Abstract Book

ESTRO 37

S562

Purpose or Objective The ImagingRing System (medPhoton, Salzburg, Austria) is a novel and unique X-ray planar and cone beam computed tomography system for in room imaging in particle therapy. The aim of this study was to establish a Monte Carlo model of the ImagingRing System for future research on scatter effects. Material and Methods The X-ray head was modelled using the Monte Carlo toolkit GATE (v8.0, GEANT4 Application for Tomographic Emission) and GEANT4 (v.10.3). In a first step experimental characterization was performed. Half- Value-Layer (HVL) values in aluminum were determined with a variety of filtration levels and types using the NOMEX multimeter (PTW, Freiburg, Germany). Next, two- dimensi onal dose distributions were measured using the scintillation based LYNX detector (IBA, Schwarzenbruck, Germany). The physical dimensions of the electron focal spot on the anode were measured using both a dedicated slit camera (PTW, Freiburg, Germany) and the detector of the ImagingRing System. The tungsten anode and its surrounding glass and oil, as well as the polycarbonate exit cone, were directly modelled using GATE, while primary collimator and flattening filter were imported from vendor supplied CAD-files. The energy spectrum was tuned by approximating the energy of the electron beam by a linear combination of discrete energies. The resulting MC based HVLs in aluminum were compared to experimental data. The best approximation for the energy spectrum was determined by minimizing the relative deviations between measured and simulated HVL. In addition, the two-dimensional dose distribution in absence of a flattening filter was simulated, and the position and size of the electron focal spot were thereby investigated. Results The average deviation between measured and simulated HVLs was within 3% in the whole clinical energy range between 80 to 120 keV. This agreement was therefore within the tolerance of the measurements. Figure 1 shows the dose distribution comparison in vertical direction. The heel effect stemming from the anode is clearly visible in both the experimental and the MC setting. The size of the simulated electron focal spot agreed within 1% with the experimentally measured data.

Results Isocenter positioning uncertainty on the EPID was < 0.1 mm. Beam angle alignment accuracy was 0.3° for both directions. Focal spot alignment accuracy was < 0.5 mm. MLC absolute position accuracy was < 0.5 mm. Gantry isocenter locus had a radius of < 0.4 mm. Conclusion The presented results are a subset of all performed tests and measurements. All were within tolerances as defined by standard protocols. Most of these tests are easy to perform and will therefore be repeated periodically to establish sufficient statistics for evaluation of the long- term performance. In conclusion, the Elekta MRI-linac geometric alignment is suitable for clinical treatments. PO-1007 Monte Carlo modelling of the ImagingRing System – a new method for realistic X-ray distribution N. Reisz 1 , P. Kuess 1,2 , H. Fuchs 1,2 , P. Steininger 3,4 , I. Messner 3 , A.D. Law 3 , H. Deutschmann 3,4 , M. Stock 5 , A. Ableitinger 5 , D. Georg 1,2 1 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria 2 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 3 medPhoton GmbH, Medical Physics, Salzburg, Austria 4 Paracelsus Medical University, Institute for Research and Development on Advanced Radiation Technologies radART, Salzburg, Austria 5 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria

Conclusion A GATE based X-ray head model was established that accurately resembles experimental measurements. The presented method was shown to provide a realistic X-ray distribution, enabling the estimation of imaging doses when implementing new clinical protocols or predict the impact of technical changes of the X-ray source. The