ESTRO 36 Abstract Book

S180 ESTRO 36 _______________________________________________________________________________________________

Conclusion MC simulations accurately modelled the dose distribution around the Bragg peak and can be used to estimate the LET at any given position of the proton beam with optimized parameters. The LET spectrum varied considerably with depth and such LET estimates are highly valuable for future studies of relative biological effectiveness of protons. OC-0343 Experimental setup to measure magnetic field effects of proton dose distributions: simulation study S. Schellhammer 1,2 , B. Oborn 3,4 , A. Lühr 1,2,5 , S. Gantz 1,2 , P. Wohlfahrt 1,2 , M. Bussmann 6 , A. Hoffmann 1,2,7 1 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology, Dresden, Germany 2 OncoRay - National Center for Radiation Research and Oncology, Medical Radiation Physics, Dresden, Germany 3 Wollongong Hospital, Illawarra Cancer Care Centre, Wollongong, Australia 4 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia 5 German Cancer Consortium DKTK, Partner Site Dresden, Dresden, Germany 6 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiation Physics, Dresden, Germany 7 Faculty of Medicine and University Hospital Carl Gustav Carus at the Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany Purpose or Objective As a first step towards proof-of-concept for MR-integrated proton therapy, the dose deposited by a slowing down proton pencil beam in tissue-equivalent material is assessed within a realistic magnet assembly. Furthermore, radiation-induced activation and demagnetization effects of the magnet are studied. Material and Methods The dose distributions of proton pencil beams (energy range 70-180 MeV) passing through a transverse magnetic field of a permanent C-shaped NdFeB dipole magnet (maximum magnetic flux density B max = 0.95 T) while being stopped inside a tissue-equivalent slab phantom of PMMA were simulated (Figure 1). The beam was collimated to a diameter of 10 mm. A radiochromic EBT3 film dosimeter was placed centrally between the two phantom slabs parallel to the beam’s central axis. 3D magnetic field data was calculated using finite-element modelling (COMSOL Multiphysics) and experimentally validated using Hall- probe based magnetometry. A Monte Carlo model was designed using the simulation toolkit Geant4.10.2.p02 and validated by reference measurements of depth-dose distributions and beam profiles obtained with Giraffe and Lynx detectors (IBA Dosimetry), respectively. The beam trajectory and lateral deflection were extracted from the film’s planar dose distribution. Demagnetization was assessed by calculating the dose deposited in the magnet elements, and by relating this to radiation hardness data from literature. A worst-case estimate of the radioactivation of the magnet was obtained by taking into account the most common produced mother nuclides and their corresponding daughter nuclides.

biological systems and endpoints studied, but also to the actual linear energy transfer (LET) in the biological systems. To provide accurate estimates of the relative biological effects of protons, high precision cell experiments are needed together with detailed knowledge of the LET at a given measurement depth. The objective of this study was to estimate the LET distribution along the depth dose profiles from a low energy proton beam, using Monte Carlo (MC) simulations adjusted to match Dose measurements were performed at the experimental proton beam line at the Oslo Cyclotron Laboratory (OCL) employing 17 MeV protons. A Markus ionization chamber and GafChromic films were used to measure the dose distribution at 28, 88 and 110 cm from the beam exit window. At each position, measurements were performed along the depth dose profile (using increasing thickness of paraffin- and Nylon6 sheets). A transmission chamber was used for monitoring beam intensity. The geometry of the experimental setup was reproduced in the FLUKA MC code. The dose profiles were calculated using FLUKA, and MC parameters relating to beam energy and beam line components were optimized based on comparisons with measured doses. LET-spectra and dose-averaged LET (LET d ) were also scored using FLUKA. Results The measured pristine Bragg peak from the OCL cyclotron covered about 200 µm (Figure 1a). The MC simulations of the beam line were validated by comparing simulated dose profiles with measured data (Figure 1a). The simulated LET d increased with depth, also beyond the Bragg peak (Figure 1a and Table 1). Also, LET d at target entrance increased with distance from the beam exit window due to the presence of air (Table 1). The LET spectrum was narrow at the target entrance, and considerably broadened at BP depth (Figure 1b). measured dose profiles. Material and Methods

Figure 1 : Simulation geometry. Results

The Monte Carlo model showed excellent agr eement with the reference measurements (mean absolute range difference below 0.2 mm). The predicted planar dose distribution clearly showed the magnetic fi eld induced beam deflection (Figure 2). The estimated in-plane