Performance evaluation of a high-speed multileaf collimator in real-time IMRT delivery to moving targets
Multileaf collimator (MLC) tracking can be used for motion management. However, on account of mechanical constraints, it is a crucial challenge for conventional MLCs (3–4 cm/s in leaf speed) to track fast targets, especially moving in 2D in the beam's eye view (BEV). Our group has recently developed a “high-speed” MLC (HS-MLC) prototype with a maximum leaf speed of 40 cm/s, which makes it possible to track the vast majority of moving targets without violation of mechanical constraints. The major innovation of the HS-MLC design is that it employs linear motors instead of rotary motors to drive leaves. This paper mainly aims to evaluate the performance of the HS-MLC in real-time intensity-modulated radiation therapy delivery to targets moving in 2D in the BEV.
A 2D real-time tracking algorithm was proposed first based on a previous superimposing leaf sequencing method. Then, simulations were performed to evaluate the delivery performance including fluence accuracy, efficiency, delivery time, and number of monitor units under various settings of limiting coefficient and dose rate for four clinical fluence maps and two target speeds. The comparisons between the HS-MLC with a “medium-speed” MLC (MS-MLC, 10 cm/s) and a “low-speed” MLC (LS-MLC, 5 cm/s) were also made. For validation, experiments were carried out on the HS-MLC prototype in the lab environment. A camera-based measurement system was set up to detect actual leaf trajectories.
Simulation results indicate that a limiting coefficient of 0.5 and a dose rate of 400 MU/min are “optimal” in the sense of getting best compromise between delivery time and number of monitor units. Under the optimal parameters, the HS-MLC achieved 100% in efficiency, 18.1 s in delivery time, and 121.2 MU in number of monitor units on average for the “fast” target speed, compared to 94%, 20.6 s, and 129.9 MU with the MS-MLC, and to 53%, 40.2 s, and 141.1 MU with the LS-MLC. The benefits of increased leaf speed were demonstrated. The experimental results agreed with the simulation ones, which further confirmed the efficacy of the HS-MLC.
The HS-MLC is superior to conventional MLCs when used for tracking, benefiting from its high leaf speed. These results indicate that the novel HS-MLC is feasible for high-accuracy and high-efficiency motion management. It also offers guidance for future MLC design.