ESTRO 38 Abstract book

S1118 ESTRO 38

EP-2037 Digital phantom for evaluating the dosimetric impact of MRI Geometric inaccuracy in MR only based RT T. Torfeh 1 , R. Hammoud 1 , S. Paloor 1 , S. Aouadi 1 , N. Al‐ Hammadi 1 1 National Center for Cancer Care & Research, Radiation Oncology, Doha, Qatar Purpose or Objective Magnetic Resonance Imaging (MRI) is increasingly being used in radiotherapy applications for tumor delineation and tracking in the presence of respiratory motion. The purpose of this work is to investigate the impact of magnetic resonance distortions on dose distributions during respiratory motion. Material and Methods An in‐house motion platform and a control point based phantom were used to calculate an MR distortion map during motion for a Cine sequence which is the standard sequence for target delineation in liver cancer RT. The calculated distortion map was used to distort original images generated from a 3D virtual phantom as shown in figure 1 which is composed of a C shape target of 5 and 7 mm of inner and outer diameters respectively. This C shape is surrounding a spherical target shape of 5 mm diameter. Another ellipsoidal organ shape with a width of 6 mm and a height of 5 mm is also included. A dataset of 42 dicom images was finally generated with a resolution of 512X512 pixels, a 3 mm slice thickness, a 0 mm gap between slices and a pixel size of 0.97x0.97 mm. Targets on the original and distorted datasets were automatically delineated. A treatment plan for liver cancer treatment was computed on EclipseTM TPS using the distorted dataset. This plan was then copied on the original dataset and the dose distribution was analysed. A highly conformal volumetric modulated arc therapy (VMAT) technique using multiple noncoplanar arcs was used. Dose Volume Histograms (DVH) including D 95 , D 50 D min and D mean were benchmarked for accuracy measurements. Results The mean magnitude of the geometric distortion was 0.5, 0.7 and 0.9mm for radial distances of 50, 100 and 150 mm respectively. Blurring was observed during motion causing an increase in the FWHM of ≈30%. Our results showed differences of less than 5% in the DVH parameters distorted and undistorted treatment plans. Results also showed that the difference in the DVH is sensitive to the size and position of the tumor. Higher differences were recorded as the distance to the isocentre increases and/or the tumor size decreases. Conclusion The present work is a preliminary study aimed at putting in place an infrastructure allowing addressing the dosimetric impact of image deformation with any anatomy of interest. Further investigations using more realistic shapes will be carried out with the idea of producing 3D patient specific models for SBRT evaluation. EP-2038 Use of deformable image registration for automatic outlining of the rectum S. Hunt 1 , S. Thomas 1 , J. McClelland 2 , K. Harrison 3 , C. Rose 1 , J. Scaife 4 , M. Sutcliffe 5 , N. Burnet 4 , R. Jena 6 1 Cambridge University Hospitals NHS Foundation Trust, Medical Physics Box 152, Cambridge, United Kingdom ; 2 University College London, Centre for Medical Image Computing, London, United Kingdom ; 3 University of Cambridge, Department of Physics, Cambridge, United Kingdom ; 4 University of Cambridge, Department of Oncology, Cambridge, United Kingdom ; 5 University of Cambridge, Department of Engineering, Cambridge, United Kingdom ; 6 Cambridge University Hospitals NHS Foundation Trust, Oncology, Cambridge, United Kingdom

Results The experimental results in the phantom are shown in Figure 1. Complex MR images are shown on one coronal slice for different dynamic acquisitions. These images show a distinct MR contrast given by the FMs for each dynamic. A CT image is shown as comparison. The differences between the distances between all FMs measured using MR and CT are within 1mm, proving correct FMs detection. The CT and MR images of the prostate with implanted FMs are shown in Figure 2 for one patient. The same variable contrast of the FMs can be seen dependent on the RF phase increment used. FMs can be clearly seen on the acquired images. Similar results were observed in the other 7 patients. The < 1 mm differences in in the distances between all FMs measured using MR and CT proves correct FMs detection. Conclusion We have presented a method for direct FMs visualization at the MR console. The method is based on phase‐cycled bSSFP imaging, providing different contrast of FMs dependent on RF phase increment used. This method does not require any additional post processing or software and can be easily done directly at the scanner. This is relevant especially for MR‐only prostate RT.