Technical Note: Real-Time MLC Control and Latency Measurement Optimization with External Verification.

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Real-time tumor-tracked radiation therapy aims to improve accuracy by adapting to tumor motion.
• The multi-leaf collimator (MLC) dynamically shapes the radiation beam but has inherent time delays.
• Characterizing MLC latency is crucial for precise target tracking and reducing treatment margins.

Purpose of the Study:

• To quantify the mechanical latency of the Alberta Linac-MR (LMR) MLC.
• To evaluate latency under various gantry angles, leaf velocities, and tumor motion patterns.
• To develop a method for real-time MLC motion control and latency verification.

Main Methods:

• A QUASAR MRI4D motion phantom simulated 1-D abdominothoracic tumor motion.
• A proportional-integral-derivative (PID) controller minimized MLC latency.
• MLC and phantom positions were timestamped and externally verified by cameras to calculate latency.

Main Results:

• MLC latency ranged from 73.2 to 80.1 ms across different motion patterns.
• For sine motion, latency was consistent across gantry angles (74.3–74.9 ms).
• Higher leaf velocities (≥10 mm/s) resulted in lower latency (76.0–76.3 ms) compared to 5 mm/s (111.4 ms).

Conclusions:

• A technique for real-time MLC latency verification was established for the LMR system.
• Gantry angle did not significantly affect MLC mechanical latency.
• Optimizing maximum leaf velocity to ≥10 mm/s is key to minimizing latency in real-time tracking.