Purpose: To characterize ionization chamber dose response as a function of angle between magnetic field direction and ionization chamber orientation.
Methods: Measurements were performed on Elekta Unity MR Linac that incorporates a 1.5 T Philips magnet and a 7 MV FFF photon beam. The response of two reference-grade chambers (Exradin A19 and A1SL paired with a PTW UE electrometer) were studied. An in-house built MR-compatible water tank and an accompanying cylindrical insert that allowed chamber rotation around the cylinderâ€™s axis was used. EPID on-board imaging helped verify the centring of the chamber exactly at the MRLinac isocentre (143.5 cm, SAD). Beam at gantry 0 degree was shaped by MLC to 10x10 cm field. Ionization chambers were rotated at 20-degree increments, and the resulting dose response was used to evaluate ionization chamber magnetic field corrections as a function of angle between directions of magnetic field and central axis of the chamber.
Results: Authors have measured a clear angular dependence of dose response for both chambers. For the A19, the most prominent differences were observed at 90Â° and 270Â°, 4.4%Â±1.3% and 5.2%Â±0.9% corrections respectively. At 90Â° the ionization chamber axis was perpendicular to directions of magnetic field and photon beam, so that the Lorentz force was sweeping electrons to the direction of ionization chamber tip. At 270Â°, electrons were swept to the stem of the chamber. For A1SL small volume chamber, we observed corrections of the order of 0.40%Â±0.07% at 90Â° and 2.6%Â±0.3% at 270Â°. Authors compared results with available Monte-Carlo studies, which have shown agreement well within uncertainties.
Conclusion: Methodology used in this work was successfully used to validate Monte Carlo studies performed by other groups and can be used to identify optimal orientation. Further ion chambers are being studied to provide data for future reference dosimetry protocols in MRLinac.