An EPID-based system for gantry-resolved MLC quality assurance for VMAT

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Comprehensive quality assurance (QA) for volumetric-modulated arc therapy (VMAT) requires precise measurement of multileaf collimator (MLC) positions relative to gantry angle.
• Relating MLC positional accuracy to patient-specific dosimetry is crucial for assessing the clinical impact of detected MLC errors.

Purpose of the Study:

• To propose and validate a novel method for verifying individual MLC trajectories during VMAT deliveries as a routine linear accelerator (LINAC) QA tool.
• To extend this method for reconstructing 3D patient dose within the treatment planning system (TPS) using measured MLC trajectories and DICOM plan data.

Main Methods:

• Extracting MLC positions from electronic portal imaging device (EPID) images acquired during clinical VMAT deliveries.
• Automatically tagging gantry angles to EPID images to generate MLC trajectories as a function of gantry angle.
• Comparing measured MLC trajectories with DICOM plan data and assessing system sensitivity to introduced MLC errors.

Main Results:

• The method achieved a maximum mean error of 0.07 mm and a maximum root-mean-square error of 0.8 mm for individual leaf positions.
• The system demonstrated sensitivity to detect random and systematic MLC errors in the range of 1-2 mm and single leaf calibration errors of 0.5 mm.
• Successful reconstruction of 3D patient dose based on measured MLC trajectories.

Conclusions:

• The developed methodology offers an efficient approach for routine LINAC MLC QA and pretreatment patient-specific QA.
• This technique enables the direct correlation of measured MLC positional errors with 3D dosimetric errors within the patient volume.
• The proposed method enhances the accuracy and reliability of VMAT delivery QA.