A dosimetric analysis of rectal hydrogel spacer use in patients with recurrent prostate cancer undergoing salvage high-dose-rate brachytherapy.

ABSTRACT

PURPOSE: We hypothesize rectal hydrogel spacer (RHS) improves rectal dosimetry in patients undergoing salvage high-dose-rate brachytherapy (HDR-BT) for intact, recurrent prostate cancer (PC). METHODS AND MATERIALS A prospectively collected institutional database was queried for recurrent PC patients treated with salvage HDR-BT from September 2015 to November 2021. Patients were offered RHS beginning June 2019. Dosimetric variables were compared between RHS and no-RHS groups for the average of two fractions using Wilcoxon rank-sum tests. Primary outcomes were rectal volume receiving 75% of prescription dose (V75%) and prostate volume receiving 100% of prescription dose (V100%). Generalized estimating equation (GEE) model was used to evaluate the association between other planning variables and rectal V75%. RESULTS: Forty-one PC patients received salvage HDR-BT, of whom 20 had RHS. All patients received 2400cGy in 2 fractions. Median RHS volume was 6.2cm3 (Standard deviation [SD] ± 3.5cm3). Median follow-up was 4 months and 17 months in the RHS and no-RHS groups, respectively. Median rectal V75% with and without RHS were 0.0cm3 (IQR 0.0-0.0cm3) and 0.06cm3 (IQR 0.0-0.14cm3), respectively (p<0.001). Median prostate V100% with and without RHS were 98.55% (IQR 97.86-99.22%) and 97.78% (IQR 97.50-98.18%), respectively (p = 0.007). RHS, rectum, and prostate volumes did not significantly affect rectal V75% per GEE modeling. There was 10% G1-2 and 5% G3 rectal toxicity in RHS group. There was 9.5% G1-2 and no G3+ rectal toxicities in the no-RHS group. CONCLUSIONS: Absolute improvement in rectal V75% and prostate V100% was significant with RHS in PC patients undergoing salvage HDR-BT, but clinical benefit is marginal.