ESTRO 2021 Abstract Book

S171

ESTRO 2021

Conclusion Rescanned IMPT_3D was found to be suitable to treat thoracic tumours in free breathing for motion up to 16 mm, in both pre-clinical 4DREM and clinical 4DREAL. Our comprehensive 4DREM was able to accurately estimate the clinical delivery. Therefore, this method can be safely used as a pre-clinical treatment course quality control tool for IMPT planning protocols. PH-0264 Quantification of organ motion during prolonged breath-holding using deformable image registration E.R. Gunwhy 1 , Z. van Kesteren 1 , M.J. Parkes 1 , M.F. Stevens 2 , J.G. van den Aardweg 3 , G. van Tienhoven 1 , A. Bel 1 , I.W. van Dijk 1 1 Amsterdam UMC – location AMC, Department of Radiation Oncology, Amsterdam, The Netherlands; 2 Amsterdam UMC – locations AMC and VUmc, Department of Anaesthesiology, Amsterdam, The Netherlands; 3 Amsterdam UMC – location AMC, Department of Pulmonology, Amsterdam, The Netherlands Purpose or Objective In radiotherapy, multiple short breath-holds (SBHs; 30s) are used to diminish organ motion, thereby mitigating effects of radiation-associated toxicity. Mechanical ventilation can safely extend breath-holds to prolonged breath-holds (PBHs; ≥300s) by combining 60% pre-oxygenation with hypocapnia. Due to gas exchange, gradual lung deflation occurs during all breath-holds, causing the chest to shrink and the diaphragm to drift upward, inducing motion of the surrounding anatomy. In this study, we establish a novel method for quantitatively assessing organ motion during PBHs upon MR images, using deformable image registration (DIR). We use this method to quantify the motion of the right diaphragm dome and the right kidney during PBHs, and investigate any correlation in this motion. Materials and Methods Seven healthy volunteers were trained to perform PBHs at end-inspiration (PIBH) and end-expiration (PEBH). 3D cine-MRIs of the upper abdomen were acquired during both manoeuvres. From each acquisition, seven dynamics (chosen for time-efficiency as the first dynamic (t=0s), three dynamics over the course of a PBH, and the three last dynamics (~15s apart)) were registered using groupwise DIR implemented in Elastix. This produced a set of deformation vector fields (DVFs) describing the global anatomical motion occurring during a PBH. Regions of interest (ROIs) were defined by delineating the liver, right diaphragm dome and right kidney upon the first dynamic. The distribution of voxel displacements within each ROI along right-left, anterior- posterior, and inferior-superior directions, and the vector magnitude (VM) of motion was then visualised using