Quantifying Dosimetric Uncertainties from Detector Rotation for Rplds In MR-Linacs: A Monte Carlo Study
Abstract
Purpose
Accurate detector alignment is essential in MR-linac reference dosimetry to account for Lorentz force–induced dose perturbations. Current protocols, such as AAPM TG-351, recommend aligning the detector’s longitudinal axis parallel to the magnetic field (B0) to minimize response variations. While the impact of detector orientation on total response has been reported, particularly for ionization chambers, the underlying physical mechanisms by which internal dose perturbations contribute to the integrated signal remain incompletely understood for solid-state dosimeters, such as radiophotoluminescent glass dosimeters (RPLDs). This study provides physical insight into these mechanisms through a high-resolution Monte Carlo (MC) analysis of internal dose distributions.
Methods
MC simulations were performed using the EGSnrc user code egs_chamber. A GD-302M RPLD was modeled in magnetic fields of 0.35 and 1.5 T. The detector’s longitudinal axis was aligned parallel to the magnetic field and perpendicular to the beam central axis. To evaluate sensitivity to setup uncertainty, the detector was rotated about its orthogonal axes in 1° increments up to ±3°. Statistical uncertainty was maintained below 0.1% (k = 1). Three-dimensional voxel-based dose distributions within the glass medium were calculated to characterize spatial dose perturbations.
Results
Despite rotational alignment errors of up to 3°, the integrated dose response varied by less than 0.2% for both magnetic field strengths. Spatial analysis revealed pronounced internal dose non-uniformities, as the Lorentz force induced systematic shifts in electron trajectories, creating localized dose gradients within the sensitive volume. When averaged over the detector volume, these opposing “hot” and “cold” regions partially cancel, explaining the observed macroscopic response stability.
Conclusion
This study provides a physical explanation for the rotational robustness of RPLDs in magnetic fields. Magnetic-field-induced internal dose perturbations were effectively mitigated by volume averaging, supporting uncertainty assessment and validating the clinical reliability of RPLDs against typical setup uncertainties in MR-linac reference dosimetry.