To compare dosimetric parameters and late gastrointestinal (GI) outcomes between patients treated with proton beam therapy (PBT) for localized prostate cancer with rectal balloon immobilization versus hydrogel rectal spacer.
Patients with localized, clinical stage T1-4 prostate adenocarcinoma were treated at a single institution using conventionally fractionated, dose-escalated PBT from 2013 to 2018. Patient-reported GI toxicity was prospectively collected while the incidence of rectal bleeding was retrospectively reviewed from patient records.
192 patients were treated with rectal balloon immobilization and 75 were treated with rectal spacer. Rectal hydrogel spacer significantly improved rectal dosimetry while maintaining excellent target coverage. The 2-year actuarial rate of Grade 2+ late rectal bleeding was 19% and 3% in the rectal balloon and hydrogel spacer group, respectively (p=0.003). In univariable analysis, the probability of Grade 2+ rectal bleeding was significantly correlated to increasing rectal dose. In multivariable analysis, only receipt of spacer hydrogel (HR 0.145, P=0.010) and anti-coagulation use (HR 5.019, P<0.001) were significantly associated with Grade 2+ bleeding. At 2 years follow-up, patient-reported EPIC-bowel quality of life (QOL) composite scores were less diminished in the hydrogel spacer group (absolute mean difference 5.5, P=0.030).
Use of rectal hydrogel spacer for prostate PBT is associated with significantly lower incidence of clinically relevant, late rectal bleeding and lower decrement in long-term, patient-reported bowel QOL compared with rectal balloon immobilization. Our results suggest that hydrogel spacer may improve rectal sparing compared to rectal balloon immobilization during PBT for prostate cancer.

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