ESTRO 37 Abstract book

ESTRO 37

S563

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 Mont e 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.

impact of technical changes of the X-ray source. The developed method can be transferred to model other commercial X-ray units. Detailed MC based investigations of the head scatter to improve imaging quality is current work in progress. PO-1008 Commissioning of IMRT/VMAT on the novel Varian Halcyon™ R. De Roover 1 , K. Poels 2 , W. Crijns 2 , A. Nulens 2 , B. Vanstraelen 2 , K. Haustermans 1,2 , T. Depuydt 1,2 1 Catholic University of Leuven, Department of Oncology, Leuven, Belgium 2 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium Purpose or Objective The novel Varian Halcyon™ O-ring linac design allows for delivery of IMRT and VMAT treatments at increased speeds by employing an encapsulated fast rotating gantry and performant dual-layer multi-leaf collimation system. The Halcyon™ is delivered with a pre-configured treatment planning system (TPS). As one of the first institutions that have implemented this novel linac system, we present an extensive dataset on the treatment delivery quality acquired with a variety of For the IMRT/VMAT commissioning of the pre-configured system, international codes-of-practice of AAPM MPPG5 and TG-119 and external IROC audits, were completed by end-to-end (E2E) tests on anthropomorphic phantoms and validation of patient-specific QA procedures. End-to-end measurements were performed for head&neck VMAT, cranial VMAT, rectum VMAT and lung IMRT. Target structures and organs-at-risk for the different treatment indications were mapped on a set of CIRS anthropomorphic phantoms. Treatment plans were generated according to Halcyon-specific class solutions. Low dose MV cone-beam CT image guidance prior to treatment delivery was integrated in treatment planning and used for phantom setup. IMRT/VMAT dose distributions were measured using multiple EBT3 film in different orientations and was supplemented with point ionization chamber (IC) measurements. Patient-specific QA of the first patient group for each treatment indication was performed using portal dosimetry, an ArCHECK diode array measurement and EBT3 film dosimetry and an A1SL IC inserted in the MultiCube phantom. Film dosimetry was compared with calculation in a relative manner. All gamma agreement scores (γAS) were determined using 3%(local)/3mm criterion and a 10% lower dose exclusion threshold. The treatment time was measured as the total beam-on time of the IMRT/VMAT delivery. Results The acquired E2E dataset included eight sites with at least two different EBT3 film planes for each. Patient- specific QA was performed for 16 patients varied across the different indications, but mainly head&neck. A selection of representative E2E results for head&neck VMAT, lung IMRT and rectum VMAT are visualized in Figure 1. The local γAS between the predicted dose by the TPS and the film measurements ranged between 90.6% and 99.8%. The relative difference of the point measurements ranged between 0.3% and 2.3% dosimetric techniques. Material and Methods

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