Whelan Brendan, Holloway Lois, Constantin Dragos, Oborn Brad, Bazalova-Carter Magdalena, Fahrig Rebecca, Keall Paul
Radiation Physics Laboratory, University of Sydney, Sydney, NSW 2006, Australia; Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia; and Liverpool Cancer Therapy Centre, Liverpool Hospital, Liverpool, NSW 2170, Australia.
South Western Clinical School, University of New South Wales, Sydney, NSW 2170, Australia; Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia; and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia.
Med Phys. 2016 Nov;43(11):5903. doi: 10.1118/1.4963216.
MRI-linac therapy is a rapidly growing field, and requires that conventional linear accelerators are operated with the fringe field of MRI magnets. One of the most sensitive accelerator components is the electron gun, which serves as the source of the beam. The purpose of this work was to develop a validated finite element model (FEM) model of a clinical triode (or gridded) electron gun, based on accurate geometric and electrical measurements, and to characterize the performance of this gun in magnetic fields.
The geometry of a Varian electron gun was measured using 3D laser scanning and digital calipers. The electric potentials and emission current of these guns were measured directly from six dose matched true beam linacs for the 6X, 10X, and 15X modes of operation. Based on these measurements, a finite element model (FEM) of the gun was developed using the commercial software opera/scala. The performance of the FEM model in magnetic fields was characterized using parallel fields ranging from 0 to 200 G in the in-line direction, and 0-35 G in the perpendicular direction.
The FEM model matched the average measured emission current to within 5% across all three modes of operation. Different high voltage settings are used for the different modes; the 6X, 10X, and 15X modes have an average high voltage setting of 15, 10, and 11 kV. Due to these differences, different operating modes show different sensitivities in magnetic fields. For in line fields, the first current loss occurs at 40, 20, and 30 G for each mode. This is a much greater sensitivity than has previously been observed. For perpendicular fields, first beam loss occurred at 8, 5, and 5 G and total beam loss at 27, 22, and 20 G.
A validated FEM model of a clinical triode electron gun has been developed based on accurate geometric and electrical measurements. Three different operating modes were simulated, with a maximum mean error of 5%. This gun shows greater sensitivity to in-line magnetic fields than previously presented models, and different operating modes show different sensitivity.
磁共振直线加速器治疗是一个快速发展的领域，要求传统直线加速器在磁共振磁体的边缘场中运行。电子枪是加速器中最敏感的部件之一，它作为束流的源。本工作的目的是基于精确的几何和电学测量，开发一个经过验证的临床三极管（或栅控）电子枪的有限元模型（FEM），并表征该电子枪在磁场中的性能。
使用三维激光扫描和数字卡尺测量瓦里安电子枪的几何形状。从六台剂量匹配的真束直线加速器直接测量这些电子枪在6X、10X和15X运行模式下的电势和发射电流。基于这些测量，使用商业软件opera/scala开发了电子枪的有限元模型（FEM）。使用在线方向上0至200 G、垂直方向上0至35 G的平行场来表征有限元模型在磁场中的性能。
有限元模型在所有三种运行模式下，将平均测量发射电流匹配到5%以内。不同模式使用不同的高压设置；6X、10X和15X模式的平均高压设置分别为15 kV、10 kV和11 kV。由于这些差异，不同运行模式在磁场中表现出不同的灵敏度。对于在线场，每种模式下第一次电流损失分别发生在40 G、20 G和30 G。这比之前观察到的灵敏度要高得多。对于垂直场，第一次束流损失分别发生在8 G、5 G和5 G，总束流损失分别发生在27 G、22 G和20 G。
基于精确的几何和电学测量，开发了一个经过验证的临床三极管电子枪的有限元模型。模拟了三种不同的运行模式，最大平均误差为5%。该电子枪对在线磁场的灵敏度比之前提出的模型更高，且不同运行模式表现出不同的灵敏度。
