Characterization and Validation of MicroDiamond Detectors in Patient-specific Quality Assurance of Magnetic Resonance-guided Stereotactic Radiotherapy.

INTRODUCTION: A dosimetric performance analysis of microDiamond (MD) detector in magnetic resonance imaging linear accelerators (MR-Linac) was performed to evaluate its utility for patient-specific quality assurance (PSQA) in stereotactic radiotherapy (SRT).

METHODS: Output factors (OFs), depth dose curves, and dose profiles for field sizes from 1 cm × 1 cm to 10 cm × 10 cm were measured in an Elekta Unity 1.5 T MR-Linac. Measurements were conducted using an MR-Safe three-dimensional (3D) water phantom with a PTW Semiflex 3D ionization chamber and a PTW MD detector. Angular response was assessed for orientations perpendicular to the beam and parallel to the magnetic field; correction factors were determined for gantry angles from 0° to 360° in 20° increments. Detectors were cross-calibrated against a 0.6 cc Farmer chamber, and patient-specific point-dose verification was performed for SRT plans.

RESULTS: Average d max measured with the MD detector (1.28 ± 0.1 cm) agreed closely with the Semiflex 3D (1.27 ± 0.1 cm). The Semiflex 3D overestimated penumbra by 1.8 ± 0.6 mm and underestimated small-field OFs, while the MD detector showed slight overresponse (0.5%). The angular dependence remains substantial and highly sensitive to field size, requiring field-size-specific angular correction factors to achieve clinical accuracy better than 2%. After cross-calibration, the difference between the measured and planned doses for the 1.2 cc target was - 6.92% for Semiflex 3D and 1.36% for MD detector.

CONCLUSION: The MD detector was successfully validated for MR-Linac. Asymmetric angular dependence due to the Lorentz force was confirmed, necessitating gantry angle-specific corrections for dose estimation. The MD detector demonstrated superior accuracy for small-field SRT targets, confirming its significance for precise PSQA.