Preliminary assessment of proton linear energy transfer distribution in patients with MRI-guided proton therapy: a simulation study

<i>Objective</i>. Magnetic resonance imaging (MRI)-guided proton therapy is under development as an advanced technique that combines proton therapy with real-time MRI imaging, offering improved tumor targeting and better protection of adjacent healthy tissues. However, clinically relevant interactions between magnetic fields and linear energy transfer (LET) remain unexplored. This study investigated the LET distributions of primary and secondary protons with an emphasis on the influence of magnetic fields on both tumors and organs at risk.<i>Approach.</i>Monte Carlo simulations were performed using the Geant4 software (version 10.1.p01) to calculate the dose-averaged LET (LET_d) at different magnetic field strengths. Treatment plans were designed for three patients with liver, head, and prostate cancers for this study. A homogeneous magnetic field perpendicular to the proton beam direction was assumed throughout.<i>Main results.</i>In conventional proton therapy (without a magnetic field), high LET_dvalues are concentrated at the distal fall-off region of proton beam. When a magnetic field is applied, these high LET_dregions are rotated along the beam deflection direction. In the prostate case with two opposing beams, the magnetic field preserved these high LET_dregions by reducing the averaging effect, thereby limiting their dilution. This preservation effect became more pronounced with increasing magnetic field strength.<i>Significance.</i>In MRI-guided proton therapy, strategies to address LET_ddistribution changes caused by the magnetic field are considered desirable, particularly in high magnetic field environments.