Experimental measurement of ionization chamber angular response and associated magnetic field correction factors in MR-linac

Purpose To measure ionization chamber dose response as a function of the angle between magnetic field direction and ionization chamber orientation in magnetic resonance-guided radiation therapy (MRgRT) system, and to evaluate angular dependence of magnetic field correction factor for reference dosimetry. Methods Measurements were performed on an Elekta MR-linac that integrates a 1.5-T Philips MRI and a 7-MV FFF photon beam accelerator. The response of four reference class chambers (Exradin-A19, A1SL, IBA FC65-G, and CC13, paired with a PTW UE electrometer) was studied. An in-house built MR-compatible water tank and an accompanying cylindrical insert that allowed chamber rotation around the cylinder's axis was used. The EPID onboard imaging was used to center chamber at the MR-linac isocenter (143.5 cm, SAD), as well as to verify position at each datapoint. Results A clear angular dependence of dose response for all chambers has been measured. The most significant effect of magnetic field on relative chamber response in the presence of magnetic field was observed in the orientation when chamber axis is perpendicular to the direction of magnetic field with the tip pointing in the same direction as Lorentz force. This effect is more pronounced for larger volume chambers; the maximum relative variation in the chamber response (between the setup described above and the one where chamber and magnetic field are parallel) is a 5.3% and 4.6% increase for A19 and FC65-G, respectively, and only 2.0% and 1.9% for smaller volume A1SL and CC13 chamber, respectively. We measured the absolute magnitude of the magnetic field correction factor kQmag for the Exradin-A19, A1SL, IBA FC65-G, and CC13 to be 0.938 +/- 1.13%, 0.968 +/- 0.99%, 0.950 +/- 1.13%, and 0.975 +/- 1.13%, respectively. The values are for perpendicular orientation of the chamber relative to magnetic field and parallel to the Lorentz force. Conclusions Experimental measurements carried out in this study have verified the optimal orientation of ionization chamber in terms of minimizing effect of magnetic field on the chamber dose response. This study provides a detailed high-resolution measurement of absolute kQmag values for four reference class chambers as a function of the angle between ionization chamber's central axis and the direction of strong magnetic field over a full 360 degrees rotation. The experimental results of this study can further be used for optimization of the actual sensitive volume of the chamber (and analysis of dead volume) in future Monte Carlo chamber simulations in the presence of strong magnetic fields. In addition, it will provide some necessary data for future reference dosimetry protocols for MR-linac.