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.

METHODS

Seven MR-compatible ionization chamber models from Standard Imaging and PTW were evaluated. Both the quality conversion factor k and the magnetic field quality conversion factor k 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. 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 with the chamber axis and magnetic field direction aligned at cardinal angles (0°, 90°, 180°, 270°).

RESULTS

Beam quality conversion (k) 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 values for their conventional chamber versions. Similarly, magnetic field quality conversion (k) 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 values to corresponding conventional chamber models demonstrating nearly equivalent chamber response. k 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.