3224 - To Space or Not to Space: The EPIC question for Prostate Stereotactic Radiation (SBRT) with or without Hydrogel Rectal Spacer (RS)

Purpose/Objective(s): Following prostate RT bowel, urinary, and sexual side effects and declines in quality of life (QOL) are common. Phase 3 trials of rectal spacers using 20 or more fractions found clinical and dose benefits and less declines in QOL. However, the role of rectal spacers using SBRT is undefined.

Materials/Methods: A prospective single institution registry of prostate SBRT between 2012-2023 was analyzed by RS use (n=290) or not (n=1815). QOL was collected via EPIC-26 at baseline, 1 mo, and serially up to 5 yrs post-RT. Treatment used CT planning with MRI fusion and 3-6 fiducials for real-time tracking with Robotic SBRT that is a frameless robotic radiosurgery system from a technology company. The CTV was the prostate plus the proximal seminal vesicles. PTV margins were 5 mm except 3 mm posteriorly. 35-36.25 Gy was delivered in 5 fractions over 1-2 weeks. No dominant nodule boost was used. Linear mixed-effects models examined the effect of time and treatment on QOL scores, accounting for within-subject variability.

Results: There were no differences in age (69 vs 72 p=0.1), PSA (7.8 vs 7.5 p=0.3), or prostate volume (39 v 38 ml p=0.9) in those with or without RS, respectively. Use of RS was associated with more intermediate/high risk (96% v 85% p<0.001), ADT use (52% v 39% p<0.001), more Caucasian patients (63% v 55% p<0.001), and treatment more recently (p<0.001). Baseline EPIC scores were not different (Table). Declines in EPIC scores following SBRT were small with the majority approaching baseline after 6 mo and remaining stable up to 5 yrs. Mean QOL decline met pre-defined thresholds for minimally important difference (MID) only for urinary obstructive, bowel, and hormonal/vitality QOL at 1 mo and then recovered. At no time did any mean meet a threshold for 2x MID. No significant differences were observed in any QOL domain for those who received RS or not (all p>0.1). Sexual function was also not different if limited to those with baseline EPIC >60 and no ADT use (p>0.3).

Conclusion: Robotic SBRT for definitive treatment of prostate cancer had minimal declines in QOL with stable and clinically insignificant outcomes for most domains beyond 6 mo. The use of RS did not influence QOL. The study is limited by non-randomized design, single institution, and may not apply to other RT delivery platforms. Further, baseline factors and medications may have influenced use of RS. Additional investigation through randomized controlled trials is warranted.