Combination of forced diuresis with additional late imaging in ⁶⁸Ga-PSMA-11 PET/CT – effects on lesion visibility and radiotracer uptake

Abstract

Purpose: Renal excretion of some prostate specific membrane antigen (PSMA)-ligands and consequently increased bladder activity can obscure locally relapsing prostate cancer (PC) lesions in PSMA-PET/CT. Furthermore, additional late imaging in PSMA-PET/CT provides a useful method to clarify uncertain findings. The aim of this retrospective study was to investigate a modified imaging protocol combining late additional imaging with hydration and forced diuresis in individuals undergoing additional late scanning for uncertain lesions or low PSA. Methods: We compared two protocols: one group of patients undergoing ⁶⁸Ga-PSMA-11-PET/CT were examined at 90 min p.i. with 1L oral hydration beginning at 30min p.i. and 20mg of Furosemide i.v. at 1h p.i. followed by additional late imaging at 2.5h p.i. without further preparation (“old” protocol). A second group received the same procedure as before, with an additional 0.5L oral hydration and 10mg of Furosemide i.v. 30min before the late imaging. 132 patients (76 “old” protocol, 56 “new” protocol) were examined with respect to urinary bladder activity (SUVmean), PC-lesion uptake (SUV_max) and lesion contrast (tumor-SUV_max÷bladder-SUVmean for local relapses and tumor-SUV_max÷gluteal-musculature-SUVmean for non-local PC lesions). Results: Bladder activity was significantly greater for the “old” protocol in the late scans compared to the “new” protocol (ratio of bladder activity (2.5h÷1.5h): 2.33±1.17 vs. 1.37±0.50, p<0.0001). Increased tumor SUV_max and contrast was seen at 2.5h compared to 1.5h (p<0.0001 “old”; P = 0.02 “new”). Increased bladder activity for the “old” protocol resulted in decreased lesion-to-bladder contrast, which was not the case for the “new” protocol. Tumor-to-background ratios increased at late imaging for both protocols, but the increase was significantly lower for the “new” protocol. For the “old” protocol, comparing the 1.5h to 2.5h acquisitions, 4 lesions in 4 patients (4/76=5.2% of the cohort) were visible at the post-diuresis 1.5h acquisition, but not visible at 2.5h, having been obscured as a result of the higher bladder activity. In the new protocol, 2/56 (3.6%) of patients had lesions visible only at late imaging and two had lesions which could be better discriminated at late imaging. Conclusion: Although the combination of diuretics and hydration can be a useful method to increase the visualisation and detectability of locally recurrent PC in standard [⁶⁸Ga]Ga-PSMA-11-PET/CT, their effects do not sufficiently continue into additional late imaging. Additional diuresis and hydration is recommended to improve visibility, detection and diagnostic certainty of local recurrences.