TH-CD-BRA-05: Proton Beam Deflection in MRI Fields: Implications for MRI-Guided Proton Therapy

Abstract

Purpose: To simulate the delivery of proton beams to the treatment zone inside a split-bore MRI-guided proton therapy system. Methods: Field maps from a split-bore 1 T MRI system are used as input to Monte Carlo simulations which model the trajectory of proton beams towards isocentre. Both inline (along the MRI bore) and perpendicular (through the split-bore gap) orientations are simulated. Monoenergetic diverging beams of energy 90 MeV, 195 MeV and 300 MeV starting from 1.5 m above isocentre were modelled. A phase space file detailing a 2D calibration pattern is used to set the particle starting positions, and their spatial location as they cross isocentre recorded. Results: Inline Orientation: The radial symmetry of the solenoidal style fringe field acts to rotate the protons around the beam’s central axis. For protons starting at 1.5 m from isocentre this rotation is 17° (90 MeV), and 8° (300 MeV). Perpendicular Orientation: Isocentre shifts of 135 mm (90 MeV) and 65 mm (300 MeV) were observed in the direction perpendicular to the main imaging field. Off-axis protons are also slightly deflected towards or away from the central axis in the direction perpendicular to the main deflection direction. This leads to a distortion of the phase space pattern, not just a shift. The distortion increases from zero at the central axis to 10 mm (90 MeV) and 5 mm (300 MeV) for a proton 150 mm off-axis. Conclusion: The complexity and energy-dependent nature of the magnetic deflection and distortion indicates the pencil beam scanning method will be the only choice for delivering a therapeutic proton beam inside a potential MRI-guided proton therapy system. Significant correction strategies will be required to account for the MRI fringe fields. The authors acknowledge funding from NHMRC Program Grant No. 1036078 and ARC Discovery Grant No. DP120100821.