Quality control of VMAT synchronization using portal imaging.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Accurate delivery in volumetric-modulated arc therapy (VMAT) relies on precise synchronization of gantry, multileaf collimator (MLC) leaf positions, and dose rate.
• Existing quality control (QC) methods for VMAT synchronization can be time-consuming and data-intensive.

Purpose of the Study:

• To implement a streamlined QC method for VMAT synchronization using portal imaging.
• To minimize the number of arcs and data handling time required for VMAT QC.
• To evaluate the sensitivity of the proposed method to common delivery errors.

Main Methods:

• A 12 mm steel bar was positioned to cast a shadow on the portal imager during a 360° VMAT arc.
• Simulated errors included gantry position (1°-9°), dose (2-20 MU), and MLC leaf position (1-6 mm).
• Inhomogeneity metrics were defined to quantify synchronization errors.

Main Results:

• Simulated delivery errors resulted in irregular patterns on portal images, contrasting with the uniformity seen in error-free deliveries.
• Inhomogeneity metrics increased significantly with simulated errors: 67% for a 4° gantry error, 33% for an 8 MU dose error, and 70% for a 2 mm MLC error.
• The method demonstrated higher sensitivity to errors at gantry angles 90°/270° compared to 0°/180°.

Conclusions:

• The portal imaging method provides a fast and effective QC for VMAT synchronization.
• This technique is suitable for regular inclusion in monthly accelerator QC programs.
• The method can identify errors in individual control point deliveries, with potential for further investigation using dynamic imaging.