A technique for achieving arbitrary machine dose rates via MLC leaf motion modulation using fixed-gantry IMRT...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Medical linear accelerators (Linacs) typically offer fixed, discrete dose rates.
• Certain clinical scenarios, like total body irradiation (TBI), require variable dose rates.
• Existing systems lack flexibility for non-standard dose rate requirements.

Purpose of the Study:

• To develop and validate a method for achieving arbitrary machine dose rates in photon beam delivery.
• To enhance treatment flexibility by modulating multi-leaf collimator (MLC) leaf motion.
• To enable customized dose rate delivery for specific radiobiological and clinical needs.

Main Methods:

• Modulating the motion of unused MLC leaves outside the jaw boundary to control dose rate.
• Adjusting leaf speed by a ratio of current to desired dose rate between control points.
• Implementing two distinct schemes for varied clinical applications.
• Verifying modulated dose rates on Varian TrueBeam and Clinac Linacs using trajectory log analysis and portal dosimetry QA (PDQA).

Main Results:

• Achieved stable and accurate dose rates with mean deviations below 0.5% across a wide range (1-600 MU/min).
• Fluctuations increased at lower dose rates, with thresholds for 1% and 10% deviations at approximately 50 and 7 MU/min, respectively.
• PDQA confirmed high accuracy with a 99% gamma pass rate (3%/2mm).
• Trajectory log analysis validated planned vs. actual treatment parameters.

Conclusions:

• A validated technique enables arbitrary machine dose rates in photon delivery.
• MLC leaf motion modulation provides a flexible solution for dose rate control.
• The method is accurate and suitable for various radiotherapy applications, including TBI and IMRT.