title: Charged particle transport in magnetic fields in EGSnrc.
abstract: OBJECTIVE
  To accurately and efficiently implement charged particle transport in a magnetic field in EGSnrc and validate the code for the use in phantom and ion chamber simulations.
  
  
  METHODS
  The effect of the magnetic field on the particle motion and position is determined using one- and three-point numerical integrations of the Lorentz force on the charged particle and is added to the condensed history calculation performed by the EGSnrc PRESTA-II algorithm. The code is tested with a Fano test adapted for the presence of magnetic fields. The code is compatible with all EGSnrc based applications, including egs++. Ion chamber calculations are compared to experimental measurements and the effect of the code on the efficiency and timing is determined.
  
  
  RESULTS
  Agreement with the Fano test's theoretical value is obtained at the 0.1% level for large step-sizes and in magnetic fields as strong as 5 T. The NE2571 dose calculations achieve agreement with the experiment within 0.5% up to 1 T beyond which deviations up to 1.2% are observed. Uniform air gaps of 0.5 and 1 mm and a misalignment of the incoming photon beam with the magnetic field are found to produce variations in the normalized dose on the order of 1%. These findings necessitate a clear definition of all experimental conditions to allow for accurate Monte Carlo simulations. It is found that ion chamber simulation times are increased by only 38%, and a 10 × 10 × 6 cm(3) water phantom with (3 mm)(3) voxels experiences a 48% increase in simulation time as compared to the default EGSnrc with no magnetic field.
  
  
  CONCLUSIONS
  The incorporation of the effect of the magnetic fields in EGSnrc provides the capability to calculate high accuracy ion chamber and phantom doses for the use in MRI-radiation systems. Further, the effect of apparently insignificant experimental details is found to be accentuated by the presence of the magnetic field.
authors.affiliation: Carleton Laboratory for Radiotherapy Physics, Physics Department, Carleton University, Ottawa, Ontario K1S 5B6, Canada.
authors.forename: V N
authors.initials: VN
authors.lastname: Malkov
authors.affiliation: Carleton Laboratory for Radiotherapy Physics, Physics Department, Carleton University, Ottawa, Ontario K1S 5B6, Canada.
authors.forename: D W O
authors.initials: DW
authors.lastname: Rogers
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D060526:Magnetic Fields; D008962:Models, Theoretical; D009010:Monte Carlo Method; D009038:Motion; D019047:Phantoms, Imaging; D011839:Radiation, Ionizing; D014867:Water
chemical_list: D014867:Water