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

The medical linear accelerator (Linac) typically operates at a constant but discrete dose rate for both 3D conformal fields and intensity modulation radiotherapy (IMRT) fields. However, in certain clinical and radiobiological scenarios, such as total body irradiation (TBI), an arbitrary dose rate outside the preset values may be desired.

This study aims to achieve arbitrary machine dose rates in photon delivery through modulation of MLC leaf motion, providing flexibility for specific needs.

The desired effective dose rate is achieved by precisely programming the motion of unused MLC leaves outside the field boundary set by the jaws, while keeping the MLC leaves within the jaw boundary same as originally planned. Between control points, the leaf speed is adjusted by multiplying the permissible limit by the ratio of the current dose rate to the desired value. During delivery, the dose rate is reduced so that the MLC leaves arrive at the expected position for the expected monitor units (MU). Two schemes were designed for different application scenarios and successfully delivered and verified. A circular 3D conformal field, a 40 cm × 40 cm TBI field, and a head-and-neck (H&N) IMRT DMLC field were modified for dose rates between 600 and 1 MU/min. These plans were successfully delivered on Varian TrueBeam, and Clinac Linacs with modulated dose rates. Trajectory log analysis and portal dosimetry QA (PDQA) were utilized to verify the accuracy.

Both schemes achieved stable and accurate dose rates, with mean deviations from the desired values remaining below 0.5% across the range from 600 to 1 MU/min. Fluctuations, represented by relative standard deviations, increased monotonically as the desired dose rate decreased. For clinically relevant dose rates above 100 MU/min, fluctuations remained below 0.5%. At lower dose rates, the thresholds for fluctuations reaching 1% and 10% were approximately 50 and 7 MU/min, respectively. Trajectory log file analysis confirmed agreement between planned and actual values of various treatment parameters, such as leaf position, speed, and MU. PDQA analysis showed 99% gamma pass rate with criteria of 3%/2 mm. For Scheme 1 with step-and-shoot, a 50% increase in delivery time due to the temporary beam-hold during step-and-shoot was both expected and observed.

A technique has been developed and validated to achieve arbitrary machine dose rates in photon delivery through the modulation of MLC leaf motion.