高速多叶准直器在实时调强放射治疗中对移动靶区进行照射时的性能评估。

Li Fang, Ye Peiqing, Zhang Hui

Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China and Beijing Key Lab of Precision/Ultra-Precision Manufacturing Equipments and Control, Tsinghua University, Beijing 100084, China.

Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China; Beijing Key Lab of Precision/Ultra-Precision Manufacturing Equipments and Control, Tsinghua University, Beijing 100084, China; and The State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.

Med Phys. 2016 Mar;43(3):1401-10. doi: 10.1118/1.4941952.

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

目的

多叶准直器（MLC）跟踪可用于运动管理。然而，由于机械限制，对于传统MLC（叶片速度为3 - 4厘米/秒）来说，跟踪快速移动目标是一项严峻挑战，尤其是在射野方向观（BEV）中二维移动的目标。我们团队最近开发了一种“高速”MLC（HS - MLC）原型，其最大叶片速度为40厘米/秒，这使得在不违反机械限制的情况下跟踪绝大多数移动目标成为可能。HS - MLC设计的主要创新之处在于采用直线电机而非旋转电机来驱动叶片。本文主要旨在评估HS - MLC在向BEV中二维移动目标进行实时调强放射治疗时的性能。

方法

首先基于先前的叠加叶片排序方法提出了一种二维实时跟踪算法。然后，针对四种临床注量图和两种目标速度，在不同的限制系数和剂量率设置下进行模拟，以评估包括注量准确性、效率、输送时间和监测单位数量在内的输送性能。还对HS - MLC与“中速”MLC（MS - MLC，10厘米/秒）和“低速”MLC（LS - MLC，5厘米/秒）进行了比较。为进行验证，在实验室环境中对HS - MLC原型进行了实验。建立了基于相机的测量系统以检测实际叶片轨迹。

结果

模拟结果表明，在输送时间和监测单位数量之间取得最佳折衷的意义上，限制系数为0.5和剂量率为400 MU/分钟是“最优”的。在最优参数下，对于“快速”目标速度，HS - MLC的效率达到100%，输送时间为18.1秒，平均监测单位数量为121.2 MU，相比之下，MS - MLC分别为94%、20.6秒和129.9 MU，LS - MLC分别为53%、40.2秒和141.1 MU。展示了提高叶片速度的益处。实验结果与模拟结果一致，进一步证实了HS - MLC的有效性。