Toward real-time verification for MLC tracking treatments using time-resolved EPID imaging

Area of Science:

• Medical Physics
• Radiation Oncology
• Image Analysis

Background:

• Multileaf collimator (MLC) tracking in radiotherapy adapts to intrafraction tumor motion, introducing risks of delivery errors undetectable by traditional methods.
• Real-time verification is crucial for ensuring accuracy in adaptive radiotherapy techniques like MLC tracking.

Purpose of the Study:

• To develop and validate a system for real-time geometric verification of MLC tracking treatments using an electronic portal imaging device (EPID).

Main Methods:

• Utilized volumetric modulated arc therapy (VMAT) with MLC tracking, acquiring images at 9.57 fps with an EPID.
• Extracted MLC positions from image frames to assess field location, size, and shape accuracy.
• Validated the technique through phantom studies and comparison with treatment log files, then applied to clinical lung SABR patient data.

Main Results:

• Phantom validation detected field location errors with sub-millimeter uncertainty (0.28-0.38 mm).
• Clinical lung SABR deliveries showed field location errors of 1.6 mm (parallel) and 4.9 mm (perpendicular) to MLC motion.
• No correlation was found between geometric errors and dosimetric errors in the target volume for the patient cohort.

Conclusions:

• A novel system for real-time delivery verification of MLC tracking was successfully developed using time-resolved EPID imaging.
• The system independently verifies the geometric accuracy of the MLC during radiotherapy, enhancing treatment safety.
• This technique provides a critical tool for quality assurance in advanced radiotherapy delivery.