Dosimetric validation of proton beam deflection and field shape deformation in a measured transverse magnetic field of an in-beam MR scanner using a novel dose calculation functionality in a research version of a commercial treatment planning system

Abstract

OBJECTIVE: Integrating in-beam magnetic resonance imaging (MRI) with proton therapy (PT) introduces magnetic fields (MFs) that affect proton beam transport and therefore require accurate incorporation into the treatment planning system (TPS) for magnetic resonance (MR)-integrated PT (MRiPT). This study experimentally validates Monte Carlo-based proton dose calculation in the presence of the MF in a research version of the commercial TPS RayStation for MRiPT, using a measured 3D magnetic vector field map of a 0.32 T in-beam MR scanner combined with a horizontal proton pencil beam scanning beam line. Approach. An MR-compatible mapping framework was used to acquire a high-resolution 3D magnetic vector field map, subsequently implemented into the TPS following coordinate transformations. Proton spot patterns and square radiation fields at 100, 150, and 220 MeV were delivered with and without the MF present. Dose distributions in air were measured using a 2D ionisation chamber array at multiple positions and compared with TPS dose calculations by analysing lateral beam shifts and field shape deformations with local γ-analysis. Clinically relevant patient treatment fields were evaluated at different water-equivalent depths (WEDs) in a phantom. Main results. Implementation of the measured magnetic vector field map preserved field vector magnitude, orientation and spatial alignment within the TPS. Dose measurements showed energy- and position-dependent MF-induced effects, which were accurately reproduced by the TPS. Agreement in lateral beam shifts was within 1-3 mm, and γ-pass rates exceeded 97% for a 3%/3 mm local criterion (10% dose threshold). Similar agreement was observed for patient treatment fields throughout all WEDs. Significance. This work validates the TPS capability to accurately model MF-induced proton beam transport in air as well as in a water-equivalent phantom. The present workflow establishes a foundation for proton treatment planning in the presence of MFs and thereby contributes to the clinical implementation of MRiPT.