Evaluation of Distorted Lateral Beam Profiles in MRI-Guided Proton Therapy
H Ueda1*, Y Fujii1,2*, K Umegaki1*, T Matsuura1*(1) Hokkaido university, Sapporo, JP(2) Hitachi Ltd., Ibaraki, JP
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PO-GePV-T-123 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: In order to realize MRI-guided proton beam therapy system, it is important to evaluate the characteristics of distorted dose and relative biological effectiveness (RBE) distributions in the MRI scanner. The deflected proton beams in the perpendicular magnetic field result in asymmetrically distorted lateral beam profiles. The purpose of this study is to clarify these profiles by analyzing dose, mean energy and dose-averaged linear energy transfer (LETd) distributions under the various magnetic fields.
Methods: Monte Carlo simulations using Geant4.10.2.p02 were performed for proton beam analysis in magnetic fields ranging from 0 to 3 T. Initial proton energies are set from 70 to 220 MeV and the proton source is assumed to have no energy spread and angular divergence. A water phantom was installed in the MRI scanner where the magnetic field was applied perpendicular to the beam incident angle. The three-dimensional dose distributions in the phantom were calculated. The lateral dose profiles were fitted using the primary Gaussian, secondary Left, and Right tail functions. Then the ratio of the coefficient of Left to Right function (γR to γL) was evaluated to quantify the magnitude of asymmetric distortion.
Results: The distortions of the lateral beam profile were observed near Bragg peak region, leading to non-negligible asymmetrical changes in the dose distributions. The asymmetric ratio of γR to γL, which indicates the magnitude of distortion, increased as the strength of the magnetic field increased. The ratio became 2.4 under the 3T magnetic field. In addition, it was shown that the mean energy and LETd distributions also have non-negligible distortions which will affect RBE distributions.
Conclusion: We quantitatively evaluated the distortion of the dose distribution in the magnetic field using the fitting functions. Non-negligible dose deformations at the Bragg peak area were identified and distortion of LETd also affected RBE.
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