TU‐F‐CAMPUS‐T‐01: Calculation of KQ for a Variety of Commercially Available Ionization Chambers in the Presence of An External Magnetic Field for MR‐Linac Dosimetry

Purpose: The strong magnetic fields associated with MRI have a significant impact on the dosimetry of radiotherapy beams as well as on the response of the detectors used to measure it. In order to calibrate the dose delivered by these beams, it is necessary to correct for these effects when performing measurements with ionization chambers. Methods: Detailed models of 16 commercially‐available ionization chambers were implemented in Geant4. To validate these models, calculations of the beam quality correction factor kQ (defined by TG‐51) were performed using the spectrum from a 6 MV Elekta SL25. kQ values were then recalculated for each detector with a 1.5 T external magnetic field applied uniformly across the geometry. Chamber, beam and B‐field were all orthogonal. Results: The comparison between the kQ calculations and TG‐51 showed agreement within 1.4%. The B‐field calculations indicate that the change in kQ values due to the B‐field is strongly affected by the volume and material of the ionization chamber. All large volume (>0.5 cm 3) ionization chambers had lower kQ values when used in a B‐field. The kQ values for small volume chambers were very sensitive to the chambers’ collecting volume and could be higher or lower when used in a B‐field. Conclusions: Of the chambers tested, the PTW 30011 demonstrated the smallest correction of just 1.2%. The next step is to calculate kQ using the energy spectrum from an actual MR‐linac. Further validation of these results against measurements will be paramount before they can be used. Funding provided by Elekta Limited. The MR‐linac spectrum will be provided by Elekta Limited based on a machine they have designed.