Impact of Rectal Spacer on Toxicity Reduction in Men Treated With Proton Versus Photon Therapy

Rectal toxicity after prostate cancer (PCa) radiotherapy may be greater with protons compared with photon intensity modulated radiotherapy (IMRT), perhaps due to lateral penumbra and end-of-range uncertainty. Rectal spacers (RS) have been shown to mitigate RT-associated acute/late rectal toxicity in men treated with photons. The relative benefit of RS in men treated with protons versus photons is unknown. We hypothesize that RS will confer greater bowel toxicity benefit in PCa treated with protons vs photons. We conducted a single institution, retrospective review of men receiving photon IMRT or pencil-beam scanning proton RT for localized PCa. Four cohorts were compared: Photon with (Ph+RS) or without (Ph-RS) RS, and proton with (Pr+RS) or without (Pr-RS) RS. Acute (< 3 months), late (≥ 3 month) & most recent toxicity was compared amongst the four cohorts. Cumulative incidence of physician-reported grade 1-2 gastrointestinal (GI) toxicity (CTCAE V5.0) was compared using Chi-square or Fisher's exact test. Patient-reported toxicity was evaluated using International Prostate Expanded Prostate Composite Index- Clinical Practice and compared using linear mixed modeling. 164 patients were eligible for analysis: 38 Ph-RS, 50 Ph+RS, 26 Pr-RS, & 50 Pr+RS. Median follow-up was 17.6 months. In proton patients, acute (6.12 vs 30.77%, p = 0.009) and most recent (4.26% vs 26.09%, p=0.01) G1-2 GI toxicity was lower with versus without RS. In photon patients there were no significant differences in toxicity with versus without RS. No significant differences in patient-reported outcomes were observed with versus without RS in photon or proton groups. RS was associated with lower G1-2 acute & most-recent GI toxicity in men treated with protons, this difference was not observed in men treated with photons. While this study is limited by sample size, a relatively greater benefit of RS with proton versus photon therapy was observed.