Standard protocol compared to a novel protocol for 68Ga-PSMA-PET/CT in patients with recurrent prostate cancer - which one is superior?

Abstract

Purpose / Introduction: Since the clinical introduction of PET-imaging with 68Ga-PSMA-11, this diagnostic tool has spread worldwide and is regarded as a breakthrough in the diagnosis of recurrent prostate cancer (PC). According to its first described clinical set-up, 68Ga-PSMA-11 PET/CT is conducted at 1h post injection (p.i.). However, further publications demonstrated that later imaging (e.g. at 3h p.i.) show the majority of PC lesions with higher contrast. In 2017, we conducted scans at 1h p.i.. However, in 2018, we changed our protocol to later imaging timing. The aim of this evaluation was to compare the standard protocol of 68Ga-PSMA-11 PET/CT with a novel protocol described below. Subjects & Methods: We retrospectively compared two patient cohorts scanned with 68Ga-PSMA-11 PET/CT in 2017 (n=94 patients) and 2018 (n=75 patients). In 2017, the scanning protocol was as follows: acquisition at 1h p.i. (targeted activity: 200 MBq) with 2 min per bed position, neither hydration nor forced diuresis. In 2018, the scans were conducted at 1.5h p.i. (also 2min per bed position and targeted activity of 200 MBq). In addition, the patients started to drink 1L of water at 0.5h p.i. and were injected with 20mg of furosemide at 1h p.i.. Rates of pathologic scans, maximum standardized uptake values (SUVmax) of tumor lesions (n=164 in 2017 and n=127 in 2018), average standardized uptake values (SUVmean) of urinary bladder as well as tumor contrast (SUVmax-tumor/SUVmean gluteal musculature) were measured in all patients. Results: Average tumor contrast was significantly (p=0.0451) higher in 2018 compared to 2017 (59.0 vs. 46.1). Average SUVmean of the urinary bladder was significantly (p<0.0001) lower in 2018 (SUVmean 7.6 ± 24.6) compared to 2017 (SUVmean 35.3 ± 7.6). Also the background activity was significantly (p<0.0001) lower in 2018. No relevant differences were detected for SUVmax of tumor and the rate of pathologic scans (2017: 80.9%; 2018: 80%; both numbers for PSA≤3.0 ng/ ml). Discussion / Conclusion: Despite highly promising results of the novel protocol including significantly higher tumor contrast and lower urinary activity which enable the assessment of tumor lesions and local recurrent PC, respectively, no higher rate of PET-positive patients were observed in the relatively low patient cohorts. However, we expect that with increasing scan numbers, more tumor lesions and more pathologic scans will be detected in a few percent of patients referred to PSMA-PET/CT thereby changing their therapeutic procedure.