Sci—Thur PM: YIS — 10: Magnetically Shielding the Linac in a Bi‐Planar Linac‐MR System

Abstract

Purpose: A low field bi‐planar linac‐MR system has been proposed to achieve real‐time image guided radiotherapy. The close proximity of the linac to the MR imager necessitates magnetic shielding to reduce the transverse magnetic fringe field magnitude. Method: Optimized magnetic shielding was designed based on simulations of linac performance in transverse magnetic fields. The linac simulation consisted of an electron gun (EGN2w, COMSOL, PARMELA), and a linac waveguide (COMSOL). The radio‐frequency field within the waveguide was calculated using COMSOL. The linac simulation generated an electron phase space at the linac target which was inputted into BEAMnrc and DOSXYZnrc to simulate dose distributions. Validation of the linac simulation was performed against measurements from a Varian 600C. Passive magnetic shielding was designed in COMSOL and active shielding was designed in the form of four current carrying coil pairs surrounding the linac. Results: The linac simulation at 0 G provided excellent agreement to all electrical and dosimetric measurements. The dosimetric effects of profile asymmetry or lateral shift caused by transverse magnetic fields on the linac were corrected through a shifting of the electron focal spot (as done in linac commissioning) and through asymmetric jaw positions. With no net dosimetric effects caused by transverse magnetic fields, linac performance was only limited by beam losses which increased with magnetic field strength. Passive and active shielding was optimized meeting all design constraints. Conclusion: Magnetic decoupling of the linac in a low field bi‐planar linac‐MR system has been accomplished using either passive or active shielding.