title: A novel electron accelerator for MRI-Linac radiotherapy.
abstract: OBJECTIVE
  MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems.
  
  
  METHODS
  Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified.
  
  
  RESULTS
  For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-width-tenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields.
  
  
  CONCLUSIONS
  Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility.
authors.affiliation: Radiation Physics Laboratory, University of Sydney, Sydney, NSW 2006, Australia and Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.
authors.forename: Brendan
authors.initials: B
authors.lastname: Whelan
authors.affiliation: SLAC National Laboratory, Menlo Park, California 94025.
authors.forename: Stephen
authors.initials: S
authors.lastname: Gierman
authors.affiliation: Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.
authors.forename: Lois
authors.initials: L
authors.lastname: Holloway
authors.affiliation: SLAC National Laboratory, Menlo Park, California 94025.
authors.forename: John
authors.initials: J
authors.lastname: Schmerge
authors.affiliation: Radiation Physics Laboratory, University of Sydney, Sydney, NSW 2006, Australia and Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.
authors.forename: Paul
authors.initials: P
authors.lastname: Keall
authors.affiliation: Department of Radiology, Stanford University, Palo Alto, California 94305.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Fahrig
mesh_terms: D004583:Electrons; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging; D010315:Particle Accelerators; D011846:Radio Waves; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation*