ESTRO 36 Abstract Book

S222 ESTRO 36 _______________________________________________________________________________________________

Material and Methods A C-shaped 0.95 T permanent neodymium (NdFeB) dipole magnet was used. The magnetic main and fringe fields were characterized using 3D automated Hall probe measurements. A method and procedure were established to perform periodic quality assurance (QA) measurements of the magnetic field’s constancy. A 3D vector field representation for the magnetic flux density distribution was calculated by finite-element modeling (FEM) using COMSOL-Multiphysics®. For irradiation experiments, proton beams of 80–225 MeV were collimated using brass apertures having circular voids of either 5 or 10 mm diameter. The beams entered a PMMA slab phantom being placed inside the magnet’s horizontal air gap, perpendicularly to the main field component. Proton beam trajectories and depth-dose curves in the presence of the magnetic field were measured with Gafchromic EBT3 film, being placed between the two slabs of the phantom. Reference trajectories were measured without magnetic field. In transmission experiments without phantom, beam deflections were measured with a 2D scintillation detector (Lynx, IBA Dosimetry) positioned perpendicular to the beam at 24 cm distally from the magnet. Results Magnetometry results (Fig. 1) validated the 3D magnetic flux density distribution as calculated by FEM. The simulation tended to underestimate the measured magnetic field strength in the plateau area by about 2% (mean difference 20 mT). In repeated QA measurements, field strength changes remained below a threshold of 3 mT. For all proton energies, the lateral beam deflection due to the magnetic field increased with depth in the phantom. Lateral displacement of the Bragg peak position increased with initial energy, from 1.1 (±0.4) mm to 10.7 (±0.8) mm, for 80 and 180 MeV (Fig. 2), respectively. In transmission measurements, only lateral deflections were measurable, ranging from 56 (±0.5) to 30 (±0.5) mm for beam energies between 80 and 225 MeV, which was in excellent agreement with theoretical predictions.

Conclusion The proposed method of analysis of isolated errors gives the magnitude of errors. In general, the most important are leaf gap width errors. Exception and further analysis is required for VMAT patients with low DR. ArcCheck positioning errors should not affect the gamma results while being in specification tolerance. PV-0421 In-magnet measurement setup for proof-of- concept and commissioning of MR integrated proton therapy A. Lühr 1,2,3 , S. Gantz 1,3 , S. Schellhammer 1,3 , O. Zarini 4 , K. Zeil 4 , U. Schramm 4 , A. Hoffmann 1,3,5 1 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology, Dresden, Germany 2 German Cancer Consortium DKTK, Partner Site Dresden, Dresden, Germany 3 OncoRay – National Center for Radiation Research in Oncology, Medical Radiation Physics, Dresden, Germany 4 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiation Physics, Dresden, Germany 5 Faculty of Medicine and University Hospital Carl Gustav Carus at the Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany Purpose or Objective There is growing interest to explore the concept of magnetic resonance integrated proton therapy (MRiPT). However, no experimental proof-of-principle has been established so far. The aim of this work was to develop an in-magnet measurement setup that facilitates to investigate the dosimetric feasibility of MRiPT and to develop a commissioning procedure for future MRiPT devices.

Conclusion An in-magnet measurement setup for first MRiPT proof-of- principle experiments has been realized. Measurements of the magnetic field and proton beam trajectory in tissue- equivalent material proofed to be feasible and facilitate the development of commissioning and QA procedures for MRiPT. Ongoing experiments focus on the impact of realistic treatment fields as well as the effect of inhomogeneous media on the dose distribution. The data