ESTRO 36 Abstract Book

S179 ESTRO 36 2017 _______________________________________________________________________________________________

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.

to 1 cm for 180 MeV in comparison to no magnetic field. No out-of-plane beam deflection was observed. Exposing the film to 2 Gy at the Bragg peak was estimated to cause a mean dose to the magnets of 20 µGy, which is expected to produce negligible magnetic flux loss. The initial activation was estimated to be below 25 kBq.

Figure 2 : Simulated dose distribution of a deflected proton beam (180 MeV, 10 7 primary particles) on a film dosimeter. Conclusion A first experimental setup capable of measuring the trajectory of a proton pencil beam slowing down in a tissue-equivalent material within a realistic magnetic field has been designed and built. Monte Carlo simulations of the design show that magnetic field induced lateral beam deflections are measurable at the energies studied and radiation-induced magnet damage is expected to be manageable. These results have been validated by irradiation experiments, as reported in a separate abstract. OC-0344 Experimental validation of TOPAS neutron dose for normal tissue dosimetry in proton therapy patients G. Kuzmin 1 , A. Thompson 2 , M. Mille 1 , C. Lee 1 1 National Cancer Institute, Division of Cancer Epidemiology and Genetics, Rockville, USA 2 National Institute of Standards and Technology, Radiation Physics Division, Gaithersburg, USA Purpose or Objective In the last several years, the popularity and use of proton therapy has been increasing due to its promise of a dosimetric advantage over conventional photon therapy. This is especially of great importance in pediatric patients who have a higher risk of developing late effects. During proton therapy 90% of scatter dose is from neutrons, which can travel out of the treatment field and can be highly biologically effective. In order to conduct epidemiological investigations of the risk of long term adverse health effect in proton therapy patients, it is imperative to accurately assess radiation dose to normal tissue. Tool for Particle Simulation (TOPAS) based on the GEANT4 Simulation Toolkit may be a computational option for normal tissue dosimetry to support large scale epidemiological investigations of proton therapy patients. While previous works have benchmarked TOPAS for proton dosimetry within treatment fields, there is a lack of validation for neutron scatter and energy spectrum. In the current study, we measured the energy spectrum of scattered neutrons using a simple physical phantom coupled with a series of Bubble Detectors irradiated by Californium-252 neutron source. 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 deflection of the Bragg peak ranged from 0 cm for 70 MeV