ESTRO 38 Abstract book

S1203 ESTRO 38

with four-dimensional information to assess tumor motion in time. The aim of this study was to determine tumor amplitude variation based on 4D CBCT data. Material and Methods Between May 2015 and August 2018 a total of 46 patients received lung SBRT treatment in 359 fractions based on 4DCT data. Abdominal compression was used in 8 patients to reduce tumor motion at lower lobe tumors. GTV was defined during treatment planning at least in 3 breathing phases to generate an ITV. Translational position corrections were determined using pre-treatment 4D CBCT at each fraction. A second 4D CBCT data set had been acquired after the treatment fraction to assess intra- fractional motion. Amplitude values in three directions were recorded both pre- and post-fractional. Results Population mean values (±SD) for pre-treatment amplitudes were: 0.12(±0.09) cm, 0.44(±0.4) cm and 0.24(±0.17) cm in Superior-Inferior (SI), Left-Right (LR) and Anterior-Posterior (AP) directions respectively for the free breathing cohort. Patients with abdominal compression the values were: 0.18(±0.1) cm, 0.87(±0.13) cm, 0.25 (±0.51) cm respectively. Post-fractional values were: 0.11(±0.09) cm, 0.4(±0.4) cm, 0.22(±0.15) cm for the free breathing and 0.18(±0.1) cm, 0.78(±0.12) cm, 0.24(±0.41) cm for the abdominal compression group. Conclusion Correlating pre- and post-treatment values in our patient cohort the complex immobilization systems combined with four-dimensional data are capable of handling tumor amplitude changes over a fraction and abdominal compression can mitigate tumor motion to a more manageable area which can lead to the reduction of the volume of ITV. EP-2178 Evaluation of a user-guided deformable registration workflow for multi-modal prostate imaging A. Lastrucci 1 , A. Barucci 2 , S. Pini 1 , S. Russo 1 , R. Barca 1 , M. Coppola 1 , S. Fondelli 1 , L. Paoletti 1 , F. Rossi 1 , P. Bastiani 1 , M. Esposito 1 1 USL Toscana Centro, Radiotherapy, Florence, Italy; 2 Ifac CNR, Medical Physics, Sesto Fiorentino, Italy Purpose or Objective Reg-refine is a tool that adds human knowledge to deformable image registration (DIR) process. Inthis study a CT-MRI deformable registration workflow for prostate Deformable and rigid alignments of axial CT and axial T2 weighted MRI scans of 8 patients were evaluated using MIM Maestro 6.7.10 (Mim Software Inc.). DIR workflow consisted of threerigid alignments, in the upper, middle, ad lower prostate part, defined by two users in consensus using Reg-refine and converted in local alignments by the DIR algorithm. Dice coefficient and Hausdorff distance werecomputed for evaluation of deformable and rigid alignments. Results Dice resulted (0.78 ±0.12; max 0.9, min 0.58) for rigid registration vs (0.89±0.05; max 0.94, min0.82). Hausdorff distance resulted resulted (11.7 mm ± 4.5 mm; max 17.1, min 5.4) for rigid registration vs(7.6 mm ± 4.1 mm; max 13.5 mm, min 3.5 mm). Box plots are reported in Figure 1. cancer was evaluated. Material and Methods Electronic Poster: RTT track: Imaging acquisition and registration, OAR and target definition

Figure 1: Box plot of Dice and Hausdorff distance (HD) for rigid and deformable CT-MRI alignments. Conclusion The DIR workflow evaluated in this study produced good results in all patients analyzed andwas able to improve the registration accuracy of rigid alignments. Reg–refine based DIR of CT and MRIscans can be used for prostate contouring. EP-2179 Estimation of intrafractional motion of intra- orbital optic nerve by MRI S. Tsuruoka 1 , Y. Hamamoto 1 , Y. Kuribayashi 2 , H. Inata 2 , T. Matsuno 2 , T. Mochizuki 1 1 Graduate School of Medicine Ehime University, Radiology, Ehime, Japan; 2 Saiseikai Imabari Hospital, Radiology, Ehime, Japan Purpose or Objective Optic nerve is one of the most significant organs at risk in radiotherapy for the intra-and/or peri-orbital tumors. Although intra-orbital optic nerve is likely to move with eye-movement, intrafractional motion of the intra-orbital optic nerve has not been well documented. To determine the optimal planning organ at risk volume, intrafractional motion of the intra-orbital optic nerve was measured by magnetic resonance imaging (MRI). Material and Methods A total of five normal volunteers were underwent MRI around the orbit. (1) One of five volunteers was underwent MRI with maximally oriented in outward, inward, upward, and downward direction at the time of both opening and closing the eyes. The displacement in each direction at three points (the adhesive part of optic nerve and eyeball, 10 mm and 20 mm from the adhesive part) was measured. (2) For all volunteers, displacement in lateral direction of the intra-orbital optic nerve from the position when gazed at the front was measured by axial cine-MR imaging. The maximum displacement of the intra-orbital optic nerve was recorded. Results (1) At the time of opening the eyes, the average of maximal displacement in four directions (outward, inward, upward, and downward) at the adhesive part of optic nerve and eyeball, 10 mm and 20 mm from the attachment were 5.20 mm (standard deviations [SD]; 0.97), 3.23 mm (SD; 0.54), and 1.23 mm (SD; 0.55), respectively. At the time of closing eyes, those were 4.11 mm (SD; 1.31), 1.91 mm (SD; 0.82), and 0.53 mm (SD; 0.24), respectively. (2) For the right eye, the average of the maximal displacement inward and outward were 2.8 mm and 2.5 mm, respectively. For the left eye, the average of the maximal displacement inward and outward were 2.6 mm and 3.0 mm, respectively. The mean amounts displacement of right and left eye on cine-MR imaging were 2.65 mm (SD; 1.14) and 2.78 mm (SD; 1.62), respectively. Conclusion Based on our results, intrafractional margins of the intra- orbital optic nerve needed at least 3 mm in outward, inward, upward, and downward direction. EP-2180 The effect of Rectal size and shape on Bladder deformation in Urinary Bladder Radiotherapy N. Hutton 1 , J. Callender 2 , D. Hutton 1 , L. Williams 1 , S. Wong 1 , H. Wong 1 , I. Syndikus 1