Assessment of Polyethylene Glycol Hydrogel Spacer and Its Effect on Rectal Radiation Dose in Prostate Cancer Patients Receiving Proton Beam Radiation Therapy

Purpose: To assess the efficacy of placing a polyethylene glycol (PEG) spacing hydrogel in patients undergoing proton beam radiation therapy for prostate cancer. This study also aims to assess the effect on rectal radiation dose of prostate–rectum separation in various anatomic planes. Methods and Materials: Seventy-two consecutive prostate cancer patients undergoing conventionally fractionated pencil beam scanning proton radiation therapy with and without hydrogel placement were compared. Magnetic resonance images taken after hydrogel placement measured prostate–rectum separation and were correlated to rectal dosing and rectal toxicity. Univariate analysis of clinical variables and radiation dosing was conducted using nonparametric Wilcoxon rank-sum test with continuity correction between groups (hydrogel spacer vs controls). Spearman's rank correlation coefficient assessed relationships between the various anatomic dimensions of perirectal space and rectal radiation dosing. Results: Fifty-one patients had hydrogel placement before therapy and 21 did not. There was a 42.2% reduction in rectal dosing (mL3 rectum) in hydrogel patients (P