Magnetic field quality conversion factors experimentally measured in clinical MR-linac beams for seven MR-compatible ionization chamber models.

Purpose: The purpose of this work was to experimentally quantify MR-compatible ionization chamber response for 1.5T Elekta Unity and 0.35T ViewRay MRIdian MR-linac systems through the determination of the magnetic field quality conversion factor, k_B,Q.

Methods: Seven MR-compatible ionization chamber models from Standard Imaging and PTW were evaluated. Both the quality conversion factor k_Q and the magnetic field quality conversion factor k_B,Q were experimentally determined through a cross-calibration method. Specifically, the ratio of absorbed dose measured with a reference A1SL chamber under reference conditions to corrected output measured with each test chamber at the same point of measurement allowed for the determination of k_B,Q. The angular dependence of the magnetic field quality conversion factor for MR-compatible chamber models was assessed for the 1.5T Elekta Unity system by measuring k_B,Q with the chamber axis and magnetic field direction aligned at cardinal angles (0°, 90°, 180°, 270°).

Results: Beam quality conversion (k_Q) factors for MR-compatible ionization chambers measured in a standard linac beam showed an average percent difference of -0.09 ± 0.18% compared to computed k_Q values for their conventional chamber versions. Similarly, magnetic field quality conversion (k_B,Q) factors for corresponding MR and non-MR ionization chamber models measured using the same cross-calibration technique demonstrated average percent differences of -0.1 ± 0.3% and 0.0 ± 0.2% for the Elekta Unity and ViewRay MRIdian, respectively. Investigation of the angular dependence of this correction factor demonstrated identical chamber response for equivalent MR-compatible and conventional chamber models.

Conclusions: This work provides critical experimental validation of MR-compatible ionization chamber performance, with a direct comparison of measured k_B,Q values to corresponding conventional chamber models demonstrating nearly equivalent chamber response. k_B,Q values determined using our experimental method will serve as an important reference for upcoming MR-linac reference dosimetry protocols and ultimately represent an important step towards accurate output calibration of MR-linac systems.