Linac-magnetic resonance (MR) systems have been proposed in order to achieve real-time image guided radiotherapy. The design of a new linac-MR system with the in-line 6 MV linac generating x-rays along the symmetry axis of an open MRimager is outlined. This new design allows for a greater MR field strength to achieve better quality images while reducing hot and cold spots in treatment planning. An investigation of linac’s performance in the longitudinal fringe magnetic fields of the MRimager is given.Methods:
The open MRimager fringe magnetic field was modeled using the analytic solution of the magnetic field generated from current carrying loops. The derived solution was matched to the magnetic fringe field isolines provided for a 0.5 T open MRimager through Monte Carlo optimization. The optimized field solution was then added to the previously validated 6 MV linac simulation to quantify linac’s performance in the fringe magnetic field of a 0.5 T MRimager. To further the investigation, linac’s performance in large fringe fields expected from other imagers was investigated through the addition of homogeneous longitudinal fields.Results:
The Monte Carlo optimization of the analytic current loop solution provided good agreement with the magnetic fringe field isolines supplied by the manufacturer. The range of magnetic fields the linac is expected to experience when coupled to the 0.5 T MRimager was determined to be from 0.0022 to 0.011 T (as calculated at the electron gun cathode). The effect of the longitudinal magnetic field on the electron beam was observed to be only in the electron gun. The longitudinal field changed the electron gun optics, affecting beam characteristics, such as a slight increase in the injection current and beam diameter, and an increasingly nonlaminar transverse phase space. Although the target phase space showed little change in its energy spectrum from the altered injection phase space, a reduction in the target current and spatial distribution peak intensity was observed. Despite these changes, the target phase space had little effect on the depth dose curves or dose profiles calculated for a field at 1.5 cm depth. At longitudinal fields larger than 0.012 T, a drastic reduction in the injection current from the electron gun was observed due to a large fraction of electrons striking the anode. This further reduced the target current, which reached a minimum of at 0.06 T. A slow increase in the injection and target currents was observed at fields larger than 0.06 T due to greater beam collimation in the anode beam tube.Conclusions:
In an effort to achieve higher quality images and a reduction in hot and cold spots in the treatment plan, a parallel configuration linac-MR system is presented. The longitudinal magnetic fields of the MRimager caused large beam losses within the electron gun. These losses may be eliminated through a redesign of the electron gun optics incorporating a longitudinal magnetic field, or through magnetic shielding, which has already been proven successful for the transverse configuration.
The authors thank PARAmed Medical Systems, Inc. (North Andover, MA) for allowing access to data for Fig. 3. Partial funding for this work was provided by the Alberta Cancer Foundation and the Natural Sciences and Engineering Research Council of Canada.
II.A. Magnetic fringe field calculations
II.B. 6 MV linac simulation
II.C. Monte Carlo simulations
III. RESULTS AND DISCUSSION
III.A. Effect of MRopen™ fringe magnetic fields on the linac
III.B. Effect of strong longitudinal magnetic fields on the linac
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