Development of An Irradiation System for MRI-Guided Proton Therapy
Y Fujii1,2*, H Ueda1, T Takayanagi2, K Umegaki1, T Matsuura1, (1) Hokkaido University, Graduate School of Engineering, Sapporo, Hokkaido, Japan. (2) Hitachi Ltd., Research and Development Group, Hitachi-shi, Ibaraki-ken, Japan.
Presentations
TH-D-TRACK 5-3 (Thursday, 7/29/2021) 2:00 PM - 3:00 PM [Eastern Time (GMT-4)]
Purpose: To investigate a proposed design for an MRI-guided proton therapy system that simultaneously achieves both the large field size (FS) for protons and large field of view (FOV) for MRI. The basic characteristics of the proton beam delivered in the proposed system were analyzed.
Methods: An open MRI design with iron magnetic poles was adopted in order to achieve both the large FS for protons and FOV for MRI and reduce the MR fringe fields at the beam scanning magnets and spot position monitor. A hole was made in the center of the yoke to ensure an unobstructed proton beam path. The shape of the magnetic poles was optimized assuming axis symmetry to ensure the magnetic field was uniform. Then the magnetic field strength map was calculated considering the yoke with a hole. A helium chamber was installed between the spot position monitor and dose monitors to reduce the beam size. Geant4 simulation (ver. 10.05.p01) was used to model these configurations and analyze the beam size with and without the helium chamber.
Results: The achieved uniformity of the magnetic field strength was less than 0.8 ppm on the FOV surface. The maximum magnetic field strength in the area upstream from the yoke was 2.8 mT. This indicated that the MR magnetic flux was successfully confined in the yoke and the fringe fields decayed rapidly. The beam sizes of a 70 MeV proton beam were 10.2 and 9.2 mm at the iso center without and with the helium chamber, respectively.
Conclusion: The proposed design of the MRI-guided proton therapy system realized both the large FS for protons and FOV for MRI, while the magnetic fringe fields were sufficiently small and did not affect the beam monitoring. Beam size was shown to be small enough to be used in clinics.
Funding Support, Disclosures, and Conflict of Interest: Authors Yusuke Fujii and Taisuke Takayanagi are employee of Hitachi Ltd., Tokyo, Japan.
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