ESTRO 36 Abstract Book

S807 ESTRO 36 _______________________________________________________________________________________________

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 and 10% in relative deviations for five key energies within the treatment area. As an example, FWHM in function of the air gap for three key energies are reported in Figure 1. Deviations observed are presented in Figure 2. Agreement achieved in terms of ranges in water is within 0.1 mm in absolute deviation for all the energies We extended a preliminary beam model based on a first predictions at nozzle entrance. The final beam model describes spot sizes within clinical tolerances of 1 mm/10%, for the treatment area considered. Detailed validation of this MC beam model is on-going and is based on beam scattering of the core pencil beam, transverse dose profiles in the low dose region (nuclear halo), absolute dose in reference conditions, evaluation of the delivery of 3D cubes (depth-dose and transverse profiles). Special emphasis will be given to non-isocentric set-up, including the use of range shifters . considered. Conclusion

EP-1505 Use of Portal dosimetry to monitor treatment consistency throughout the course of treatment S. Deshpande 1 , A. Sutar 1 , S. Naidu 1 , M. Vikram 1 , V. Anand 1 , R. Bajpai 1 , V. Kannan 1 1 P.D. Hinduja National Hospital, oncology, Mumbai, India Purpose or Objective Use of portal dosimetry software to check treatment delivery consistency and to monitor changes in patient anatomy during course of treatment. Material and Methods Varian portal dosimetry software and Electronic Portal Imaging Device (EPID) aS1200 were used to study consistency of treatment. Patients undergoing VMAT treatment were enrolled in this study. Patient plan was delivered after correcting set up error and transmitted images were acquired by the EPID aS 1200 during the treatment. The transmitted dose images were acquired by EPID after the beam passes through patient. Images were acquired in continuous mode at source to imager distance SID = 150cm on the 1,2,3,5,10,15,20,25 fraction number. Before measuring transmitted dose images cone beam CT was performed to eliminate any set up error. Day one transmitted dose images were defined as base line images. On an average 8 images were acquired during treatment for each patient. These images were compared with base line image. Gamma index evaluation was performed with 1mm and 1% parameter using Varian portal dosimetry software. Results For the first five images i.e. up to tenth fraction we got average gamma index passing 98.3% which is within action level threshold of 97%. Depending upon the site of treatment we observed gamma passing percentage varies during fag end of treatment Conclusion Dosimetric measurement during treatment is good tool to investigate error during the treatment. Portal vision is mostly used for patient set up and pre treatment QA of patient. We found that portal dosimetry is useful tool for checking consistency of treatment delivery and monitoring changes in patient contours. EP-1506 Temperature dependent dose readout of Gafchromic EBT3 and EBT-XD film and clinical relevance in SRT K. Buchauer 1 , L. Plasswilm 1 , J. Schiefer 1 1 Kantonsspital St. Gallen, Departement of Radiation Oncology, St Gallen, Switzerland Purpose or Objective Modern radiation therapy modalities regularly produce SRT/SRS/SBRT plans with highly irregular and steep dose