WE-E-BRB-06: Monte Carlo Calculations of the Skin Dose for Longitudinal Linac-MR System Using Realistic...

Area of Science:

• Medical Physics
• Radiation Oncology
• Magnetic Resonance Imaging

Background:

• Linac-MR systems combine linear accelerators with MRI scanners for integrated radiotherapy.
• The strong magnetic fields in MR systems can influence electron transport during radiotherapy.
• Understanding these interactions is crucial for patient safety and treatment efficacy.

Purpose of the Study:

• To quantify the impact of longitudinal magnetic fields on patient skin dose in linac-MR systems.
• To investigate the role of electron confinement in magnetic field-induced dose increases.
• To assess the clinical significance of these effects using realistic modeling.

Main Methods:

• Monte Carlo simulations using EGSnrc (BEAMnrc and DOSXYZnrc) were performed.
• Realistic 3D magnetic field maps from Opera-3D were incorporated.
• Simulations included a Varian 600C linac and MR magnet assembly at 0.6T and 1T.
• Skin dose was analyzed for various air gaps and field sizes.

Main Results:

• Electron containment and increased skin dose were observed, primarily in uniform magnetic field regions.
• Skin dose increases ranged from 1% to 13% depending on air gap (5-31 cm).
• Dose increases also varied with field size, from 3% (20x20 cm²) to 11% (5x5 cm²).

Conclusions:

• Realistic 3D magnetic field modeling, accounting for fringe field decay, shows a minor increase in patient skin dose.
• The calculated increases in skin dose for longitudinal linac-MR systems are clinically insignificant.
• These findings support the safe integration of linac-MR technology in radiotherapy.