First steps towards dosimetric commissioning of an in-beam MR system for proton pencil beam scanning

Introduction: A prototype system for MR‐integrated proton therapy (MRiPT) with an in-beam MR scanner is currently under investigation at our facility. The commissioning thereof requires an accurate beam model, a 3D map of the full static magnetic field (MF) of the MR scanner and consideration of their interaction. This work describes measurements of the proton beam, the MF and beam modeling performed to set up a proton beam model for the MRiPT prototype system. Materials & Methods: Measurements of central proton beam spot sizes in air at 10 distances from the beam isocenter (between -15 and 54 cm) and integral depth-dose profiles (100 to 226.7 MeV) in water were obtained without MF using a scintillation detector and a water phantom with a Bragg peak chamber, respectively. A beam model was fitted to these measurements by utilizing an automated regularization-based optimization process. A 3D magnetic field map of the 0.33 T open MR scanner was acquired using spatially resolved Hall probe measurements. Results: In the absence of the MF, the optimized beam model reproduced the measured beam spot sizes in air with an error <0.5 mm for 100 – 226.7 MeV and depth dose curves in water with an error <0.1 g/cm² for 100 – 226.7 MeV. The magnetic field measuring approach delivers reproducible results with high accuracy and an uncertainty <±5 mT. Summary: A proton beam model for the MRiPT prototype system without MF was established and a high-precision method for mapping the 3D MF of the MR scanner was developed. The MF map and the beam model will be used to establish an experimental treatment planning system providing a correction algorithm accounting for the influence of the MF of the MR scanner on proton beams. Further investigations are necessary to validate the beam model in the presence of the MF.