ESTRO 2020 Abstract Book

S324 ESTRO 2020

simulations mimicked the influence of the Nozzle by offsetting and superimposing mono-energetic beams such that measured depth dose distributions were resembled. Treatment plans (TPs) were generated for five patient cases and four box shaped targets in water at varying depths and side lengths using the TPS clinical beam model and subsequently the RBE weighted dose was re- computed using the fluence tables based on Geant4. Results The fluence spectra of the primary and secondary particles simulated with Geant4 and FLUKA agreed generally well, but exhibiting two major systematic differences (see Fig.1): the lithium and beryllium yield over the entire energy range and the hydrogen and helium fluence below about 1 MeV was considerably lower in Geant4 compared to FLUKA. However, these differences were not expected to have an impact on RBE, as the former had the lowest fluence of all secondary fragments and the latter, hyperthermal ions, was reported to have a negligible clinical relevance (Elsässer et al. , 2009). Using the two energy spectra (FLUKA vs. GEANT4) to calculate the RBE weighted dose distributions resulted in average in deviations of less than 1% for each patient or water cases in the entrance up to the end of the target region, with a maximum local deviation of 3% at the distal edge of the target. In the fragmentation tail higher discrepancies up to 5% in average were found for deep seated targets.

Fig. 2 Energy deposition spectra for calibration (top) and measurements in water (bottom). Conclusion Commercially available Timepix detector placed in an in- house developed waterproof holder enables accurate measurements of energy deposition spectra produced in water along longitudinal and lateral profiles of therapeutic proton pencil beams. A good agreement was obtained when comparing experimental results with MC simulations. The ongoing development of particle identification methods will enable detailed investigations of LET spectra for different particle types. Experimental validation of LET spectra is an essential step towards RBE-based treatment planning in proton therapy. OC-0577 Impact of beamline-specific particle energy spectra on clinical plans in carbon ion beam therapy A. Resch 1 , N. Lackner 2 , T. Niessen 3 , S. Engdahl 3 , A. Elia 2 , D. Boersma 4 , L. Grevillot 2 , H. Fuchs 1 , G. Kragl 2 , L. Glimelius 3 , D. Georg 1 , M. Stock 2 , A. Carlino 2 1 Medizinische Universität Wien, Radiation Oncology, Vienna, Austria ; 2 MedAustron Ion Therapy Centre, Medical Physics, Wiener Neustadt, Austria ; 3 RaySearch Laboratories AB, Physics, Stockholm, Sweden ; 4 ACMIT Gmbh, Research and Development, Wiener Neustadt, Austria Purpose or Objective In carbon ion therapy, the physical dose is scaled by the relative biological effectiveness (RBE) to account for the different biological effect with respect to photon therapy. The Local Effect Model v1 (LEM I) is applied clinically across Europe to quantify the RBE and requires the full particle fluence spectrum differential in energy in each voxel as input parameter. The treatment planning systems (TPSs) use beamline specific look-up tables generated with FLUKA2011 or FLUKA2008, which is not available anymore (Parodi et al. , 2012). The purpose of this study was to quantify the clinical impact of using fluence spectra provided either by the TPS vendor or simulated in-house using FLUKA2011 and GATE/Geant4, respectively. Material and Methods The fluence differential in energy was scored dividing the sum of track lengths by the volume (Papiez and Battista, 1994), which was subsequently used to calculate the energy spectrum at 500 depths in water in 1 mm steps for 58 initial carbon ion energies (ranging from 120.0 to 402.8 MeV/u in 5 MeV/u steps). While a dedicated beam model was applied including the full description of the Nozzle using GATE-RTionV1.0 (Geant4.10.03), the FLUKA2011

Conclusion Using energy spectra derived from two different Monte Carlo codes and methods to account for the nozzle contribution resulted in clinically acceptable agreement with respect to RBE weighted dose. The results confirmed, as an independent validation of the clinical beam model, that the open source and publicly available Geant4 code can also be used to generate basic beam data required by the LEM I model. OC-0578 Ultrasound-guided PBS proton beam tracking in lung using a statistical motion model M. Krieger 1,2 , A. Giger 3,4 , A. Duetschler 1,2 , C. Jud 3,4 , P.C. Cattin 3,4 , R.V. Salomir 5,6 , O. Bieri 3,7 , D.C. Weber 1,8,9 , A.J. Lomax 1,2 , Y. Zhang 1 1 Paul Scherrer Institute, Centre for Proton Therapy, Villigen PSI, Switzerland ; 2 ETH Zurich, Department of Physics, Zurich, Switzerland ; 3 University of Basel, Department of Biomedical Engineering, Allschwil, Switzerland ; 4 University of Basel, Center for medical Image Analysis & Navigation, Allschwil, Switzerland ; 5 University of Geneva, Image Guieded Interventions Laboratory, Geneva, Switzerland ; 6 University Hospitals of Geneva, Radiology Division, Geneva, Switzerland ; 7 University Hospital Basel, Department of Radiology, Basel, Switzerland ; 8 Inselspital Bern, Department of Radiation Oncology, Bern, Switzerland ; 9 University