title: Effect of transverse magnetic fields on a simulated in-line 6 MV linac.
abstract: The effects of a transverse magnetic field on an in-line side-coupled 6 MV linear accelerator are given. The results are directly applicable to a linac-MR system used for real-time image guided adaptive radiotherapy. Our previously designed end-to-end linac simulation incorporated the results from the axisymmetric 2D electron gun program EGN2w. However, since the magnetic fields being investigated are non-axisymmetric in nature for the work presented here, the electron gun simulation was performed using OPERA-3d/SCALA. The simulation results from OPERA-3d/SCALA showed excellent agreement with previous results. Upon the addition of external magnetic fields to our fully 3D linac simulation, it was found that a transverse magnetic field of 6 G resulted in a 45 +/- 1% beam loss, and by 14 G, no electrons were incident on the target. Transverse magnetic fields on the linac simulation produced a highly asymmetric focal spot at the target, which translated into a 13% profile asymmetry at 6 G. Upon translating the focal spot with respect to the target coordinates, profile symmetry was regained at the expense of a lateral shift in the dose profiles. It was found that all points in the penumbra failed a 1%/1 mm acceptance criterion for fields between 4 and 6 G. However, it was also found that the lateral profile shifts were corrected by adjusting the jaw positions asymmetrically.
authors.affiliation: Department of Physics, University of Alberta, 11322-89th Avenue, Edmonton, Alberta T6G 2G7, Canada.
authors.forename: J
authors.initials: J
authors.lastname: St Aubin
authors.forename: S
authors.initials: S
authors.lastname: Steciw
authors.forename: B G
authors.initials: BG
authors.lastname: Fallone
mesh_terms: D003198:Computer Simulation; D004583:Electrons; D006801:Humans; D008280:Magnetics / Q000295:instrumentation* / Q000379:methods; D015233:Models, Statistical; D009010:Monte Carlo Method; D009038:Motion; D009369:Neoplasms / Q000628:therapy; D010315:Particle Accelerators / Q000295:instrumentation*; D011878:Radiotherapy / Q000379:methods*; D015203:Reproducibility of Results; D012542:Scattering, Radiation