Effect of Hydrogel Rectal Spacer on Intrafraction Prostate Motion During Radiotherapy of Prostate Cancer Patients

Abstract

<h3>Purpose/Objective(s)</h3> Insertion of hydrogel rectal spacer may potentially stabilize the prostate by reducing intrafraction prostate motion. The purpose of this study was to investigate the effect of hydrogel rectal spacer on intrafraction prostate motion. <h3>Materials/Methods</h3> A retrospective review was conducted of prostate cancer patients treated from September 2015 to November 2018 who underwent external beam radiation therapy (EBRT) with intrafraction motion management using implanted electromagnetic transponders. There were 87 patients (73%) who received conventional fractionation, 16 (13%) moderate hypofractionation, 10 (8%) stereotactic body radiation therapy (SBRT) and 7 (6%) EBRT plus brachytherapy. The threshold for gating was set at 4 mm for conventional fractionation, 3 mm for moderate hypofractionation, and 2 mm for SBRT. Maximum prostate motion was measured by displacement outside of the gating threshold during treatment per fraction. Independent samples t-test was performed to compare prostate motion with and without hydrogel rectal spacer. <h3>Results</h3> We identified 120 patients with a total of 4447 fractions eligible for analysis. 49 patients (41%) were treated with hydrogel spacer and 71 (59%) without hydrogel rectal spacer. Mean displacement of prostate outside of the gating threshold was 4.9 mm (range, 2.1 – 12.8) and 5.6 mm (4.1 – 14.6) anteriorly, 2.6 mm (1.1 – 7.6) and 19.4 mm (4.0 – 25.8) posteriorly, 4.7 mm (2.2 – 8.3) and 12.5 mm (4.0 – 20.8) superiorly, 3.1 mm (1.1 – 8.2) and 14.2 mm (3.7 – 22.7) inferiorly, 4.3 mm (3.0 -12.4) and 4.5 mm (4.2 -7.6) left and 3.1 mm (3.1 – 4.1) and 4.3 mm (4.1 – 11.6) right for patients treated with and without hydrogel rectal spacer, respectively. There was a significant reduction of mean displacement in the posterior (2.6 mm vs. 19.4 mm, <i>P</i> = 0.04) and inferior direction (3.1 mm vs. 14.2 mm, <i>P</i> = 0.04) in patients with hydrogel rectal spacer compared to those without hydrogel rectal spacer during "beam on time". <h3>Conclusion</h3> The use of hydrogel rectal spacer can mitigate intrafraction prostate motion in posterior and inferior directions, suggesting a potential dosimetric advantage with the use of hydrogel rectal spacer. However, longer follow-up may be needed to confirm whether this translates into significant clinical benefit outweighing potential risk.