ESTRO 36 Abstract Book

S499 ESTRO 36 _______________________________________________________________________________________________

Results The digital phantom simulations showed good agreement with analytical solutions, with only small discrepancies due to pixelation of the phantoms. For a bandwidth of 440 Hz at 3 T, the calculated distortions in the patient-based 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]

and to establish the performance of the approach on a much larger patient cohort. PO-0903 Patient-induced susceptibility effects simulation in magnetic resonance imaging J.A. Lundman 1 , M. Bylund 1 , A. Garpebring 1 , C. Thellenberg Karlsson 1 , T. Nyholm 1 1 Umeå University, Department of Radiation Sciences, Umeå, Sweden Purpose or Objective The role of MRI is increasing in radiotherapy. A fundamental requirement for safe use of MRI in radiotherapy is geometrical accuracy. One factor that can introduce geometrical distortion is patient-induced susceptibility effects. This work aims at developing a method for simulating these distortions. The specific goal being to objectively identify a balanced acquisition bandwidth, keeping these distortions within acceptable A simulation algorithm based on Maxwell’s equations and calculations of shift in the local B-field was implemented as a dedicated node in Medical Interactive Creative Environment (MICE), which is available as a free download. The algorithm was validated by comparison between the simulations and analytical solutions on digital phantoms. Simulations were then performed for four body regions using CT images for eight prostate cancer patients. For these patient images, CT Hounsfield units were converted into magnetic susceptibility values for the corresponding tissues, and run through the algorithm. limits for radiotherapy. Material and Methods

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

Figure 1: Simulated normalized local B-field for one of the patients [ppm].