Impact of PSMA PET/CT on SRT planning: Preliminary results from the randomized phase III trial NCT03582774.

Abstract

30 Background: The purpose of this trial is to evaluate the success rate of salvage radiation therapy (SRT) for recurrence of prostate cancer (PCa) after radical prostatectomy with and without planning based on prostate specific membrane antigen (PSMA) positron emission tomography (PET). Methods: This is a multicenter, prospective, randomized, controlled, open-label, Phase 3 clinical imaging trial powered for clinical outcome at 5 years. UCLA is the leading central site in which PSMA PET, clinical follow-up and data management are being done. UCSF was a participating site in which PSMA PET imaging can be done. SRT can be performed anywhere, patients are followed remotely by the UCLA investigators. Patients scheduled for SRT for recurrence after primary prostatectomy and with PSA ≥ 0.1ng/ml at time of enrollment were eligible. Patients were randomized to proceed with standard SRT allowing for any conventional imaging aside from PSMA PET/CT (control arm) or undergo a 68Ga-PSMA-11 PET/CT scan prior to SRT planning (investigational arm). The primary endpoint is the success rate of SRT at 5 years in patients who undergo SRT. We report here the preliminary results of a secondary endpoint: the impact of PSMA PET on SRT planning by comparing the pre-randomization RT plans prospectively obtained on surveys before randomization to the actually delivered RT plans obtained after follow-up. Results: Enrollment of the trial was complete. 193 patients were enrolled from 09.06.2018 to 08.17.2020. 7/90 patients (9%) in the control arm dropped-out the study because they underwent a PSMA PET at another institution, while 1/103 (1%) patients of the intervention arm dropped-out due to COVID-19 related complications. After a median follow-up of 13.3 months (last follow-up date 09/01/2020), delivered RT plans were obtained in 60/83 (72%) and 70/102 (69%) of patients of the control and the PSMA arms, respectively. Median PSA at enrollment was 0.32 ng/ml (IQR 0.17-1.35) and 0.22 ng/ml (IQR 0.14-0.50) in the control and PSMA arms, respectively. There was a change between the intended pre-randomization RT plan and the actually delivered RT plan in 17/60 (28%) and 40/70 (57%) of the patients in the control and PSMA arms, respectively (p = 0.002). SRT was aborted in favor of systemic therapy and/or metastasis directed RT for extra-pelvic M1 disease in 2/60 (3%) and 12/70 (17%) of the control and PSMA arms, respectively (p = 0.17). Dose prescription and/or target volume delineation was changed in 2/60 (3%) and 1/70 (26%) in the control and PSMA arms, respectively (p = 0.001). Conclusions: In this prospective randomized phase 3 study, PSMA PET had an impact on the SRT plan in more than half of the patients. Long-term follow-up will show if the impact of PSMA PET on SRT planning translates into improved outcome or not. Clinical trial information: NCT03582774.