Study the Impact of Magnetic Field on Dosimetry of Proton Therapy Using Monte Carlo Simulation

Abstract

<h3>Purpose/Objective(s)</h3> Using Monte-Carlo simulation to study the dosimetry of proton therapy under the environment of magnetic field. The results of this study will provide knowledge for us to design and build the MRI system that is integrated to proton therapy to form an MRI-guided proton therapy system using our new developed medium temperature superconductors (MTS) and high temperature superconductors (HTS). <h3>Materials/Methods</h3> We performed Monte Carlo calculations using version 3.26 of the PHITS code to simulate and investigate various proposed configurations of MRI-guided proton therapy. The first thing to study is the dosimetry under the environment of magnetic field. In simulation we used a cylindrical water phantom 60 cm in diameter and 2 m in length to imitate a patient and irradiated it with the proton beam with an energy of 250 ± 5 MeV under the environment of a dipole magnetic field. We started the study first with simple and uniform magnetic fields, and then with a more realistic magnetic field that is close to or similar to what we are going to design: 1) simple and uniform magnetic fields with different strength varies from 0.5 to 3.5 Tesla along X, Y or Z direction in respectively; 2) a more realistic magnetic field of a designed magnet using MTS and HTS technology. The track of particles and the dose distribution were investigated. <h3>Results</h3> For magnetic field in only X or Y direction (Bx≠0 or By≠0), the proton beam was bent and the coordinates of the maximum absorbed dose (Bragg peak) position shifted. The displacement of Bragg peak position increases with the magnetic field strength. For magnetic field along Z direction (Bz≠0, Bx=By=0), the change in Bragg peak position and the absorbed dose are small. For the magnetic field using MTS and HTS technology and the magnet was aligned along the beam direction, our simulation shows no significant change in Bragg peak position as well as the absorbed dose such as the maximum dose D_max. <h3>Conclusion</h3> Using Monte Carlo simulation to study the dosimetry of the proton beam interacting with medium (water) under the environment of magnetic fields with different directions and various strengths allowed us to understand the trajectories of proton beam and other secondary particles as well as the impact to the absorbed dose, which provide us important information to assist the design of MRI-guided proton therapy system.