Enhancement of electron beam conformity in MRI-guided radiotherapy with parallel magnetic fields: a Monte Carlo analysis.

MRI-guided radiotherapy systems employ magnetic fields that fundamentally alter dose distributions, yet parallel field configurations remain unexplored. Unlike prior studies limited to small fields or single magnetic strengths, this work presents the first comprehensive parametric analysis mapping the full interplay between magnetic field strength (0–3 T), beam energy (6–15 MeV), and field size (3 × 3–15 × 15 cm²) in a clinically realistic setting. We systematically investigated parallel magnetic field effects on clinical electron beams. Monte Carlo simulations (GATE v9.2) modeled 6, 12 and 15 MeV electron beams under 0-3T parallel magnetic fields for three field sizes (3 × 3, 6 × 6, 15 × 15 cm²). We analyzed depth-dose parameters (R100, R50, R90), lateral penumbra, and 2D dose distributions. Novel conformity metrics—Electron Conformity Index and Penumbra Ratio—were introduced to quantify the focusing effects of parallel magnetic confinement. 6 MeV beams demonstrated exceptional stability (R100 variation < 8.2%) with systematic penumbra improvement (50.5% reduction at 3T). Conversely, 12 and 15 MeV beams showed dramatic field-size-dependent instability, with R100 varying 76.7% for 3 × 3 cm² fields but stabilizing for larger fields. All configurations exhibited progressive lateral dose confinement, achieving 32–50% penumbra reduction. These results reveal nonlinear dose–field interactions not captured in earlier work, identifying a 1.5–2 T therapeutic window where conformity and stability are maximized. Parallel magnetic fields offer superior dose conformity through helical electron confinement, contrasting with problematic perpendicular configurations. This study establishes a complete parametric framework and quantitative reference for MR-linac optimization, providing the first theoretical evidence that parallel-field MRI-guided electron therapy can achieve clinically actionable margin reductions (2–3 mm) while maintaining beam stability across energies.