Electron contamination modeling and reduction in a 1 T open bore inline MRI-linac system

Area of Science:

• Medical Physics
• Radiation Oncology
• Magnetic Resonance Imaging

Background:

• Inline MRI-linac systems integrate magnetic resonance imaging (MRI) with linear accelerators (linacs).
• A potential side effect is electron contamination focusing, leading to elevated skin dose.
• This study investigates mitigation strategies for an open bore 1 T MRI system.

Purpose of the Study:

• To model the efficiency of an electron contamination deflector (ECD) in purging electron contamination.
• To assess the impact of a helium gas region on reducing air-generated contamination.
• To reexamine skin dose predictions for an open bore 1 T MRI system.

Main Methods:

• 3D magnetic field maps were generated using magnetic modeling of the 1 T MRI.
• Geant4 Monte Carlo simulations were performed, incorporating linac head, ECD, and water phantom.
• 2D skin doses at 70 μm depth were calculated for various beam sizes and field configurations.

Main Results:

• The ECD was highly efficient at purging electron contamination, though some scattering occurred.
• Using helium gas significantly minimized air-generated contamination.
• An optimal ECD and helium gas combination moderately increased skin dose in small hot spots (12-65% of Dmax).

Conclusions:

• An efficient ECD coupled with a helium gas region can ameliorate skin dose increases in MRI-linac systems.
• The ECD is practical, with correctable MRI imaging distortion and manageable mechanical forces.
• This approach offers a viable solution to mitigate electron contamination side effects in MRI-guided radiotherapy.