Technical Note: Consistency of PTW30013 and FC65-G ion chamber magnetic field correction factors.

PURPOSE: Reference dosimetry in a strong magnetic field is made more complex due to (a) the change in dose deposition and (b) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, relative orientations of field, chamber and radiation, and minor variations in ion chamber stem or electrode construction. The PTW30013 and IBA FC65-G detectors are waterproof Farmer-type ion chambers that are suitable for reference dosimetry. The magnetic field correction factors have previously been determined for these chamber types. The aim of this study was to assess the chamber-to-chamber variation and determine whether generic chamber type-specific magnetic field correction factors can be applied for each of the PTW30013 and FC65-G type ion chambers when they are oriented anti-parallel (ǁ) to, or perpendicular (⊥) to, the magnetic field. METHODS: The experiment was conducted with 12 PTW30013 and 13 FC65-G chambers. The magnetic field correction factors were measured using a practical method. In this study each chamber was cross-calibrated against the local standard chamber twice; with and without magnetic field. Measurements with 1.5 T magnetic field were performed with the 7 MV FFF beam of the MRI-linac. Measurements without magnetic field (0 T) were performed with the 6 MV conventional beam of an Elekta Agility linac. A prototype MR-compatible PTW MP1 phantom was used along with a prototype holder that facilitated measurements with the chamber aligned 90° counter-clockwise (⊥) and 180° (ǁ) to the direction of the magnetic field. A monitor chamber was also mounted on the holder and all measurements were normalized so that the effect of variations in the output of each linac was minimized. Measurements with the local standard chamber were repeated during the experiment to quantify the experimental uncertainty. Recombination was measured in the 6 MV beam. Beam quality correction factors were applied. Differences in recombination and beam quality between beams are constant within each chamber type. By comparing the results for the two cross calibrations the magnetic field correction factors can be determined for each chamber, and the variation within the chamber-type determined. RESULTS: The magnetic field correction factors within both PTW30013 and FC65-G chamber-types were found to be very consistent, with observed standard deviations for the PTW30013 of 0.19% (ǁ) and 0.13% (⊥), and for the FC65-G of 0.15% (ǁ) and 0.17% (⊥). These variations are comparable with the standard uncertainty (k = 1) of 0.24%. CONCLUSION: The consistency of the results for the PTW30013 and FC65-G chambers implies that it is not necessary to derive a new factor for every new PTW30013 or FC65-G chamber. Values for each chamber-type (with careful attention to beam energy, magnetic field strength and beam-field-chamber orientations) can be applied from the literature.

A formalism for reference dosimetry in photon beams in the presence of a magnetic field.

A generic formalism is proposed for reference dosimetry in the presence of a magnetic field. Besides the regular correction factors from the conventional reference dosimetry formalisms, two factors are used to take into account magnetic field effects: (1) a dose conversion factor to correct for the change in local dose distribution and (2) a correction of the reading of the dosimeter used for the reference dosimetry measurements. The formalism was applied to the Elekta MRI-Linac, for which the 1.5 T magnetic field is orthogonal to the 7 MV photon beam. For this setup at reference conditions it was shown that the dose decreases with increasing magnetic field strength. The reduction in local dose for a 1.5 T transverse field, compared to no field is 0.51% ± 0.03% at the reference point of 10 cm depth. The effect of the magnetic field on the reading of the dosimeter was measured for two waterproof ionization chambers types (PTW 30013 and IBA FC65-G) before and after multiple ramp-up and ramp-downs of the magnetic field. The chambers were aligned perpendicular and parallel to the magnetic field. The corrections of the readings of the perpendicularly aligned chambers were 0.967 ± 0.002 and 0.957 ± 0.002 for respectively the PTW and IBA ionization chambers. In the parallel alignment the corrections were small; 0.997 ± 0.001 and 1.002 ± 0.003 for the PTW and IBA chamber respectively. The change in reading due to the magnetic field can be measured by individual departments. The proposed formalism can be used to determine the correction factors needed to establish the absorbed dose in a magnetic field. It requires Monte Carlo simulations of the local dose and measurements of the response of the dosimeter. The formalism was successfully implemented for the MRI-Linac and is applicable for other field strengths and geometries.