ESTRO 36 Abstract Book

S796 ESTRO 36 2017 _______________________________________________________________________________________________

patient. The MCNP5 Monte Carlo code was used for simulation of the phantoms, applicators, and the sources. Results The results of this study showed no significant differences between the results of superposition method, and the MC simulations for different dosimetry points. The difference in all important dosimetry points were found to be less than 4%. The maximum dose differences were found at the tip of the detectors. Conclusion According to the results, the superposition method, adding the dose of each source obtained by the TG-43 algorithm, can estimate the dose to point A, B, bladder,and rectum points with good accuracy. EP-1504 Monte Carlo modeling of non-isocentric proton pencil beam scanning treatments A. Elia 1,2 , L. Grevillot 1 , A. Carlino 1,3 , T. Böhlen 1 , H. Fuchs 1,4,5 , M. Stock 1 , D. Sarrut 2 1 EBG MedAustron GmbH, Medical Department, A-2700 Wiener Neustadt, Austria 2 CREATIS- Université de Lyon- CNRS UMR5220- Inserm U1044- INSA-Lyon- Université Lyon 1, Centre Léon Bérard, 69007 Lyon, France 3 University of Palermo, Department of Physics and Chemistry, 90128 Palermo, Italy 4 Medical University of Vienna / AKH, Department of Radiation Oncology, Vienna, Austria 5 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria Purpose or Objective Monte Carlo (MC) calculation is the gold standard to support dose calculation analytically performed by Treatment Planning Systems (TPS). This work is built upon a preliminary beam model of a fixed beam line based mainly on measurements performed at isocenter. For non- isocentric treatments, accurate description of beam spot size for reduced air-gaps is of paramount importance for accurate treatment planning. This work extends the previous beam model based on final medical commissioning data, with special emphasis on beam optics modeling in non-isocentric conditions. Material and Methods GATE 7.2 based on GEANT4 10.02, using physics-builder QBBC_EMZ and both range cut and step limiter of 0.1 mm were used. Mean energy and energy spread were optimized in order to match the clinical range (R80) and the Bragg peak width measured in water. An initial set of beam optics parameters (beam size, divergence and emittance) was predicted at nozzle entrance (1.3 m upstream the isocenter) for five key energies. At this step of the study, a symmetrical proton pencil beam was considered. A sensitivity study in order to understand the influence of beam optics parameters at nozzle entrance on the spot size in air for different air gaps was performed. The beam optics parameters were then adjusted empirically, in order to reach 1 mm in absolute deviation or 10% in relative deviation within a treatment area (defined from 58 cm upstream the isocenter to the isocenter). Eventually, optical parameters were extrapolated for 20 clinical energies. Results Differences obtained between simulated spot sizes and the measured spot sizes seem to be due to systematic differences in the modeling of beam scattering through the nozzle and air gap. These differences are most probably due to combined intrinsic uncertainties from Multiple Coulomb Scattering (MCS) algorithm and nozzle geometry implemented in the simulation. The achieved agreement between measured and simulated spot FWHM is within clinical tolerances of 1 mm in absolute deviation

Conclusion Brass bolus may be used for surface dose enhancement in external beam radiotherapy with megavoltage photons. The surface dose increased from 20 % to 57 % of dose at d- max for a 10 cm x 10 cm 6 MV field. The non-uniform surface dose distribution should have minimal clinical impact for multi-fraction radiotherapy regimes where multiple layers and the random orientation of brass links relative to skin surface will vary with daily setup. EP-1503 The effect of tandem-ovoid applicator on the dose distribution in GYN brachytherapy using Ir-192 M.H. Sadeghi 1 , A. Mehdizadeh 1 , M. Tafi 1 , R. Faghihi 1 , S. Sina 2 , A.S. Meigooni 3 , A. Shabestani Monfared 4 1 Shiraz University, nuclear engineerning department, Shiraz, Iran Islamic Republic of 2 Shiraz University, Radiation Research Center, Shiraz, Iran Islamic Republic of 3 Comprehensive cancer center of Nevada, Las Vegas- Nevada, USA 4 Babol University of Medical Sciences, Babol, Iran Islamic Republic of Purpose or Objective The dosimetry procedures by simple superposition accounts only for the source shield, and does not take in to account the attenuation of photons by the applicators. The purpose of this investigation is estimation of the effects the tandem ovoid applicator on the dose distribution inside the phantom by MCNP5 Monte Carlo simulations. Material and Methods In this study, the superposition method is used for obtaining the dose distribution in the phantom for a typical GYN brachytherapy. Then the sources are simulated inside the tandem ovoid applicator, and the dose at points A, B, bladder and rectum was compared with the results of supper position. The exact dwell positions, and times of the source, and positions of the dosimetry points were determined from images of a