ESTRO 2020 Abstract book

S343 ESTRO 2020

ADC resulted in smaller volumes compared to T2 weighted MRI in GTV delineation, reaching an almost perfect agreement between the two observers, a radiologist and a radiation oncologist. These preliminary results could suggest that incorporating the use of ADC maps may obtain GTVs different from those based only on T2 weighted images. The role of ADC could be evaluated as an option for target functional contouring, with an acceptable variability between different observers. This could be investigated in order to improve delineation reproducibility for image-guided adaptive radiotherapy. OC-0568 The effect of external beam radiotherapy on bowel motility in gynaecological cancer patients J. Laan 1 , D. Barten 1 , Z. Van Kesteren 1 , B. Pieters 1 , A. Bel 1 , H. Westerveld 1 1 University Medical Center Amsterdam, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Many gynaecological cancer patients develop bowel cramps and diarrhea during external beam radiotherapy (EBRT). Movement of the bowels (bowel motility) in- and outside the high-dose area may be an important factor to consider during treatment planning. However, bowel motility and its possible effect on bowel dose-volume parameters have never been investigated in this patient group. We developed a new method to visualize and quantify bowel motion in a non-invasive way based on 3D cine-MRI. The purpose of this study was to evaluate the feasibility of this new tool. Secondly, the differences in bowel motility before and after definitive pelvic EBRT for gynaecological cancer were investigated. Material and Methods Patients treated with definitive radiotherapy for gynaecological cancer were included in this prospective observational study. Patients were received radiotherapy to the pelvis with volumetric arc therapy (VMAT) to a dose of 45 Gy in 25 fractions. Before and after five weeks of EBRT a special 3D motility MRI scan was performed using a balanced turbo field echo MRI sequence (TE = 1.39 ms and TR = 2.8 ms). This sequence acquires 3D images in 3.7 seconds, called dynamics. During a 10 minute scan, 160 dynamics were acquired. Subsequent dynamics were deformable registered in order to determine the mean motion for each voxel between two dynamics. A motility map displaying the mean motion every 3.7 seconds within 10 minutes MRI acquisition was made (Figure 1). The median bowel motility (M50%), which is the median motility of the bowel bag per dynamic, was determined for each MRI session. A Wilcoxon signed-rank test was used to analyse the intra-patient differences in bowel motility before and after EBRT. Results Fifteen patients were included in this study. Figure 1 shows an example of a MRI motility map before and after EBRT in two patients. One patient (patient 10) had a large difference in bowel motility before and after EBRT, while the other patient did not (patient 6). For the entire group, the M50% of the bowel bag before and after EBRT ranged from 1.2mm to 5.3mm, and 0.6mm to 2.9mm every 3.7s, respectively. The motility map of the individual patients clearly shows sub regions in the bowel bag with even higher motility (≥10mm) (Figure 1). For the majority of patients (14 out of 15) the median motility (M50%) of the bowel bag was significantly higher before EBRT compared to after EBRT (Figure 2).

Conclusion