ESTRO 36 Abstract Book

S493 ESTRO 36 _______________________________________________________________________________________________

Oncology, Dresden, Germany 7 Institute of Radiooncology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Purpose or Objective In neuro-oncology, 3 Tesla (3T) MRI is the current clinical standard for tumor localization, radiotherapy volume delineation and stereotactic (radio)surgery, sometimes complemented by amino acid PET imaging. With superior SNR and image resolution, anatomical 7T MRI can visualize micro-vascularization in glioblastomas potentially allowing improved target volume delineation. However, concerns about geometrical distortion (GD) with increasing field strength (B 0 ) are detrimental for applications of 7T MR in image-guided interventions. For high-precision treatment strategies, the spatial integrity of anatomical images needs to be warranted within ±1mm. The aim of the study was to evaluate B 0 - and sequence- related GD in clinically relevant 7T sequences and compare it to equivalent 3T sequences and CT images Material and Methods To quantify B 0 - and sequence-related GD in T1-GRE, T1- TFE, T2-TSE, T2-TSE FLAIR on 7T and 3T sequences, a dedicated anthropomorphic head-phantom (CIRS Model 603A) was used. The phantom is composed of bone-/soft- tissue equivalent materials and contains a 3D grid (3mm rods spaced 15mm apart). System-based distortion correction methods were applied to restore the gradient uniformity of 3T and 7T. For all CT and MR images, 436 points of interests (POIs) were defined by manual reconstruction of the 3D grid points in the respective images. GD was assessed in 3 ways. Firstly, global GD was estimated by the mean absolute difference (MAD global ) between the measured and the true Euclidian distances of all unique combinations of POIs, independent of location within the phantom. Secondly, local GD was assessed by MAD local between the measured and the true Euclidian distances of all POIs relative to the magnetic field isocenter. Thirdly, a distortion map was created by evaluating 3D displacement vectors for each individual grid point in 3T and 7T images ranged from 0.19−0.75mm and 0.27−1.91mm, respectively, and was more pronounced than in CT images. CT was not entirely free of GD with MAD global ranging from 0.14−0.64mm. B 0 -related GD was larger in 7T than in 3T MRI with MAD local ranging from 0.21-1.81mm and 0.11-0.73mm, respectively (p<0.05). MAD local increased with increasing distance from the magnetic isocenter and largest GDs were noted at the level of the skull (Fig. 1). MAD local was <1mm for all sequences up to 68.7mm from the isocenter. Sequence- related GD at 7T was prominent in T1-TFE and significantly differed from other 7T sequences (p<0.001). Figure 2 shows an anisotropic distribution of GD in T1-TFE with increasing GD along the frequency-encoding direction Results MAD global

Conclusion System-related GD was present in all 3T and 7T MR images but remained within the 2mm tolerance limit. Near the magnetic isocenter, 7T anatomical images showed no difference in geometric reliability to 3T MR images. Careful selection of 7T sequences and judicious use of GD correction methods can warrant the geometrical quality required for incorporation of 7T MR into image-guided interventions PO-0895 MRI-based analysis of volumetric changes of healthy brain tissue in glioma patients after photon RT A. Gommlich 1,2,3 , H. Wahl 4 , F. Raschke 2 , M. Baumann 1,2,3,5,6 , M. Krause 1,2,3,5,6 , E.G.C. Troost 1,2,3,5,6 1 Institute of Radiooncology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany 2 OncoRay - Center for Radiation Research in Oncology, Translational and Clinical Radiation Oncology, Dresden, Germany 3 NCT - National Center for Tumor Diseases- partnersite Dresden- Germany, partnersite, Dresden, Germany 4 Institute of Neuroradiology- University Hospital Carl Gustav Carus, Institute of Neuroradiology, Dresden, Germany 5 University Hospital Carl Gustav Carus, Department of Radiotherapy and Radiation Oncology, Dresden, Germany 6 DKTK - German Cancer Consortium, Partnersite, Dresden, Germany Purpose or Objective State-of-the-art Linac-based photon beam irradiation achieves highly conformal target volume coverage in glioma patients, but is also known to cause side-effects to surrounding tissues and organs. Apart from subjective measures (e.g., questionnaires, function tests) objective means to quantify tissue damage, e.g., anatomical or functional magnetic resonance imaging (MRI) are urgently needed to compare different treatment techniques and beam qualities (e.g., protons vs. photons) and to develop predictive measures for optimal sparing of normal brain tissue. As initial part of our program for dose-dependent spatial mapping of structural and functional radiation induced brain damage, we assessed here a retrospectively collected MRI-dataset in order to potentially detect volumetric changes of the healthy brain tissue (gray and white matter) in the non-affected hemisphere of glioma patients treated with photon irradiation.