Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations

Abstract

The aim of this work was to determine magnetic field correction factors that are needed for dosimetry in hybrid devices for MR-guided radiotherapy for Farmer-type ionization chambers for different magnetic field strengths and field orientations. The response of six custom-built Farmer-type chambers irradiated at a 6 MV linac was measured in a water tank positioned in a magnet with magnetic field strengths between 0.0 T and 1.1 T. Chamber axis, beam and magnetic field were perpendicular to each other and both magnetic field directions were investigated. EGSnrc Monte Carlo simulations were compared to the measurements and simulations with different field orientations were performed. For all geometries, magnetic field correction factors, [Formula: see text], and perturbation factors were calculated. A maximum increase of 8.8% in chamber response was measured for the magnetic field perpendicular to chamber and beam axis. The measured chamber response could be reproduced by adjusting the dead volume layer near the chamber stem in the Monte Carlo simulations. For the magnetic field parallel to the chamber axis or parallel to the beam, the simulated response increased by 1.1% at maximum for field strengths up to 1.1 T. A complex dependence of the response was found on chamber radius, magnetic field strength and orientation of beam, chamber axis and magnetic field direction. Especially for magnetic fields perpendicular to beam and chamber axis, the exact sensitive volume has to be considered in the simulations. To minimize magnetic field correction factors and the influence of dead volumes on the response of Farmer chambers, a measurement set-up with the magnetic field parallel to the chamber axis or parallel to the beam is recommended for dosimetry.

Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality

“…Recalculating with the updated blueprint‐based model used in this work and a 1.5 mm long “dead" volume region reduces the difference between the simulation and experiment to 0.35%. Spindeldreier et al . compared their experimental measurements with EGSnrc simulations (using a different magnetic field code) to show that, by adjusting the size of the sensitive volume used in the simulation, agreement with a root‐mean‐square deviation of 0.2% and a maximum difference 0.9% could be achieved for the six chambers in their study.…”

“…As mentioned above, when a magnetic field is present the true size of the ion chamber's collection volume plays an important role in the simulated dose‐to‐air . This is due to the charge collection electric field of the chamber having a fringing effect near the stem and guard electrode, and a portion of “dead" air is formed from which the charge released is not collected .…”

“…The second approach is to introduce an additional magnetic field correction factor, k B , such that:k B accounts only for the change in the calibration coefficient due to the magnetic field. This is the approach explored by a few groups for specific chambers, magnetic fields, and beam qualities . A potential advantage of using k B instead of k is a reduced dependence of the correction factor on beam quality, but a disadvantage is the added complexity in using two correction factors.…”

“…The dependence of both %dd(10) x and TPR on magnetic fields for several beam qualities are explored in this work. Furthermore, the effect of the true sensitive collection volume of the ion chamber has been found to impact the MC determined chamber response, that is, dose to the chamber's air cavity, when a magnetic field is present . Here the variation in k due to changes in the sensitive volume are looked at, and a resolution of the issue is sought through finding an optimal orientation with respect to the magnetic field.…”

Relative dosimetry with an MR‐linac: Response of ion chambers, diamond, and diode detectors for off‐axis, depth dose, and output factor measurements

“…This magnetic field disrupts the deposition of dose (in a patient or a phantom) by deflecting the paths of the secondary electrons via the Lorentz force . In addition, the dose–response of the detectors used to characterize and calibrate the beam is also affected …”