Whelan Brendan, Gierman Stephen, Holloway Lois, Schmerge John, Keall Paul, Fahrig Rebecca
Radiation Physics Laboratory, University of Sydney, Sydney, NSW 2006, Australia and Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.
SLAC National Laboratory, Menlo Park, California 94025.
Med Phys. 2016 Mar;43(3):1285-94. doi: 10.1118/1.4941309.
MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems.
Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified.
For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-width-tenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields.
Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility.
磁共振成像引导放疗是一个快速发展的领域；然而，当前的电子加速器并非设计用于在磁共振成像扫描仪的磁边缘场中运行。因此，当前的磁共振成像 - 直线加速器系统需要磁屏蔽，这会降低磁共振图像质量并限制系统灵活性。这项工作的目的是开发并测试一种新型医用电子加速器概念，该概念对于在线磁共振成像 - 直线加速器系统在磁场中的运行具有内在的鲁棒性。
利用计算模拟对加速器进行建模，包括热电子发射过程、加速结构内的电磁场以及粒子在这些场中的轨迹。在加速器靶处对电子束的空间和能量特性进行量化，并与传统加速器的已发表数据进行比较。然后将该模型与模拟的1T超导磁体的场耦合，并求解阴极到等中心距离在1.0至2.4米之间的情况；对电子束的影响进行量化。
对于零场解，靶处的平均电流为146.3毫安，中位能量为5.8兆电子伏特（四分位间距为0.1兆电子伏特），光斑尺寸直径为全宽十分之一最大值时为1.5毫米。这样的电子束适用于治疗，与传统系统的已发表数据相比具有优势。模拟的加速器在在线磁场中运行时显示出更高的鲁棒性，最大电流损失为3%，而传统系统在相同磁场中的电流损失为85%。
计算模拟表明，用基于射频的源替代传统的直流电子源可用于开发对在线磁场运行具有鲁棒性的医用电子加速器。这将能够开发直线加速器周围无磁屏蔽的磁共振成像 - 直线加速器系统，减少对磁边缘场优化的要求，简化高场磁体的设计，并提高系统灵活性。
