Beam-axis stability during gantry rotation measured with scintillation dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Volumetric modulated arc therapy (VMAT) demands high mechanical precision due to continuous gantry rotation.
• Ensuring beam-axis stability is critical for accurate dose delivery in VMAT.

Purpose of the Study:

• To investigate beam-axis stability during VMAT delivery.
• To evaluate scintillation-based imaging as a complementary quality assurance (QA) tool for VMAT.

Main Methods:

• Beam-axis deviations were measured using a scintillation plate dosimeter at 5 frames per second.
• Measurements were performed on a phantom with conformal arc beams under varying electric portal imaging device (EPID) and x-ray volume imaging (XVI) configurations.
• Short- and long-term reproducibility of beam-axis deviations were assessed, and three-dimensional displacements were calculated.

Main Results:

• Short-term reproducibility showed maximum standard deviations (SDs) below 0.11 mm for all tested energies.
• Long-term reproducibility exhibited maximum SDs up to 0.19 mm.
• Three-dimensional displacements between the radiation dose center and beam axis remained below 1 mm across all energies, with no significant impact from EPID/XVI configurations.

Conclusions:

• Scintillation-based imaging provides real-time, high-resolution assessment of beam-axis stability during VMAT.
• The observed deviations are consistently within submillimeter accuracy.
• This technique is suitable as a complementary QA method to conventional tests for VMAT.