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Novel MRI Compatible Electron Accelerator for MRI-Linac Radiotherapy

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B Whelan

B Whelan1*, S Gierman2 , J Schmerge2 , L Holloway3 , P Keall1 , R Fahrig4 , (1) University of Sydney, Sydney, Australia, (2) SLAC National Accelerator Laboratory, Palo Alto, California, (3) Ingham Institute, Sydney, NSW, (4) Stanford University, Stanford, CA


WE-G-BRD-9 (Wednesday, July 15, 2015) 4:30 PM - 6:00 PM Room: Ballroom D

Purpose:MRI guided radiotherapy is a rapidly growing field; however current linacs are not designed to operate in MRI fringe fields. As such, current MRI-Linac systems require magnetic shielding, impairing MR image quality and system flexibility. Here, we present a bespoke electron accelerator concept with robust operation in in-line magnetic fields.

Methods:For in-line MRI-Linac systems, electron gun performance is the major constraint on accelerator performance. To overcome this, we propose placing a cathode directly within the first accelerating cavity. Such a configuration is used extensively in high energy particle physics, but not previously for radiotherapy. Benchmarked computational modelling (CST, Darmstadt, Germany) was employed to design and assess a 5.5 cell side coupled accelerator with a temperature limited thermionic cathode in the first accelerating cell. This simulation was coupled to magnetic fields from a 1T MRI model to assess robustness in magnetic fields for Source to Isocenter Distance between 1 and 2 meters. Performance was compared to a conventional electron gun based system in the same magnetic field.

Results:A temperature limited cathode (work function 1.8eV, temperature 1245K, emission constant 60A/K/cm²) will emit a mean current density of 24mA/mm² (Richardson’s Law). We modeled a circular cathode with radius 2mm and mean current 300mA. Capture efficiency of the device was 43%, resulting in target current of 130 mA. The electron beam had a FWHM of 0.2mm, and mean energy of 5.9MeV (interquartile spread of 0.1MeV). Such an electron beam is suitable for radiotherapy, comparing favourably to conventional systems. This model was robust to operation the MRI fringe field, with a maximum current loss of 6% compared to 85% for the conventional system.

Conclusion:The bespoke electron accelerator is robust to operation in in-line magnetic fields. This will enable MRI-Linacs with no accelerator magnetic shielding, and minimise painstaking optimisation of the MRI fringe field.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by US (NIH) and Australian (NHMRC & Cancer Institute NSW) government research funding. In addition, I would like to thank cancer institute NSW and the Ingham Institute for scholarship support.

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