ESTRO 36 Abstract Book

S495 ESTRO 36 2017 _______________________________________________________________________________________________

images showed maximal 95th percentile distortions of 0.39, 0.32, 0.28, and 0.25 pixels for the neck, lungs, thorax with the lungs excluded, and pelvic region, respectively. In order to accommodate other field strengths and bandwidths, normalized displacement values were also simulated for these body regions. Table 1: Simulated displacement values normalized to field strength and bandwidth [pixels * BW / B0]

stomach and suppress signal from the stomach filling. Gadolinium was used as intravenous contrast agent for the patient only. Results Visual inspection showed that for TD and staging, T2w exhale respiratory navigator triggered, rather than a respiratory sensor, provides excellent contrast with limited motion artifacts. For TP, mDixon with a large FoV, a high signal to noise ratio (SNR) and HR in one BH is feasible. For motion modeling, 4D T2w MRI resulted in a good slice ordering, high SNR and HR. For MM, TSE Cine- MRI gave a good SNR and HR without artifacts. For staging and treatment response monitoring, FB DWI with an increased number of averages gave the best result, only limited motion and susceptibility artifacts were visible. FB 4D THRIVE DCE resulted in a good temporal resolution and limited motion artifacts. (Figure 1)

Conclusion The 95th percentile of the patient-induced susceptibility distortions can be kept below 0.5 pixels for a 3 T system and 440 Hz bandwidth. With the provided normalized data, distortions for other field strengths and bandwidths can be calculated. The developed simulation software can also be used to quickly and easily estimate the susceptibility-based distortions from a given series of patient CT images that are converted into susceptibility values, or directly from a susceptibility map. PO-0904 Development of an MRI-protocol for radiotherapy treatment guidance in gastric cancer V.W.J. Van Pelt 1 , M.F. Kruis 1 , T. Van de Lindt 1 , L.C. Ter Beek 2 , M. Verheij 1 , U.A. Van der Heide 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands 2 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiology, Amsterdam, The Netherlands Purpose or Objective Because of the superior soft-tissue contrast of MRI, integration of MRI in pre-operative radiotherapy (RT) for gastric cancer, is expected to improve the identification of shape and position of the target volume. MRI of the stomach is technically challenging due to respiratory, cardiac and bowel motion. In this study we therefore developed a scan protocol consisting of anatomical and functional sequences for staging and target delineation (TD), for treatment planning (TP) including motion modeling and for intra-fraction motion monitoring (MM). Material and Methods For staging and TD we compared high resolution (HR) T2- weighted (T2w) turbo spin echo (TSE) MRI, applying either navigator or respiratory sensor triggering during the exhale position of the diaphragm to reduce motion artifacts. For TP, the feasibility of a fast 3D HR mDixon with a large Field of View (FoV) within one exhale breath- hold (BH) was evaluated. For motion modeling, a 4D T2w MRI with retrospective self-sorting reconstruction was tested for robustness [1] . For intra-fraction MM, 2D T1w dynamic turbo field echo (TFE), fast field echo (FFE) and TSE Cine-MRI with a refocusing pulse were compared. For staging and treatment response monitoring, a single-shot echo planar Diffusion Weighted Imaging (DWI) was tested using b-values of 0, 200 and 800 s/mm², applying either free-breathing (FB), BH, navigator or respiratory triggering. For Dynamic contrast enhanced (DCE) MRI, FB T1w spoiled gradient echo, 4D mDixon and 4D THRIVE with keyhole technique were compared. Subtraction images were reconstructed to show the uptake of intravenous contrast agent. The sequences were tested on healthy volunteers and one patient using a 3T MR system (Ingenia; Philips Healthcare, The Netherlands) and reviewed by two MR-experts and one radiologist. Pineapple juice was given orally to distend the

Conclusion We developed a comprehensive imaging protocol for the entire RT guidance treatment chain. The complex motion artifacts were reduced by applying either navigator triggering or BH techniques. The new gastric cancer protocol looks therefore very promising and will be used for MR-based delineation for RT. [1] van de Lindt T, et al. ESTRO 35 2016 Abstract-book:PV- 0325; 171-172 PO-0905 (Semi-)Automatic contouring strategies for rectal boost treatment on the MR-Linac C.N. Nomden 1 , M.P.W. Intven 1 , A.N.T.J. Kotte 1 , I.H. Kiekebosch 1 , S. Mook 1 , I.M. Jürgenliemk-Schulz 1 , G.G. Sikkes 1 , L.T.C. Meijers 1 , E.N. De Groot 1 , G.H. Bol 1 , B. Van Asselen 1 , L.G.W. Kerkmeijer 1 , B.W. Raaymakers 1 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective The MR-Linac enables online treatment adaptations in response to changes in anatomy. This stresses the need for fast contouring strategies for target and OARs. Unfortunately, manual delineation in an online workflow is time consuming and therefore suboptimal. The purpose of this study was to investigate whether automatic and semi-automatic contouring strategies result in clinical acceptable contours for an online workflow on the MR- Linac. Material and Methods Fifteen patients with early staged rectal cancer were scanned at an 1.5T MRI for five consecutive days. The scan consisted of a T2 weighted MRI; voxelsize 0.63x0.63mm, slice thickness 4 mm and a total number of 30 slices. For each scan the following contours were delineated by an experienced radiation oncologist (manual contours): GTV, mesorectum, bladder, rectum, sphincter, gynecological volume (in one contour: vagina, cervix and uterus), left and right femur. The manual contours of the first day were used as input for the automatic/semi-automatic contouring strategies. Automatic contouring software (ADMIRE research v1.13.5 Elekta AB, Stockholm, Sweden) was used for MR based deformable registration and