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.

Electron contamination modeling and skin dose in 6 MV longitudinal field MRIgRT: Impact of the MRI and MRI fringe field

“…9,10 In terms of engineering, the transverse MRI-linac system faces some issues with changes to the gun, waveguide, and multileaf collimator operation. [12][13][14][15] Magnetic shielding is required to reduce the effect of the MRI fields down to low enough levels for proper operation of the linac. 14 There have been some recent studies on the improved dosimetry that a parallel or longitudinal MRI-linac system would offer over the current transverse field systems.…”

Magnetic shielding investigation for a 6 MV in-line linac within the parallel configuration of a linac-MR system

“…In category 2, the linac and the MR (biplanar, open, or split-field) rotate in unison around the patient. 1 In this design, the linac and MR magnets are stationary with respect to each other 1,5,6 which eliminates any eddy currents induced in the linac as the gantry rotates, and allows for an unattenuated treatment beam. There are two possible linac-to-MR configurations in this design which allows for the B 0 field to be either perpendicular or parallel to the axis of the treatment beam.…”

FEM design and simulation of a short, 10 MV, S‐band Linac with Monte Carlo dose simulations

“…19 The electron phase space injected into the waveguide was produced by the same Opera 3D/SCALA simulated electron gun as our group's previous work. 20 The PARMELA reference particle was started at a phase of −45 • , to ensure it remained in the center of the traveling bunch, and RF phase was T III. Accelerating cavity radii (R Cav ) and coupling cavity postlengths (L P ) after tuning the unshifted and the 1.45 mm shifted (denoted by *) accelerator models.…”