Image guidance in radiation therapy has three goals: 1. to decrease the geometric uncertainties of the dose distribution, 2. to improve the sparing of healthy tissue, and 3. to facilitate additional boost treatment to the macroscopic tumour. An ideal imaging system for image guidance would provide real-time imaging at 3-5 frames per second, excellent soft tissue contrast for tumour visualization, and geometric fidelity of 2 mm, all with n additional non-therapeutic radiation dose to the patient. We hypothesize that real-time magnetic resonance image guidance provides the requisite image quality for a wide variety of tumour types and locations in the body. In support of this hypothesis, we have aligned with a multi-institution collaborative program to research, develop and evaluate all aspects of the technology required to bring MR guidance into the radiation therapy world. In this proposal, we bring our experience with electron optics and x-ray tube development to bear on the challenge of researching and building an MR-compatible linear accelerator for use with the 'in-line MR-Linac'system, as well as for use with our new concept of Robotic Linac Adaptation or 'RLA MRI-Linac'. Our hypothesis is that an electron gun designed for optimal operation in an in-line magnetic field can function as effectively and efficiently as current electron gun designs in non- magnetic field conditions.
The specific aims of this project are: 1. to develop an optimized electron gun design for use within a parallel fringe field with strengths up to 0.2 T and 2. to build and verify the us of this gun when placed in parallel at up to 0.2 T fringe magnetic field in the In-Line MRI-Linac and/or the RLA MRI- Linac.
The first aim will be accomplished using Finite Element modeling to refine our new, MR-compatible electron gun design to include grid control as well as to ensure compatibility between the gun and the accelerating waveguide.
The second aim will be accomplished by working with an electron gun manufacturer to build our optimized design, and then by measuring and verifying the performance of the gun in various external fields using a Faraday cup measurement apparatus. The general problem we address is the focusing of radiation beams on tumors as their anatomy and physiology changes during treatment. We believe that successful completion of this MRI-Linac program could have a direct impact on the treatment and lives of cancer patients in the near future.

Public Health Relevance

The goal of radiation therapy is to direct the maximum dose to tumors while avoiding the surrounding healthy tissue. Magnetic resonance imaging could be used to guide radiation therapy since MR images clearly show tumor, and also can be used to follow the change in shape and location of tumors during breathing and other motions. We will design and build an MR-compatible linear accelerator gun so that radiation therapy can be delivered with MR image guidance in the most flexible and useful way possible, with the goal of improving the treatment of cancer patients in the near future.

National Institute of Health (NIH)
National Institute of Biomedical Imaging and Bioengineering (NIBIB)
Exploratory/Developmental Grants (R21)
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Biomedical Imaging Technology Study Section (BMIT)
Program Officer
Krosnick, Steven
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Stanford University
Schools of Medicine
United States
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Wang, Jinghui; Trovati, Stefania; Borchard, Philipp M et al. (2017) Thermal limits on MV x-ray production by bremsstrahlung targets in the context of novel linear accelerators. Med Phys 44:6610-6620
Whelan, Brendan; Holloway, Lois; Constantin, Dragos et al. (2016) Performance of a clinical gridded electron gun in magnetic fields: Implications for MRI-linac therapy. Med Phys 43:5903
Whelan, Brendan; Gierman, Stephen; Holloway, Lois et al. (2016) A novel electron accelerator for MRI-Linac radiotherapy. Med Phys 43:1285-94
Constantin, Drago? E; Holloway, Lois; Keall, Paul J et al. (2014) A novel electron gun for inline MRI-linac configurations. Med Phys 41:022301