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

Abstract

Background Radiation therapy is continuously moving towards more precise dose delivery. The combination of online MR imaging and particle therapy, for example, 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, and dosimetry. Purpose To present a workflow for commissioning of a light ion beam line with an integrated dipole magnet to perform MR‐guided particle therapy (MRgPT) 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 dose perturbations. Methods The research beam line at MedAustron was coupled with a resistive dipole magnet positioned at the isocenter. Beam data were measured for proton and carbon ions with and without an applied magnetic field of 1 T. Laterally integrated depth‐dose curves (IDC) 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 experimental data. Results A 3D magnetic field volume covering fringe fields until 50 mT was found to be sufficient for an accurate beam trajectory modeling. The effect on particle range retraction was 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 field area. Conclusion The presented approach based on in silico modeling and measurements allows to commission a beam line for MRgPT while providing benchmarking data for the magnetic field modeling, required for state‐of‐the art dose calculation methods.

MR‐guided ion therapy: Detector response in magnetic fields during carbon ion irradiation

“…For photons, this is typically not the case 11 . For charged particles, a small change in the particle range was observed earlier 15 . A more prominent contribution may be expected due to the lateral displacement of the irradiation field due to the Lorentz force.…”

“…The magnet is capable of producing magnetic field strengths up to 1 T. Positioning was such that the magnetic field center coincided with the room isocenter (see Figure 1). A detailed characterization of the beam line, the magnetic fields and the corresponding beam path deviations due to the Lorentz force were performed earlier 15 . The beam line is equipped with a clinical nozzle and allows spot scanning with a maximum field size of $$20\times 20 \nobreakspace \mathrm{cm^2}$$ over the clinical energy range from 62.4 to 252.7 MeV and 120.0 to 402.8 MeV/u for protons and carbon ion beams, respectively.…”

“…A detailed description of the magnet and measurement set-up can be found elsewhere, 12,15 in the following only a short overview is given. A resistive dipole magnet (Danfysik A/S, Taastrup, Denmark) with a pole gap of 13.5 cm and a pole diameter of 25 cm was placed in front of the horizontal research beam line at the MedAustron ion therapy center (Wr.…”

“…11 For charged particles, a small change in the particle range was observed earlier. 15 A more prominent contribution may be expected due to the lateral displacement of the irradiation field due to the Lorentz force. To investigate a potential change of the dose to water at the point of measurement, Monte Carlo simulations were performed using our commissioned Monte Carlo tool GATE.…”

Technical note: Experimental dosimetric characterization of proton pencil beam distortion in a perpendicular magnetic field of an in‐beam MR scanner

“…While homogeneous media is likely to alter the dose spot size, orientation, and eccentricity in a nontrivial way, for inhomogeneous media, the modification of dose spot attributes additionally depends on the composition of the materials. For this reason, it is necessary to carry out further measurements in homogeneous and inhomogeneous materials, which include depth dose and lateral beam profile measurements utilizing a water phantom as proposed by Fuchs et al 25 . Second, this work solely considered the measured dosimetric effects arising from the MRiPT prototype setup that was used in our institution.…”