Monte Carlo modeling of the origin of contaminant electrons on a 0.5T bi-planar Linac-MR

Abstract

Introduction: In the last decade, hybrid linear accelerator magnetic resonance imaging (Linac-MR) devices have evolved into FDA-cleared clinical tools, facilitating magnetic resonance guided radiotherapy (MRgRT). The addition of a magnetic field to radiation therapy has previously demonstrated dosimetric and electron effects regardless of magnetic field orientation.

Purpose: This study uses Monte Carlo simulations to investigate the importance and efficacy of the magnetic field design in mitigating surface dose enhancement in the Aurora-RT, focusing specifically on contaminant electrons, their origin, and energy spectrum.

Methods: The Aurora-RT 0.5 T Biplanar Linac-MR device was modeled using the BEAMnrc package using the updated EM macros, a magnetic field map generated from Opera 3D. Simulation generated phasespace data at the distal side of the first magnetic pole plate (89 cm) and at machine isocenter (120 cm) were analyzed with respect to electron energy spectra and electron creation origins, both with and without the static magnetic field.

Results: The presence of the main magnetic field was verified to affect the origin and distribution of contaminant electrons, removing them from the air column up to 60 cm from the target, and focusing them along the CAX within the region below. Analysis of the remaining electron energy fluence reveals the net removal of electrons with energies > 2 MeV and generation of electrons with energies < 2 MeV in the presence of the static magnetic field as compared to no magnetic field. Moreover, in the presence of the magnetic field the integral energy contained in the contaminant electrons increases from 89 cm to isocenter but is still 15% less overall than the integral energy contained in contaminant electrons without the magnetic field.

Conclusion: This study provides an analysis of contaminant electrons in the Aurora-RT 0.5 T Linac-MR, emphasizing the role of magnetic field design in successfully minimizing electron contaminants.

Keywords: MR‐Linac; electron contamination; surface dose.

FIGURE 1

Linac‐MR beam path section highlighting the yoke and first magnetic pole plate positioning with respect to the target, beam collimation, isocenter, and magnetic field direction.

FIGURE 2

The simulated Z (CAX) position of origin for all electrons scored in a phasespace in the Linac‐MR in a 20 cm x 20 cm collimated field, (a) without the magnetic field at the distal side of the first pole plate (89 cm), (b) with the magnetic field at the distal side of the first pole plate (89 cm), (c) without the magnetic field at isocenter (120 cm), (d) with the magnetic field at isocenter (120 cm). Vertical lines depict the entry to the yoke (50 cm) and the distal side of the first pole plate (89 cm). Simulation uncertainty is displayed, but for the majority of points is smaller than the data points.

FIGURE 3

Radial histogram of uncollimated electrons in the plane of the pole plate (89 cm) in the presence of the main magnetic field.

FIGURE 4

2D (horizontal red dashed line represents distal side of the first pole plate) scatter plot of electron positions of creation [scoring plane at isocenter (120 cm)] in a 10 cm x 10 cm collimated beam in the presence of the main magnetic field, colour scale represents electron energy in MeV, z axis is along the beam central axis (CAX).

FIGURE 5

Electron energy fluence of a 20 cm X 20 cm field scored at isocenter (120 cm) and the distal side of the first pole plate (89 cm) with and without the main magnetic field. Simulation uncertainty is displayed, but for the majority of points is smaller than the data points.