ESTRO 37 Abstract book

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ESTRO 37

differed significantly between the 20% and 70% phases was 5 and 4 for left and right lungs, respectively, there were 4 common features and only 1 uncommon feature. The number of features that differed significantly between the 0% and 50% phases and the 20% and 70% phases was 2. Conclusion The respiration state caused a significant difference in certain CT radiomic features and the influence of breathing motions should be considered in feature extraction. EP-2084 Feasibility of CBCT positioning in an MRI only Prostate Treatment Planning A. Garcia Perez 1 , S. Reigosa Montes 1 , A. Lopez Medina 1 , A.G. Teijeiro Garcia 1 , J. Vazquez Rodriguez 1 , F.J. Salvador Gomez 1 , A. Gonzalez Castro 2 , M. Martinez Agra 2 , V.M. Munoz Garzon 2 1 Hospital do Meixoeiro, Radiofisica y PR, Vigo Pontevedra, Spain 2 Hospital do Meixoeiro, Radioterapia, Vigo Pontevedra, Spain Purpose or Objective The MRI-only workflow aims to base the treatment planning solely on magnetic resonance imaging (MRI), hence excluding the traditional computed tomography (CT) scan. Several studies 1-2 have mainly focused on the equivalence in calculated dose based on CT and synthetic CT (sCT or MRCAT: MR for Calculating ATtenuation, in Philips nomenclature). The aim of this study is to compare if any difference in patient positioning based on conventional CT or based on sCT is found, when the patient is accurately relocated based on Cone Beam CT (CBCT). Material and Methods We performed an MRI scan (Philips Ingenia 3T) and a conventional CT scan (Philips Big Bore) for two prostate patients. The first patient had two different treatment phases and they were considered independently because they had different isocentres. We compared the averaged Hounsfield units (HU) inside the external contour for both datasets, because dose calculation is basically affected by HU of each voxel. We calculated the Dice similarity coefficient (DSC) - defined as the ratio between the double of the intersection of two volumes and the sum of both volumes - of the external volume and the bone volume, obtained both by automatic segmentation in CT and in sCT using Eclipse (Varian). Finally, we registered the daily CBCT firstly with the CT and then with the sCT computing the shift between the two datasets along the 3 axes. The positioning error was separated in systematic (Σ) and random error 4 (σ). Results The averaged HU for the external volumen were CT:4 ± 171; sCT: -17 ± 178 for the first patient, and CT: 12 ± 189; sCT: -22 ± 150 for the second one. In both cases, the difference in HU implies a very small difference in relative electron density, which is more important in dose calculation (less than a 2%). The DSC for external volumen was: 0.98 for both patients, and for the bone volume was 0.81 for the patient 1 and 0.85 for the patient 2. In all cases the DSC value is much greater than 0.7, which is considered the limit for a good overlap between two volumes 3 . The systematic errors and random errors are shown in the table for the three cases. In the figure the translational displacement calculated from sCT and CBCT is represented versus the displacement calculated from CT and CBCT. The values are almost the same and, as expected, the slopes of the linear regressions of data are very close to 1.

Conclusion sCT can be used in an MRI only workflow radiotherapy department, not only for patient delineation, and dose calculation, but also for treatment positioning in prostate cancer treatment. 1 Persson, E. et al . Int J Radiat Oncol Biol Phys 2017 99: 692-700 2 Paradis E , et al. Int J Radiat Oncol Biol Phys 2015;93:1154-1161. 3 Zijdenbos AP, et al. IEEE Trans Med Imaging 1994;13: 716–24. 4 van Herk, M., et al. Int J Radiat Oncol Biol Phys, 2000 47: 1121-1135. EP-2085 Simplification of relative electron density information in synthetic CT images for dose calculation J. Handrack 1,2 , M. Bangert 1,2 , C. Möhler 1,2 , T. Bostel 2,3,4 , S. Greilich 1,2 1 German Cancer Research Center DKFZ, Department of Medical Physics in Radiation Oncology, Heidelberg, Germany 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany 3 University of Heidelberg, Department of Radiation Oncology, Heidelberg, Germany 4 German Cancer Research Center DKFZ, Clinical Cooperation Unit Radiation Oncology, Heidelberg, Germany Purpose or Objective The interest in using magnetic resonance (MR) images for dose calculation in radiotherapy is currently growing quickly. However, MR images lack essential information on electron density (ED), which is studied to be overcome