ESTRO 36 Abstract Book

S428 ESTRO 36 2017 _______________________________________________________________________________________________

lines. Negative MLC errors were not performed at Institution 2, due to differing equipment. The automated VMAT plans from institution 3 were similar in pass rate to the manually planned VMAT for collimator errors, despite the difference (higher magnitude for manual VMAT plans) in error magnitude. This could be caused by the higher MLC modulation in the automated plans. Conclusion Not all deliberately introduced errors were discovered for VMAT plans using a typical 3%/3mm global gamma pass rate (for 10% threshold with correction off). Consistency between institutions was low for plans assessed utilising differing devices and software. A 2%/2mm global analysis was most sensitive to errors. PO-0809 A 3D polymer gel dosimeter coupled to a patient-specific anthropomorphic phantom for proton therapy M. Hillbrand 1 , G. Landry 2 , G. Dedes 2 , E.P. Pappas 3 , G. Kalaitzakis 4 , C. Kurz 2 , F. Dörringer 2 , K. Kaiser 2 , M. Würl 2 , F. Englbrecht 2 , O. Dietrich 5 , D. Makris 3 , E. Pappas 6 , K. Parodi 2 1 Rinecker Proton Therapy Center, Medical Physics, Munich, Germany 2 Ludwig-Maximilians-Universität München, Department of Medical Physics, Munich, Germany 3 National and Kapodistrian University of Athens, Medical Physics Laboratory- Medical School, Athens, Greece 4 University of Crete, Department of Medical Physics, Heraklion, Greece 5 Ludwig-Maximilians-Universität München, Department of Radiology, Munich, Germany 6 Technological Educational Institute, Radiology & Radiotherapy Department, Athens, Greece Purpose or Objective The high conformity of proton therapy (PT) dose distributions, attributed to protons stopping in the target, is also the main source of uncertainty of the modality. PT is sensitive to errors in relative stopping power to water (RSP) uncertainties and to density changes caused by organ motion. The ability to verify PT dose distributions in 3D with a high resolution is therefore a key component of a safe and effective PT program. Existing 2D dosimetric methods suffer from shortcomings attributed to LET dependence, positioning uncertainties, limited spatial resolution and their intrinsic 2D nature. Recent advances in polymer gel dosimetry coupled to 3D printing technology have enabled the production of high resolution, patient specific dosimetry phantoms. So far this approach has not been tested for PT. Material and Methods A 3D-printed hollow head phantom derived from real CT data was filled with VIPAR6 polymer gel and CT scanned for pencil beam scanning (PBS) treatment planning, following RSP characterization of the gel and the 3D printer bone mimicking material (see Figure 1). All irradiations of phantoms were carried out at the Rinecker Proton Therapy Center in Munich, which is dedicated for PBS. An anterior oblique SFUD plan was used to cover a centrally located cerebral PTV, following the standard operating procedures of the PT facility. The field was crossing the paranasal sinuses (see Figure 2A) to test the TPS modelling of heterogeneities. 3D maps of the T2 relaxation time were obtained from subsequent MR scanning of the phantom and were converted to relative dose. The dose response linearity and proton range were verified using separate mono-energetic irradiations of cubic phantoms filled with gel from the same batch. Relative dose distributions were compared to the TPS predictions using gamma analysis.

Figure 1. 3D printed patient-specific head phantom filled with dosimetric gel during the treatment planning process. Results Results from mono-energetic irradiation of the cubic phantoms showed proton range agreement to the TPS within 1 mm for 90 MeV and 115 MeV, supporting the SPR gel characterization accuracy. Dose-response linearity was confirmed for the delivered dose range, except at the Bragg peak position where a LET dependence was revealed. Gamma index and relative dose distribution profiles showed good agreement between TPS and gel, as shown in in Figure 2.

Figure 2. (A) Slice of the 3D SFUD dose distribution converted from a T2 relaxation map obtained from MR scanning the irradiated 3D printed head phantom filled with polymer gel. The PTV is indicated in white. (B) Gel (RTsafe) and TPS dose profiles along the path marked in red in (A). (C) 3%/2mm gamma index along the profile. Conclusion In this work we have shown that patient-specific 3D polymer gel dosimetry is applicable to PT using PBS. Further characterization and correction of the LET dependence and comparison to MC dose calculations will PO-0810 Absolute dose pre-treatment Portal Dosimetry using the Varian MAASTRO implementation A. Taborda 1 , J. Stroom 1 , C. Baltes 2 , A. Seabra 1 , K. Dikaiou 2 , C. Greco 1 1 Champalimaud Centre for the Unknown, Clinical Department, Lisboa, Portugal 2 Varian Medical Systems, Varian Medical Systems Imaging Laboratory, Baden-Dättwil, Switzerland be carried out and presented. Acknowledgements: DFG-MAP