Commissioning a beam line for MR-guided particle therapy assisted by in-silico methods.

Abstract

Radiation therapy is continuously moving towards more precise dose delivery. The combination of on-line MR imaging and particle therapy, e.g. radiation therapy using protons or carbon ions, could enable the next level of precision in radiotherapy. In particle therapy, research towards a combination of MR and particle therapy, is well underway, but still far from clinical systems. The combination of high magnetic fields with particle therapy delivery poses several challenges for treatment planning, treatment workflow, dose delivery anddosimetry. To present a workflow for commissioning of a light ion beam line with an integrated dipole magnet to perform MR-guided particle therapy research, producing not only basic beam data but also magnetic field maps for accurate dose calculation. Accurate dose calculation in magnetic field environments requires high-quality magnetic field maps to compensate for magnetic field dependent trajectory changes and doseperturbations. The research beam line at MedAustron was coupled with a resistive dipole magnet positioned at the isocenter. Beam data was measured for proton and carbon ions with and without an applied magnetic field of 1 T. Laterally integrated depth dose curves as well as beam profiles were measured in water while beam trajectories were measured in air. Based on manufacturer data, an in-silico model of the magnet was created, allowing to extract high-quality 3D magnetic field data. An existing GATE/Geant4 Monte Carlo (MC) model of the beam line was extended with the generated magnetic field data and benchmarked against experimentaldata. A 3D magnetic field volume covering fringe fields until 50 mT was found to be sufficient for accurate beam trajectory modeling. The effect on particle range retraction were found to be 2.3 and 0.3 mm for protons and carbon ions, respectively. Measured lateral beam offsets in water agreed within 0.4 and -0.5 mm with MC simulations for protons and carbon ions, respectively. Experimentally determined in-air beam trajectories agreed within 0.4 mm in the homogeneous magnetic fieldarea. The presented approach based on in-silico modelling and measurements allows to commission a beam line for MR-guided particle therapy while providing benchmarking data for the magnetic field modeling, required for state-of-the art dose calculationmethods. This article is protected by copyright. All rights reserved.