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Curative radiotherapy for gynaecological cancer frequently results in bowel toxicity. Bowel loop motion is not taken into account in radiotherapy planning, while it may influence the actual dose received by individual bowel loops. Our primary aim is to analyse bowel loop motion before External Beam Radiotherapy Treatment (EBRT), after EBRT, and shortly before brachytherapy treatment with an applicator in situ. Our secondary aim is to analyse the effect of different patient and treatment characteristics that might influence bowel loop motion.

A total of 20 women with gynaecological cancer treated with definitive radiotherapy consisting of EBRT and brachytherapy were included. During the treatment period three 10-minute 3D cine-MRI scans with an image acquisition each 3.7s were obtained. The 1st before EBRT (MR1), the 2nd at the end of EBRT (MR2), and the 3rd shortly after the brachytherapy applicator insertion under general anaesthesia (MR3).

The median bowel loop motion during the 10-minute scan was computed for all voxels inside the bowel bag (Fig. 1). Motion Volume Histograms were created and from these, the M50% (minimum motion per 3.7s of 50% of the bowel bag volume) was computed as a measure for bowel loop motion (Fig. 1).

The difference in M50% before and after EBRT (MR1 vs MR2), and the influence of fasting, general anaesthesia, and the brachytherapy applicator (MR2 vs MR3) on M50% were analysed by the Wilcoxon’s signed rank test. In addition, M50% before EBRT (MR1) was compared between patients with and without previous abdominal surgery and between patients with a BMI ≤25 vs >25 kg/m2 at baseline. Furthermore, the influence of chemotherapy and BMI on the relative difference in M50% before and after EBRT (MR1 and MR2) were analysed by Mann-Whitney U tests.

The median M50% (in mm per 3.7 seconds) of all patients was 1.7 for MR1, 1.1 for MR2, and 0.7 for MR3, showing a significant decrease in M50% between consecutive scans (p=0.001 for MR1 and MR2, and p=0.008 for MR2 and MR3)(Fig. 2AB). Neither previous abdominal surgery nor BMI were associated with baseline M50% (Fig. 2C). In addition, BMI was not associated with decreased M50% at the end of EBRT (Fig. 2D). There was, however, a relative decrease in M50% of 31% at the end of EBRT in patients who received chemotherapy, which was only 3% in patients who did not receive chemotherapy (p=0.063)(Fig. 2D).

In our small cohort of patients treated for gynaecological cancer, the median bowel loop motion decreased at the end of EBRT, and this was even more pronounced at time of brachytherapy with an applicator in situ. In addition, there is a trend towards a further decelerated bowel loop motion after concurrent chemotherapy. This study strengthens the need for further research aiming to control for bowel loop motion in our future pelvic radiotherapy planning.