Beam-axis stability during gantry rotation measured with scintillation dosimetry

Volumetric modulated arc therapy (VMAT) requires high mechanical precision due to continuous gantry rotation. This study aims to investigate beam-axis stability during VMAT delivery and evaluate scintillation-based imaging as a complementary quality assurance (QA) tool.

Beam-axis deviations were measured using a scintillation plate dosimeter, acquiring images at 5 frames per second in axial and coronal planes. Nine conformal arc beams with a field size of 1 × 1 cm² were delivered to a phantom under three configurations of the electric portal imaging device (EPID) and x-ray volume imaging (XVI) systems: middle, retracted, and extended. Short- and long-term reproducibility was assessed through repeated measurements. Deviations between the radiation dose center and the beam axis were analyzed in X, Y, and Z directions, and three-dimensional (3D) displacements were calculated.

Short-term reproducibility showed maximum standard deviations (SDs) of 0.05, 0.06, and 0.11 mm for 4 MV, 6 MV, and 6 FFF beams, respectively. Long-term reproducibility exhibited maximum SDs of 0.15, 0.13, and 0.19 mm, respectively. Angle-dependent variations were most prominent at gantry angles of 0° and ±180°, consistent with gantry sag under gravity. The 3D displacements between the radiation dose center and the beam axis remained below 1 mm across all energies: the average (maximum) displacement ranged from 0.47 mm (0.96 mm) to 0.52 mm (0.97 mm) for 4 MV, 0.41 mm (0.68 mm) to 0.44 mm (0.76 mm) for 6 MV, and 0.42 mm (0.88 mm) to 0.47 mm (0.95 mm) for 6FFF. No relevant differences were observed among the three EPID/XVI configurations, indicating that these devices have a minimal impact on gantry mechanical stability.

Scintillation-based imaging enables real-time, high-resolution evaluation of beam-axis stability during VMAT. Deviations were consistently within submillimeter accuracy, supporting its use as a complementary QA method to conventional tests.