3346 - Optimizing Perirectal Spacer Use: Risk Factors for Rectal Toxicity and Bowel Quality of Life after Hypofractionated Radiation Therapy for Prostate Cancer

Purpose/Objective(s):

Prostate radiotherapy (RT) has improved significantly in the modern era to reduce gastrointestinal (GI) toxicity, with intensity-modulated RT, advanced image guidance, and rectal spacers. We evaluated GI toxicity and bowel quality of life (QOL) in men treated with conventional (CRT) or hypofractionated (HFX) RT to inform decisions regarding spacer placement.

Materials/Methods:

Men treated with curative-intent RT for prostate cancer from 2006-2023 were analyzed from an institutional IRB-approved registry. Men with prior spacer placement, brachytherapy, or radical prostatectomy were excluded. Median CRT dose was 78 Gy/39 fx and 60 Gy/20 fx for HFX (predominantly treated 2019-2023) RT. In the HFX group, 64%, 14%, and 22% had doses =60, >60 to =64, and >64 Gy including nodule boosts to 67+ Gy, respectively. Most HFX men had image-guided RT utilizing fiducial markers and triggered kV imaging (89%) to correct for intrafraction motion. Covariates included diabetes (DM), anticoagulant medications at consultation (ACAP, including aspirin use), age, RT dose, and rectal dosimetric parameters. Prospectively recorded GI toxicity (CTCv4.0) and bowel QOL (EPIC-26) were evaluated by univariate (UVA) and multivariable (MVA) analysis, including change >minimum clinically important difference (MCID, 5 points). Median follow-up was 64 and 33 months for CRT and HFX groups, respectively.

Results:

410 men were treated with CRT and 187 men were treated with HFX RT. Cumulative freedom from grade 2+ toxicity (FFG2+) at 2-yr was 89% in CRT group and 92% in the HFX group (p=0.08), occurring at a median 13 mo (IQR 6-21). At last follow up, only 1% had gr2+ GI toxicity. On UVA, significantly associated covariates were RT dose (p= 0.01) and DM (p= 0.01) for CRT, and dose >60 Gy (p=0.03), ACAP (p=0.02), age 69+ (p= 0.01), and rectal V60 Gy =3 cc (p=0.01) for HFX RT. In the HFX group, significantly associated covariates on MVA included rectal V60 Gy (RR 7.1, p=0.01), ACAP (RR 4.2, p=0.03), and dose >60 Gy (RR 5.8, p=0.01) but not age (RR 1.05 per year, p=0.29). FFG2+ toxicity-2-yr associated with ACAP was less severe for men with rectal V60 Gy <3 cc (94% vs 100%, p=0.08) than =3 cc (81% vs 94%, p=0.07). Similarly, RT dose >60 Gy was associated with less G2+ toxicity if V60 Gy <3 cc (97% vs 98%, p=0.70), than if =3 cc (77% vs 98%, p=0.01).

No statistically significant differences were observed for bowel QOL between CRT and HFX RT. In the HFX group, median global score at 2-years was unchanged from baseline (both 95.8), and 30% and 21% had MCID1x and 2x change, respectively. No tested covariates were associated with QOL.

Conclusion:

GI toxicity following moderate HFX RT, with or without simultaneous intraprostatic nodule boosting, is infrequent without perirectal spacers, especially at last follow-up. Bowel toxicity may be mitigated by limiting rectal V60 Gy <3 cc, or with selective use of perirectal spacers in men on anticoagulant medicines or receiving doses >64 Gy/20 fx.