EMPTY-RT: A Phase 2 Trial of Prostate Stereotactic Body Radiotherapy with an Empty Bladder Approach

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

Eligible patients had histologically confirmed prostate cancer and planned for SBRT (35-36.25 Gy/5# to prostate ± 25 Gy to pelvic nodes, alternate days or once-weekly). Participants voided immediately before planning CT scan and each SBRT fraction. Interfraction bladder volume variability was evaluated with on-board cone beam CT. Primary endpoint was the rate of acute urinary grade 2+ AE (CTCAE v5.0), with a non-inferiority margin of +10% compared to a 32% reference rate from the XXX randomized trial (NCTXXXXXX) using similar planning constraints.

Simulation scan was acquired with an empty bladder for 139 patients, of which planning constraints were achieved for 118 patients (simulation bladder volume ≤10cc 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 cc (IQR 59 - 110). Interfraction bladder volume variation for individual patients was 18%. Grade ≥2 urinary AE occurred in 28.1% (90% CI 21.2-35.8), meeting the non-inferiority threshold. Grade 3 AE were rare (urinary n=1, 0.9%; GI = 0), and 7.9% had grade 2 GI AE. Higher bladder volume at simulation was associated with a higher risk of gr2+ urinary AE [Odds ratio 1.02 (95% CI 1.01 - 1.03), p = 0.002]. Once-weekly SBRT was associated with lower GI AE (0% vs 12.9%, p = 0.03) but not urinary AE.

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