Impact of PSMA PET/CT on prostate cancer salvage radiotherapy management: Results from the prospective randomized phase 3 trial [PSMA SRT NCT03582774].
Authors: Armstrong Wesley R , Kishan Amar Upadhyaya , Booker Kiara M , Fendler Wolfgang Peter , Hope Thomas A. , Nickols Nicholas George , Czernin Johannes , Calais Jeremie 
Published: June 2022

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Background: The purpose of the randomized PSMA SRT trial is to compare the success rate of salvage radiation therapy (SRT) for recurrence of prostate cancer (PCa) after radical prostatectomy (RP) with (intervention arm) and without (control arm) planning based on PSMA PET/CT. Here we report the secondary endpoint of the trial: impact of PSMA PET/CT on the treatment plan. Methods: This is a Randomized, controlled, prospective, open label, phase 3 clinical trial with institutional funding. 193 patients were randomized to proceed with standard SRT with any conventional imaging aside from PSMA PET/CT (control arm) or undergo a 68Ga-PSMA-11 PET/CT scan prior to SRT (investigational arm). The following information was collected on case-report forms before randomization (intended SRT plan) and after treatment: Radiation field region (prostate fossa (PF), pelvic lymph node (PLN)), total dose, dose per fraction, duration, ADT use and duration, PSMA influence on target volume, or other (free-text). Changes between SRT plan before randomization and delivered treatment were classified as Major, Minor or No Change. Major change: change of ADT duration ≥3 months, change of standard RT volumes (PF and PLN), target volume delineation beyond standard RT field, simultaneous-integrated boost (SIB) beyond standard RT fields, and initiation of advanced systemic therapy (novel ADT agents, chemotherapy). Minor change: SIB within standard RT fields. Fisher exact test was used to compare prevalence of events between study arms. Results: Enrollment is complete. 193 patients enrolled from 09.06.2018 to 8.17.2020: 90 and 103 randomized to the control group and PSMA group. Median Time from RP to enrollment and median PSA was 20.3 months (IQR 1.4–245) and 0.3 ng/ml (IQR 0.2-10.3), and 28.3 months (IQR 1.2–21) and 0.23 ng/ml (IQR 0.1-29.9), respectively. The control arm had 13 dropouts (17%) while the intervention had one (1%). PSMA was positive in 38/102 (38%): 12/102 (12%) outside of pelvis, and 20/102 (20%) in PLNs. Pre-randomization RT plan and Delivered RT plan were available in 193/193 (100%), (77/90 control (86%) and 102/103 PSMA (99%), (p = 0.0004)respectively. There were 0/77 (0%) and 7/102 (7%) minor changes in the control and PSMA groups (p = 0.02). There were 17/77 (22%) and 45/102 (44%), major changes (p = 0.004); 32/45 (71%) were PSMA-related. Treatment escalation occurred in 7/17 (41%) and 36/52 (69%) (p = 0.048), and de-escalation in 10/17 (59%) and 10/52 (19%) (p = 0.004). Nine/102 patients (9%) received advanced systemic therapy in relation to PSMA findings whereas only 1/77 (1%) patient in the control received advanced therapy (p = 0.044). Conclusions: In this prospective randomized phase 3 study, proportion of major changes between the pre-randomization SRT plan and the delivered RT plan was 44% in the PSMA intervention group and 22% in the conventional imaging control group. Clinical trial information: NCT03582774.