The magnetic field dependent displacement effect and its correction in reference and relative dosimetry

Objective.This study investigates the perturbation correction factors of air-filled ionization chambers regarding their depth and magnetic field dependence. Focus has been placed on the displacement or gradient correction factorPgr.Additionally, the shift of the effective point of measurementPeffthat can be applied to account for the gradient effect has been compared between the cases with and without magnetic field.Approach.The perturbation correction factors have been simulated by stepwise modifications of the models of three ionization chambers (Farmer 30013, Semiflex 3D 31021 and PinPoint 3D 31022, all from PTW Freiburg). A 10 cm × 10 cm 6 MV photon beam perpendicular to the chamber's axis was used. A 1.5 T magnetic field was aligned parallel to the chamber's axis. The correction factors were determined between 0.4 and 20 cm depth. The shift ofPefffrom the chamber's reference pointPref,Δz,was determined by minimizing the variation of the ratio between dose-to-waterDwzref+Δzand the dose-to-airD¯airzrefalong the depth.Main Results.The perturbation correction factors with and without magnetic field are depth dependent in the build-up region but can be considered as constant beyond the depth of dose maximum. Additionally, the correction factors are modified by the magnetic field.Pgrat the reference depth is found to be larger in 1.5 T magnetic field than in the magnetic field free case, where an increase of up to 1% is observed for the largest chamber (Farmer 30013). The magnitude ofΔzfor all chambers decreases by 40% in a 1.5 T magnetic field with the sign ofΔzremains negative.Significance.In reference dosimetry, the change ofPgrin a magnetic field can be corrected by applying the magnetic field correction factorkQmsrBwhen the chamber is positioned with itsPrefat the depth of measurement. However, due to the depth dependence of the perturbation factors, it is more convenient to apply theΔz-shift during chamber positioning in relative dosimetry.