Reduction of bowel loop motion during radiotherapy for gynaecological cancer assessed by 3D cine-MRI.

Introduction: Gastrointestinal toxicity is a major concern after radiotherapy for gynaecologic cancer. Strong dose-effect relationships for the bowel and individual loops are lacking, which might be explained by challenges in accurate radiation dose estimation due to continuous motion of the bowel loops. The objective of this study was to evaluate whether changes in bowel loop motion occur over the course of radiotherapy, with the use of three-dimensional (3D) cine-MRI. Methods and Materials: This study prospectively enrolled patients with gynaecologic cancer undergoing primary radiotherapy with curative intent. To measure bowel loop motion, three 3D cine-MRI scans were acquired: before external beam radiotherapy (EBRT), in the last week of EBRT, and on the day of brachytherapy. A MR image was acquired every 3.7 s during a 10-minute scan. Deformable image registration was used to generate a motion map and a motion volume histogram. Changes in bowel loop motion over the course of treatment were assessed by the Wilcoxon signed-rank test using the median bowel loop motion in millimetres per 3.7 s of 50% of all voxels. Results: For 15 of the 22 patients, 3D cine-MR scans were available at all three time points. Median bowel loop motion per 3.7 s was 1.8 mm (range 0.6-3.3 mm) before EBRT, 1.0 mm (range 0.6-2.3 mm) in the last week of EBRT, and 0.7 mm (range 0.4-1.1 mm) at brachytherapy. Most patients (14 out of 15) showed reduced bowel loop motion in the last week of EBRT compared to the first MR, with the lowest motion at brachytherapy in 13 patients (87 %). Median bowel loop motion decreased significantly with each subsequent MRI scan ( Conclusions: The results of this study demonstrate a statistically significant decrease in bowel loop motion over the course of primary radiotherapy for gynaecological cancer.