Empty-RT: A Phase 2 Trial of Prostate Stereotactic Body Radiation Therapy With an Empty Bladder Approach

To evaluate the feasibility and dosimetric safety of prostate stereotactic radiation therapy (SBRT) with an empty bladder approach.

Eligible patients had histologically confirmed prostate cancer and were planned for stereotactic body radiation therapy (SBRT) (35-36.25 Gy/5# to prostate ± 25 Gy to pelvic nodes, alternate days or once-weekly). Participants voided immediately before the planning computed tomography scan and each SBRT fraction. Interfraction bladder volume variability was evaluated with onboard cone beam computed tomography. The primary endpoint was the rate of acute urinary grade 2+ adverse event (Common Terminology Criteria for Adverse Events version 5.0), with a noninferiority margin of +10% compared with a 32% reference rate from the PRIME randomized trial (NCT03561961) using similar planning constraints.

Simulation scans were acquired with an empty bladder for 139 patients, of which planning constraints were achieved for 118 patients (simulation bladder volume ≤ 10 cm³ deemed unsuitable for empty bladder workflow, n = 7; planning constraints not met, n = 14). These 118 patients were treated with empty bladder and constituted the trial cohort evaluable for safety. Median simulation empty bladder volume was 77 cm³ (IQR, 59-110). Interfraction bladder volume variation for individual patients was 18%. Grade ≥2 urinary adverse events (AE) occurred in 28.1% (90% CI, 21.2-35.8), meeting the noninferiority threshold. Grade 3 AE were rare (urinary n = 1, 0.9%; gastrointestinal [GI] = 0), and 7.9% had grade 2 GI AE. Higher bladder volume at simulation was associated with a higher risk of grade 2+ urinary AE (odds ratio, 1.02; 95% CI, 1.01-1.03; P = .002). Once-weekly SBRT was associated with lower GI AE (0% vs 12.9%; P = .03) but not urinary AE.

Prostate SBRT delivered with an empty bladder was observed to be dosimetrically feasible and reproducible, with acute safety comparable with standard full bladder protocols. If validated in larger studies, it may offer a time-efficient and patient-friendly alternative in routine prostate SBRT practice.