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s / Physica Medica 30 (2014) e45ee74 e66 whereas for RPLDs the response is linear with dose. Fading effect was checked and results show that for TLDs and OSLDs signal stabilizes in around 10 and 2 days after irradiation, respectively. For RPLDs, when the preheating treatment is applied, the signal stabilizes and there is no further change. Energy corrections for a range of photon beams (6 18MV) and electron beams (6 20 MeV) relative to a Co-60 beam, are of the order of 35% for all three detectors for highest beam energies. Discussion: All three systems, especially TLD and RPLD systems need very careful handling procedures in order to achieve good reproducibility. The advantage of RPLDs and OSLDs is that the reading process can be repeated if necessary. RPLD does not need a non-linearity and fading corrections. Assuming that all corrections are properly applied, all three systems can be used for dosimetry audit in radiotherapy. DOSE CALCULATION ACCURACY AT PRESENCE OF DENTAL IMPLANTS ON UNCORRECTED AND METAL ARTIFACT REDUCED COMPUTED TOMOGRAPHY DATA Manuel Maerz, Pia Mittermair, Oliver Koelbl, Barbara Dobler. Department of Radiotherapy, RegensburgUniversity Medical Center, Germany Background: Dose calculation in modern treatment planning is based on Computed Tomography (CT) data of the patient. Metallic dental implants cause severe streaking artifacts, which inhibit the correct representation of shape and density of the metal and the surrounding tissue.Wewill present the influence of dental implants, metal artifacts and the benefit of the metal artifact reduction algorithm IFS (iterative frequency split) on dose calculation accuracy depending on treatment technique and dose calculation algorithm. Materials and Methods: The study is conducted on cylindrical phantoms consisting of water equivalent material surrounding dental implant materialand other heterogeneities (e.g. air, muscle) in various geometric arrangements. Computed Tomography images of the phantoms are generated and corrected using the IFS algorithm. Several plans are irradiated to the phantoms equipped with Gafchromic EBT 3 films: five beams IMRT (IntensityModulatedRadiationTherapy), nine beams IMRTunddual arcVMAT (Volumetric Modulated Arc Therapy). The measured dose distributions are compared to calculations on corrected and uncorrected CT data using the dose calculation algorithms Pencil Beam and Collapsed Cone, implemented inOncentra External Beamv.4.3 and theMonte Carlo simulationXVMC. The Pencil Beam and XVMC algorithms report dose towater, whereas Collapsed Cone reports dose to media. To compare film measurements to Collapsed Cone calculations the film measurements which are calibrated to dose to water are rescaled to dose to media. Inaccuracies caused by artifacts or not adequately corrected CT images and inaccuracies caused by incorrect modeled transmission of radiation through metal implants are separated. Results: Depending on the dose calculation algorithm and the treatment technique artifacts lead to inaccuracies in dose calculation that can be reduced by application of the IFS algorithms. The accuracy of the calculation on the IFS corrected data and the improvement with respect to the uncorrected data depends on the dose calculation algorithm and the composition of material in the phantom. Conclusion: Metal artifact reduction leads to an improvement in accuracy of dose calculations. The application of a metal artifact reduction algorithm is recommended to reduce dose uncertainties. Acknowledgments:The work was supported by the Wilhelm Sander Foundation. TEST, VALIDATION AND UPGRADE OF THE MD ANDERSON ANALYTICAL MODEL PREDICTING SECONDARY NEUTRON RADIATION IN PROTON THERAPY FACILITIES J. Farah , A. Bonfrate , A. De Oliveira , S. Delacroix , J. H erault , F. Martinetti , S. Piau , F. Trompier , I. Vabre , I. Clairand . a Institute for Radiological Protection and Nuclear Safety (IRSN), Human Health Division, 31 ave de la Division Leclerc, 92260 Fontenay-aux-Roses; France; b Institut Curie e Centre de Protonth erapie d’Orsay (ICPO), Campus universitaire bâtiment 101, 91898 Orsay, France; Centre Antoine Lacassagne (CAL) Cyclotron biom edical, 227 avenue de la Lanterne, 06200 Nice, France; d Institut de Physique Nucl eaire d’Orsay (IPNO), Service de dosim etrie, Campus universitaire bâtiment 104, 91406 Orsay, France Purpose: This study follows the MD Anderson approach to build an analytical model predicting leakage neutrons within the local 75 MeV ocular proton therapy facility. Its main goal is to test, validate and upgrade the model to clinically relevant configurations. Methods: Using Monte Carlo (MC) calculations, neutron ambient dose equivalents, H*(10), were simulated at different positions inside the treatment room while considering a closed final collimator and pristine Bragg peak delivery as per the MD Anderson method. Using this data, a facility specific analytical model was developed and tested. Starting from H*(10) values at isocentre, this model attempts to reproduce the neutron decrease with axial and lateral distance to isocentre while separately accounting for the contribution of intranuclear cascade, evaporation, epithermal and thermal neutrons. To validate the model, simulated H*(10) values were considered as well as experimental measurements previously unavailable at MD Anderson. The model was also expended in the vertical direction to enable a full 3D mapping of H*(10) inside the treatment room. Results: The work first proved that it is possible to build ones’ own analytical model following the MD Anderson approach which however requires a MCmodel of the local proton therapy facility. Validation showed that the analytical model efficiently reproduced simulated H*(10) values with a maximum difference below 10%. In addition, it succeeded in predicting measured H*(10) values with differences |
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