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Proton beam deflection in MRI fields: Implications for MRI-guided proton
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This paper investigates, via magnetic modeling and Monte Carlo simulation, the
ability to deliver proton beams to the treatment zone inside a split-bore
MRI-guided proton therapy system.
Field maps from a split-bore 1 T MRI-Linac system are used as input to
geant4 Monte Carlo simulations which model the trajectory of proton
beams during their paths to the isocenter of the treatment area. Both inline
(along the MRI bore) and perpendicular (through the split-bore gap) orientations
are simulated. Monoenergetic parallel and diverging beams of energy 90, 195, and
300 MeV starting from 1.5 and 5 m above isocenter are modeled. A phase space file
detailing a 2D calibration pattern is used to set the particle starting positions,
and their spatial location as they cross isocenter is recorded. No beam
scattering, collimation, or modulation of the proton beams is modeled.
In the 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 isocenter, this rotation is 19° (90 MeV) and 9.8° (300
MeV). A minor focusing toward the beam’s central axis is also seen, but only
significant, i.e., 2 mm shift at 150 mm off-axis, for 90 MeV protons. For the
perpendicular orientation, the main MRI field and near fringe field act as the
strongest to deflect the protons in a consistent direction. When starting from 1.5
m above isocenter shifts of 135 mm (90 MeV) and 65 mm (300 MeV) were observed.
Further to this, off-axis protons are slightly deflected toward 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. This
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. In both orientations, there is a small but
subtle difference in the deflection and distortion pattern between protons fired
parallel to the beam axis and those fired from a point source. This is indicative
of the 3D spatially variant nature of the MRI fringe field.
For the first time, accurate magnetic and Monte Carlo modeling have been used to
assess the transport of generic proton beams toward a 1 T split-bore MRI.
Significant rotation is observed in the inline orientation, while more complex
deflection and distortion are seen in the perpendicular orientation. The results
of this study suggest that due to the complexity and energy-dependent nature of
the magnetic deflection and distortion, the pencil beam scanning method will be
the only choice for delivering a therapeutic proton beam inside a potential
MRI-guided proton therapy system in either the inline or perpendicular
orientation. Further to this, significant correction strategies will be required
to account for the MRI fringe fields.
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