ESTRO 2020 Abstract book

S351 ESTRO 2020

maximizing it in the tumor region. Proton linear energy transfer (LET), characteristic with depth is not precisely reflected in radiobiological effectiveness (RBE) of 1.1, which is currently used in clinical routine and causes uncertainties in biological dose calculations. In fact, RBE depends on LET as well as tissue type, dose, dose-rate, fractionation scheme etc. The purpose of this work was to characterize the particle field produced by proton pencil beam at different positions in water to validate Monte A hybrid semiconductor pixel detector was used to characterize the energy deposition spectra of mixed radiation field produced by therapeutic proton beams. The Timepix ASIC enables to measure the energy deposition, position and direction of energetic charged particles by high-resolution spectrometric tracking of single particles. In-situ measurements were performed with a Timepix detector equipped with a 300 um silicon sensor in a miniaturized MiniPIX-Timepix camera placed in an in- house designed, thin and waterproof PMMA holder (Fig. 1) and positioned inside a water phantom (BluePhantom, IBA). The calibration was performed with the primary beam in air and was followed by the energy deposition measurements along longitudinal and lateral pencil beam profiles in water. The energy deposition spectra acquired with Timepix were compared to MC simulations performed with GATE MC and GPU-accelerated MC code FRED. Carlo (MC) simulations. Material and Methods

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

Fig. 1.Experimental setup. Results

Fig. 2 (top) shows the spectrum of energy deposited by therapeutic proton beams obtained from calibration measurements in air. Fig. 2 (bottom) shows spectrum of energy deposited by all particles produced by 150 MeV proton beam, at ¾ of its range (117 mm), and 6 cm away from the beam core. The corresponding MC simulation shows energy deposited by charged particles, gammas and electrons. The summary of 40 measurements performed at different depths along the pencil beam lateral profiles for primary protons of 100, 150 and 200 MeV will be reported and the discrepancy between experimental and simulation results will be discussed.