Purpose: Provide physical insights on detector dose-response to multiple megavoltage photon beam sizes coupled to magnetic fields and determine optimal orientations for measurements.
Methods: Small-cavity detector (solid-state: PTW60012 and PTW60019, ionization chambers: PTW31010, PTW31021, and PTW31022) dose-responses in water are determined with Monte Carlo simulations (EGSnrc), using Elekta Unity 7 MV FFF phase-spaces of field widths between 0.25 and 10 cm, for four detector axis orientations: perpendicular to the B-field (1.5 T) with the beam parallel (orientation 1) or perpendicular (orientation 2 or 3 with FLorentz towards stem or tip); perpendicular to the beam and parallel to B-field (orientation 4). The B-field effect on perturbation factors is evaluated. The overall perturbation factor (P_MC) accounts for the extracameral components, atomic composition, and density. For each orientation, the density (P_ρ) and volume averaging (P_vol) perturbation factors and quality correction factors (k_(Q(B))) are calculated.
Results: For solid-state detectors, P_ρ is unaffected by the B-fiteld and field size. For chambers in B=1.5 T, it increases up to 1.564±0.001 with decreasing field size over all orientations. For chambers, the B-field effect on P_MC is only significant for field widths <1 cm, increasing with decreasing field size in all orientations. For solid-state detectors, the B-field effect on P_MC exhibits different trends with orientation, indicating that the beam incident angle and geometry play a crucial role. A maximal effect of 1.221±0.005 occurs in orientation 2. P_vol remains the most significant perturbation both with and without magnetic fields. In most cases, the B-field effect on P_vol is 1% or less, except for chambers PTW31021 and PTW31022 that over-respond and under-respond in orientations 2 and 3, respectively, at smaller fields.
Conclusion: Solid-state detectors dose-response is strongly affected by the B-field in all orientations. In general, ionization chambers yield a k_(Q(B)) closer to unity, especially in orientations 2 and 4.
Magnetic Fields, Small Fields, Perturbation Factor
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined computational dosimetry: Monte Carlo