ESTRO 2020 Abstract book

S1010 ESTRO 2020

Next stage is to test the technique for a beam passing through different tissues. Developments are ongoing to construct a spectrometer prototype. The ultimate goal is a clinically compliant system for on-line, real-time range verification. PO-1729 Quality assurance of deformable image registration for bowel motility quantification K.J. Nelissen 1 , D.L.J. Barten 2 , J.J. Laan 2 , H. Westerveld 2 , A. Bel 2 , Z. Van Kesteren 2 1 VU University, Faculty of Science - Master Biomedical Technology and Physics, Amsterdam, The Netherlands ; 2 Amsterdam UMC - location AMC, Department of Radiotherapy, Amsterdam, The Netherlands Purpose or Objective In ~5-13% of gynaecological cancer patients, severe late bowel toxicity develops as a side effect of radiation treatment. Higher radiation dose during treatment leads to an increase in bowel toxicity. Therefore, taking bowel motion (motility) into account might reduce dose to the bowel. Deformable image registration (DIR) could be a suitable tool for quantifying this motility. Before the implementation of this technique, specific validation of the applied DIR method needs to be developed as suggested by the AAPM Task group 132 in 2017. In the context of bowel motility calculation, many DIR operations are performed and an automized method is needed for efficient quality assurance (QA). The purpose of this study is to introduce and analyze a QA method for DIR validation applied to quantification of bowel motility in gynecological cancer patients, and the determination of cutoff values for automatic selection of errors in registrations. Material and Methods Seventeen gynaecological cancer patients treated with external beam radiotherapy and brachytherapy were included in this study. Patients received up to four MRI- sessions during their treatment. One motility MRI acquisition consisted of 160 dynamics, acquired with a balanced turbo field echo MRI sequence of 3.7s/3D image (TE=1.39ms and TR=2.8ms), resulting in a 10-minute scan. Motility mapping in 3D was done by applying DIR on subsequent dynamics, using a B-spline transformation model. From this registration 159 corresponding deformation vector fields (DVFs) were obtained, which described the translation for each voxel between two dynamics. Four voxel layers were excluded on the borders of each dynamic to avoid possible artefacts and truncation. As QA metrics, the Harmonic energy (HE) and Jacobian determinant (JAC) were used to calculate the following values for each DVF: the mean HE (µHE), percentage of voxels with JAC>2 (JAC2) and percentage of voxels with JAC<0 (JAC0). The cutoff value for these QA metrics was determined at median ±2*inter quartile range (IQR) over all registrations. As validation, a selection of the 159 registrations per patient were visually verified. Results In total 42 MRIs were done, resulting in 6678 registrations. Figure 1 shows the distribution of the QA metrics with the cutoff values determined at µHE=4.0, JAC0=3.9%, JAC2=4.6%. Visual inspection of registrations confirmed the efficacy of these values; Figure 2 shows three typical examples. Regions with high µHE per voxel coincided with regions containing registration errors. Finally, visual inspection performed on samples with QA metrics smaller and larger than the cutoff value, confirmed the expediency of these cutoff values to identify both registration errors and correct registrations. Conclusion The introduction of µHE, JAC2 and JAC0 as QA metrics with cut off values at median ±2*IQR enables the identification of registration errors. Therefore, these QA metrics and associated methods are promising for QA of the DIR method to quantify bowel motility.

PO-1730 Development of a framework to quantify bowel motility in 3D using MRI D. Barten 1 , Z. Van Kesteren 1 , J. Visser 1 , J. Laan 1 , H. Westerveld 1 , A. Bel 1 1 Amsterdam UMC - AMC, Radiotherapie, Amsterdam, The Netherlands Purpose or Objective Patients with locally advanced gynecological cancer are treated with external beam radiotherapy (EBRT) and brachytherapy (BT). As a consequence, a part of the bowel is irradiated, yielding risk of severe bowel toxicity. So far no clear dose-effect relationships are found for the bowel/sigmoid, possibly due to inter- and intrapatient bowel motility differences. In addition, it is unknown how and when the bowel is moving in and outside the high dose

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