Proton beam deflection in MRI fields: Implications for MRI-guided proton therapy

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Magnetic Resonance Imaging

Background:

• MRI-guided proton therapy offers advanced precision in cancer treatment.
• Split-bore MRI systems present unique challenges for beam delivery due to complex magnetic fields.

Purpose of the Study:

• To investigate proton beam delivery within a split-bore 1 Tesla (T) MRI-guided proton therapy system.
• To model magnetic effects on proton trajectories using Monte Carlo simulations.

Main Methods:

• Utilized field maps from a 1 T split-bore MRI-Linac system.
• Employed Geant4 Monte Carlo simulations to track proton paths.
• Simulated both inline and perpendicular beam orientations at various energies and starting points.
• Recorded proton spatial locations at the isocenter.

Main Results:

• Inline orientation: Proton rotation (up to 19° at 90 MeV) and minor focusing observed due to solenoidal fringe fields.
• Perpendicular orientation: Significant deflection (up to 135 mm at 90 MeV) and phase space distortion (up to 10 mm at 90 MeV) caused by main and fringe fields.
• Observed subtle differences between parallel and point-source beams, indicating 3D field variations.

Conclusions:

• MRI fringe fields significantly impact proton beam transport, causing rotation, deflection, and distortion.
• Pencil beam scanning is likely the only viable delivery method for MRI-guided proton therapy.
• Advanced correction strategies are essential to compensate for magnetic field effects.