Feasibility Study of Proton Scanning System with Perpendicular Magnetic Field by Open MRI
H Ueda1*, Y Fujii2,3, T Takayanagi3, K Nishioka4, T Hashimoto4, H Aoyama4, K Umegaki1, T Matsuura1, (1) Faculty of engineering, Hokkaido University, Japan, (2) Graduate School of Engineering, Hokkaido University, Japan, (3) Hitachi Ltd., Japan, (4) Department of Radiation Oncology, Faculty and Graduate School of Medicine, Hokkaido University, Japan
Presentations
PO-GePV-M-84 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To achieve both large field-size (FS) of protons and large field-of-view (FOV) in MRI-integrated proton therapy (MRiPT) system, an open MRI design has been proposed, where a hole is made in the center of the yoke through which the proton beam travels to patient. This study identified the offset rotation angle of the MR scanner around the beam’s central axis to ensure the large FOV and explored the feasibility of a prostate plan with the proposed MRiPT assembly.
Methods: Proton scanning nozzle with its design slightly modified from the existing system and the magnetic field map modeled for 0.3-T MRiPT system were used in the Geant4 simulation. The yoke hole is open over 20 degrees in a direction vertical to the magnetic field. 70 and 220-MeV pencil beams were delivered to a water phantom with the Bragg peaks positioned at the isocenter depth, laterally shifted by ±20 cm from the isocenter. The offset rotation angle of the MR scanner was determined so that the yoke hole ensures an unobstructed proton beam path. Then, by assuming the assembly, prostate plans were made using the Geant4-based inverse optimization tool, and the impact of the beam deflection by the magnetic field was assessed by comparing the DVH parameters of CTV and OARs (rectum and bladder).
Results: The irradiation field of 40 × 30 cm² was successfully scanned with 3 degrees of scanner’s rotation. The differences in DVH parameters of both CTV and OARs were all less than 2.5%, therefore, the impact of the magnetic field on the dose distribution was considered to be small.
Conclusion: Large FS of proton beam was realized with a small rotation of the open MRI scanner. The treatment planning for prostate cancer was shown to be feasible with the proposed MRiPT assembly.
Funding Support, Disclosures, and Conflict of Interest: Yusuke Fujii and Taisuke Takayanagi are paid by Hitachi Ltd.
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