title: Proton beam deflection in MRI fields: Implications for MRI-guided proton therapy.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Illawarra Cancer Care Centre (ICCC), Wollongong, NSW 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, Australia.
authors.forename: B M
authors.initials: BM
authors.lastname: Oborn
authors.affiliation: Shoalhaven Cancer Care Centre, Nowra, NSW 2541, Australia.
authors.forename: S
authors.initials: S
authors.lastname: Dowdell
authors.affiliation: Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, Australia and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.
authors.forename: P E
authors.initials: PE
authors.lastname: Metcalfe
authors.affiliation: School of Information Technology and Electric Engineering, University of Queensland, St. Lucia, QLD 4072, Australia.
authors.forename: S
authors.initials: S
authors.lastname: Crozier
authors.affiliation: Department of Radiation Oncology, MD Anderson, Houston, Texas 77030.
authors.forename: R
authors.initials: R
authors.lastname: Mohan
authors.affiliation: Sydney Medical School, University of Sydney, Sydney, NSW 2006, Australia and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.
authors.forename: P J
authors.initials: PJ
authors.lastname: Keall
mesh_terms: D003198:Computer Simulation; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008962:Models, Theoretical; D009010:Monte Carlo Method; D061766:Proton Therapy / Q000295:instrumentation / Q000379:methods*; D011522:Protons
chemical_list: D011522:Protons