EPID-based verification of the MLC performance for dynamic IMRT and VMAT

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Multileaf collimator (MLC) performance verification is crucial for advanced radiotherapy techniques like intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT).
• Existing geometric quality assurance (QA) methods for MLCs can be extended for more comprehensive evaluations.

Purpose of the Study:

• To develop robust algorithms for fast, accurate, and efficient quantitative investigation of MLC performance.
• To extend existing measurement methods for geometric QA of MLCs to more comprehensive evaluation techniques.

Main Methods:

• Analysis of integrated electronic portal imaging device (EPID) images from picket fence tests in step-and-shoot mode.
• Analysis of cine EPID images from sliding gap patterns in dynamic mode under various conditions (leaf speeds, gantry angles, arc delivery, motion direction).
• Testing accuracy by introducing intentional errors in delivery patterns.

Main Results:

• Developed algorithms achieved sub-pixel accuracy for leaf position, gap width, and skewness detection.
• MLC performance metrics (gap width, leaf position deviation, skewness) remained stable within 0.5 mm and 0.2° over 5 months for tested linear accelerators (linacs).
• Sliding gap analysis accurately determined leaf velocity and acceleration/deceleration, with a slight accuracy decrease at higher leaf speeds. Method accuracy assessed at 0.01 mm.

Conclusions:

• The study provides fast, easy, and accurate methods for routine MLC performance QA.
• These methods facilitate quicker troubleshooting of MLC issues in IMRT and VMAT treatments.