Impact of magnetic field orientation and strength on dose distributions in 6 MV photon beams: A Monte Carlo study.

Abstract

This study investigates the influence of magnetic field orientation (Ox, Oy, Oz) and strength (0-3 T) on percentage depth dose (PDD) and dose profile parameters for 6 MV clinical photon beams using Monte Carlo simulations. Monte Carlo simulations were performed using GATE (version 9.2) with a validated 6 MV phase space source. Three field sizes (10 × 10, 15 × 15, and 20 × 20 cm²) were evaluated under magnetic fields ranging from 0 to 3 T in increments of 0.5 along three orthogonal directions (Ox, Oy, Oz). PDD parameters including R₁₀₀, D₁₀₀, D₂₀₀, surface dose (Ds), and quality index (Qi) were analyzed. Dose profile characteristics including field size and penumbra width were evaluated in both X and Y directions. Magnetic field orientation and strength significantly affected dose distributions. For Ox and Oy orientations, R₁₀₀ decreased progressively with increasing field strength: from 16 to 17 mm at 0 T to 7-9 mm at 3 T across all field sizes. Surface dose increased substantially from 40 to 45% at 0 T to 58-65% at 3 T. For Oz orientation, R₁₀₀ showed minimal variation (10-13 mm across all field strengths), while surface dose increased more substantially to 56-66%. Penumbra width and field size demonstrated orientation-dependent variations, with minimal changes for Ox and Oy, but systematic reductions for Oz orientation. It is concluded that magnetic field presence substantially alters dose deposition patterns in an orientation-dependent manner. These findings are critical for MRI-guided radiotherapy systems and require specific dosimetric considerations for treatment planning and quality assurance protocols.