Design and simulation of a short, 10 MV, variable energy linear accelerator for use in linac-MR systems

• Author / Creator

  Baillie, Devin MJ

• Hybrid linac-MR devices are currently being designed and constructed by several groups around the world which combine a linear accelerator with an MRI in order to enable real-time 3D imaging to visualize tumor movement during external beam radiotherapy treatments. Each of the current systems is limited to a single x-ray energy (6 MV or 8 MV) for treatments which can result in suboptimal treatments for many patients. In conventional radiotherapy, a variety of energies are used to achieve optimal treatments. Larger patients with deeper tumors can be better treated with higher energy x-rays, while superficial tumors or those located near an air cavity can benefit from lower energy x-rays. Current multi-energy medical linacs are too long to be incorporated into linac-MR systems. In order to enable linac-MR systems to treat every patient optimally, this research aimed to design a new linear accelerator capable of producing a variety of energies from 4 to 10 MV that short enough to be compatible with the Alberta linac-MR system, which currently uses a 27.5 cm long Varian 600C (6 MV) linear accelerator. A previously designed finite element model (FEM) of the Varian 600C linear accelerator was used to investigate the feasibility of designing a linear accelerator the same length as the current 6 MV accelerator, but capable of producing 10 MV. The input power to the FEM model was increased, and the resulting electron and photon beams were simulated using particle-in-cell (PIC) and Monte Carlo (MC) methods, respectively. It was shown that, with some modification to the waveguide, a 10 MV x-ray beam can be produced without risk of electric breakdown within the waveguide. A new, short linear accelerator was then designed capable of producing a 10 MV x-ray beam. This was accomplished by first designing a single accelerator cavity based on published electric breakdown experiments, and using this as a basis for a full accelerator structure. The cavity dimensions were matched to the breakdown study using a stochastic approach and FEM simulations. FEM simulations were also used to tune the full waveguide structure and calculate the RF fields within. PIC and MC simulations were again used to simulate the electron beam through the waveguide and the x-ray beam produced. The results of the MC simulations were then used to optimize the waveguide geometry until an x-ray beam matching the energy of currently used 10 MV linear accelerators was produced. Percent depth dose curves from the new linac’s x-ray beam closely matched that produced by a Varian 10 MV linac, while the fields within the waveguide remained below the cavity-specific breakdown threshold. The newly designed 10 MV accelerator was then modified to allow the x-ray energy to be reduced, in order to produce a variety of x-ray energies from a single accelerator. This was accomplished by the addition of a tuning cylinder to the first coupling cavity of the waveguide, allowing the power in the first cavity to be varied independently from the power in the remaining cavities. By reducing the input power, the electron energies were reduced, and by adjusting the position of the tuning cylinder the first cavity fields were optimized for electron capture. The input power and cylinder position were optimized to so that the new 10 MV linac would produce x-ray beams with energies of 4, 6, 8, and 10 MV, with the 4, 6, and 10 MV energies all beam-matched to Varian accelerators of the same energy. Final depth dose curves from the new variable-energy linac showed excellent agreement compared to Varian accelerators over all optimized energies (4, 6, and 10 MV).

• Subjects / Keywords

  • Waveguide
  • Radiotherapy
  • Linac

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3DB7W426

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.