A Multi-Institutional Phase II Trial of High-Dose SAbR for Prostate Cancer Using Rectal Spacer

ABSTRACT Purpose High-Dose Stereotactic Ablative Radiotherapy (SAbR) for prostate cancer (PCa) offers the radiobiologic potency of the most intensified radiation therapy regimens, but was associated with >90% rates of ulceration of the anterior rectal wall on endoscopic assessment that infrequently progressed to severe rectal toxicity in prior prospective series. A multi-institutional Phase II prospective trial was conducted to assess whether placement of a peri-rectal hydrogel spacer would reduce acute periprostatic rectal ulcer events following high dose (> 40 Gy) SAbR. Methods Eligible patients included men with stage ≤ T2c localized Grade Group 1-3 PCa, PSA ≤ 15, with AUA scores ≤ 18 and gland volume ≤ 80 cm3. Patients underwent peri-rectal hydrogel spacer placement, followed by 45 Gray (Gy) in 5 fractions every other day SAbR to the prostate only. Androgen deprivation was not allowed except for cytoreduction. The rectal wall was directly assessed by serial anoscopy during followup to determine if the spacer would reduce acute periprostatic rectal ulcer events from >90% to Results Forty-four men were enrolled and forty-three were eligible for protocol analysis. Median followup for surviving patients was 48 months. Acute peri-prostatic ulcers were observed in 6/42 (14.3%, 95% CI 6.0-27%, P Grade 3 toxicities occurred in any domain. Four year freedom from biochemical failure was 93.8% (95% CI 85.2-100.0%). Conclusion Temporary hydrogel spacer placement prior to high-dose SAbR treatment for localized PCa and use of strict dose constraints are associated with a significant reduction in the incidence of rectal ulcer events compared to prior Phase I/II trial results.