ESTRO 36 Abstract Book

S790 ESTRO 36 2017 _______________________________________________________________________________________________

do play a major role. This resulted in in noticeable differences in the dose gradients regions. Absolute dose differences where below 1.5%. Conclusion Recalculating doses based on EPID and machine based parameters is a possible way for QA in an adaptive treatment approach. As QA parameters are taken from information that is given anyway or that can be easily generated, it does not complicate the procedure of frequent replanning. Results are as expected quite good for the machine file approach while higher discrepancies were found using EPID data. Main problem we face here is that especially for the machine file based version we do not have full independent data sources. EP-1494 The MedAustron proton gantry: nozzle design recommendations based on Monte Carlo simulations H. Fuchs 1,2 , L. Grevillot 2 , A. Elia 2 , A. Carlino 2,3 , J. Osorio 2 , V. Letellier 2 , R. Dreindl 2 , M. Stock 2 , S. Vatnitsky 2 1 Medizinische Universität Wien Medical University of Vienna, Department of Radiation Oncology & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 2 MedAustron lon Therapy Center, Department of Medical Physics, Wr. Neustadt, Austria 3 University of Palermo, Department of Physics and Chemistry, Palermo, Italy Purpose or Objective MedAustron is equipped with one vertical and three horizontal fixed beam lines and one proton gantry based on the PSI gantry 2 design for patient treatments. This work focuses on simulations and design considerations for the proton gantry nozzle, allowing an optimization of beam delivery properties at isocenter. Material and Methods Different gantry nozzle designs were evaluated using Gate/Geant4 Monte Carlo (MC) simulations: air filled nozzle, helium filled nozzle, full vacuum nozzle, moving snout, compacting of nozzle elements (vacuum window and monitors). Design considerations were based on the use of similar components as in the other beam lines as well as a static beam monitoring system. Simulations were performed for 5 representative energies (62, 96.5, 157.4, 205, and 252.3 MeV) also considering the use of range shifter and ripple filter. Nozzle designs were based on the MC model of the MedAustron fixed beam line nozzle, which was verified with measurements at isocenter for 20 representative energies. Results The influence of the gantry nozzle filling with vacuum or helium (as used in some commercial systems) was found to have only a minor impact on spot size (< 2%). Compacting all nozzle elements towards the nozzle exit and reducing the nozzle dimension in beam direction by 25% lead to a reduction of the spot size of up to 20%, depending on the initial energy as depicted in Fig. 1. Using higher energies in combination with range shifter also decreased the delivered spot size for shallow seated tumors, as already demonstrated in other studies (Parodi, et al. PMB 57(12), 2012).

Conclusion To more sophisticated analysis the gamma criteria should be less than 3%/3mm or/and local dose method should be used. The resolution of used detector is crucial and should be high for better interpretation of results. Gamma method presents some statistic data, for scrutiny analysis the clinical interpretation should be assessed.

EP-1493 Machine record parameters or Epid based data for ART QA. A comparison of two scenarios.

P. Haering 1 , C. Lang 1 , M. Splinter 1 1 DKFZ, E040, heidelberg, Germany

Purpose or Objective Using machine record files and Epid based dosimetry is popular for machine and patient related QA, as this may also work for adaptive treatment approaches. The Siemens Artiste treatment machine used here, allows a comparison of both methods in one session. Exit images and all relevant machine parameters are included in the image header collected during treatment. Here we present results of a comparison between QA dose recalculations based on the two sources, exit images and machine recorded parameters. Material and Methods A software tool was developed that allows for the extraction of the relevant parameters (MLC-positions, MU, etc.) from the machine records as well as from the Epid measured exit fluencies. While machine data had to undergo a reformat to be used for recalculation, the exit fluencies need more attention. Here both, the delivered fluence as well as the absorption in the patient do play a role. Therefore both have to be separated to receive reliable MLC positions. The algorithm used first generates an image containing only absorption information for the beam using this to remove this influence on the MLC positions. MUs were used from the parameter file, as the fluence uncertainties on the EPID images have shown to be to large to be used for that purpose. The extracted parameters are then inserted in a newly generated Dicom RT-Plan file that then can be used in the treatment planning system (here Raystation, Raysearch) to recalculate the dose. Dose distributions (Epid based, parameter file based and originally planned) are then compared. Results Measuring exit doses with the EPID was a simple task and could be done for all coplanar field sets. The software tool made it simple to extract all the needed parameter from the files and images resulting in 2 new Dicom plan files. Dose recalculation was done by just importing the new plan files to Raystation. Comparing the original dose distribution to the machine file based one showed almost no difference at all (< 0.7%), as MU and leaf position differences where quite small. This might also be grounded in the used calculation grid of 2mm size. MLC positions derived from EPID images show much larger differences. Here detection uncertainties, EPID positioning and the resulting image resolution of 0.3mm