Effect of transverse magnetic fields on a simulated in-line 6 MV linac

Phys Med Biol. 2010 Aug 21;55(16):4861-9. doi: 10.1088/0031-9155/55/16/015. Epub 2010 Aug 3.


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.

MeSH terms

  • Computer Simulation
  • Electrons
  • Humans
  • Magnetics / instrumentation*
  • Magnetics / methods
  • Models, Statistical
  • Monte Carlo Method
  • Motion
  • Neoplasms / therapy
  • Particle Accelerators / instrumentation*
  • Radiotherapy / methods*
  • Reproducibility of Results
  • Scattering, Radiation