distinct imaging pitfall patterns. Our study aims to investigate the underlying mechanisms of the varying detectability of PSMA ligands labelled with different radionuclides. ABSTRACT: Background: Several studies indicate, particularly in the case of [18F]PSMA-1007, a relatively high rate of detection of ganglia in PSMA PET imaging. Ganglia are an integral part of the sympathetic portion of the autonomous nervous system. To date, no studies have directly compared [68Ga]Ga-PSMA-11 and [18F]PSMA-1007 ganglionic uptake intra-individually and analyzed the underlying molecular and physical mechanisms of different detection rates. With this monocentric retrospective study, we sought to evaluate the intra-individual physiological ganglion uptake of these different PSMA ligands in evidence-based imaging for prostate cancer. Methods: Our cohort consists of 19 male patients (median age 72 ± 9 with a range of 56–85) with biochemical recurrence of prostate cancer who underwent both [68Ga]Ga-PSMA-11 and [18F]PSMA-1007 PET/CT in our clinic on the same scanner per standard care between March 2015 and March 2022. Tracer uptake was quantified according to maximum standardized uptake value (SUV(max)) for both [68Ga]Ga-PSMA-11 and [18F]PSMA-1007 PET/CT scans. Ganglia-to-background ratios (GBRs) were determined to quantify the image contrast through dividing the SUV(max) of the ganglia by the background value (SUV(max) of blood pool in the descending aorta, fatty tissue, and skeletal muscle in gluteal region). We used descriptive analyses for demographics and tumor characteristics and performed two-way repeated-measures ANOVA (analysis of variance) for SUV metrics including GBR measurements. Results: In total, we examined 101 ganglia with [(18)F]PSMA-1007 scanning, localized mostly in pairs as stellate, coeliac, and sacral, of which 76 were also detected with [(68)Ga]Ga-PSMA-11 PET/CT scanning. There was no statistically significant difference in PSMA uptake in terms of SUV(max) between [(18)F]PSMA-1007 and [(68)Ga]Ga-PSMA-11 (p value: 0.052). In contrast, the comparison of GBRs revealed a higher detectability rate of ganglia with [(18)F]PSMA-1007 imaging (p < 0.001). Furthermore, a separate comparison of ganglia with respect to their anatomical location also demonstrated statistically significant differences both within and between [(18)F]PSMA-1007 and [(68)Ga]Ga-PSMA-11 PET/CT scans. Conclusion: Given the impression of more accentuated [(18)F]PSMA-1007 uptake in ganglia compared with (68)Ga-labelled counterparts, our study demonstrated that the better detectability of ganglia is not due to more intense [(18)F]PSMA-1007 uptake by these small structures but to much more favorable physical properties of the radionuclide (18)F. The most relevant limitations of our study are its retrospective design and the small patient cohort.