Department of Physics, University of Alberta, 11322-89 Avenue, Edmonton, Alberta T6G 2G7, Canada.
Med Phys. 2010 Feb;37(2):466-76. doi: 10.1118/1.3276778.
The design of a 3D in-line side-coupled 6 MV linac waveguide for medical use is given, and the effect of the side-coupling and port irises on the radio frequency (RF), beam dynamics, and dosimetric solutions is examined. This work was motivated by our research on a linac-MR hybrid system, where accurate electron trajectory information for a clinical medical waveguide in the presence of an external magnetic field was needed.
For this work, the design of the linac waveguide was generated using the finite element method. The design outlined here incorporates the necessary geometric changes needed to incorporate a full-end accelerating cavity with a single-coupling iris, a waveguide-cavity coupling port iris that allows power transfer into the waveguide from the magnetron, as well as a method to control the RF field magnitude within the first half accelerating cavity into which the electrons from the gun are injected.
With the full waveguide designed to resonate at 2998.5 +/- 0.1 MHz, a full 3D RF field solution was obtained. The accuracy of the 3D RF field solution was estimated through a comparison of important linac parameters (Q factor, shunt impedance, transit time factor, and resonant frequency) calculated for one accelerating cavity with the benchmarked program SUPERFISH. It was found that the maximum difference between the 3D solution and SUPERFISH was less than 0.03%. The eigenvalue solver, which determines the resonant frequencies of the 3D side-coupled waveguide simulation, was shown to be highly accurate through a comparison with lumped circuit theory. Two different waveguide geometries were examined, one incorporating a 0.5 mm first side cavity shift and another with a 1.5 mm first side cavity shift. The asymmetrically placed side-coupling irises and the port iris for both models were shown to introduce asymmetries in the RF field large enough to cause a peak shift and skewing (center of gravity minus peak shift) of an initially cylindrically uniform electron beam accelerating within the waveguide. The shifting and skewing of the electron beam were found to be greatest due to the effects of the side-coupling irises on the RF field. A further Monte Carlo study showed that this effect translated into a 1% asymmetry in a 40 x 40 cm2 field dose profile.
A full 3D design for an in-line side-coupled 6 MV linear accelerator that emulates a common commercial waveguide has been given. The effect of the side coupling on the dose distribution has been shown to create a slight asymmetry, but overall does not affect the clinical applicability of the linac. The 3D in-line side-coupled linac model further provides a tool for the investigation of linac performance within an external magnetic field, which exists in an integrated linac-MR system.
本文设计了一种用于医疗用途的 3D 直线侧边耦合 6MV 直线加速器波导，并研究了侧边耦合和端口光阑对射频（RF）、束流动力学和剂量学解的影响。这项工作的动机是我们对直线加速器-MR 混合系统的研究，该系统需要在存在外加磁场的情况下为临床医疗波导提供准确的电子轨迹信息。
对于这项工作，使用有限元方法设计了直线加速器波导。这里概述的设计采用了必要的几何变化，以纳入带有单个耦合光阑的全端加速腔、允许从磁控管向波导传输功率的波导-腔耦合端口光阑，以及一种控制注入枪的电子进入的第一个半加速腔中的 RF 场幅度的方法。
将全波导设计为在 2998.5 +/- 0.1 MHz 处谐振，得到了完整的 3D RF 场解。通过将一个加速腔的重要直线加速器参数（Q 因子、并联阻抗、传输时间因子和谐振频率）与基准程序 SUPERFISH 计算的结果进行比较，估计了 3D RF 场解的准确性。发现 3D 解与 SUPERFISH 的最大差异小于 0.03%。通过与集总电路理论进行比较，证明了确定 3D 侧边耦合波导模拟的谐振频率的特征值求解器非常准确。研究了两种不同的波导几何形状，一种带有 0.5mm 的第一侧边腔偏移，另一种带有 1.5mm 的第一侧边腔偏移。结果表明，不对称放置的侧边耦合光阑和两个模型的端口光阑在 RF 场中引入了足够大的不对称性，导致在波导中加速的初始圆柱形均匀电子束的峰值偏移和倾斜（质心减去峰值偏移）。发现由于侧边耦合光阑对 RF 场的影响，电子束的偏移和倾斜最大。进一步的蒙特卡罗研究表明，这种效应导致 40x40cm2 场剂量分布的 1%不对称。
给出了一种用于模拟常见商用波导的直线加速器的全 3D 侧边耦合设计。研究表明，侧边耦合对剂量分布的影响会产生轻微的不对称性，但总体上不会影响直线加速器的临床适用性。3D 直线侧边耦合直线加速器模型进一步为研究外加磁场中直线加速器的性能提供了工具，该外加磁场存在于集成的直线加速器-MR 系统中。
