纵向磁场对模拟直线加速器中 6MV 射线的影响。

Effect of longitudinal magnetic fields on a simulated in-line 6 MV linac.

机构信息

Department of Physics, University of Alberta, 11322-89 Avenue, Edmonton, Alberta T6G 2G7, Canada.

出版信息

Med Phys. 2010 Sep;37(9):4916-23. doi: 10.1118/1.3481513.

DOI:10.1118/1.3481513

PMID:20964210

Abstract

PURPOSE

Linac-magnetic resonance (MR) systems have been proposed in order to achieve realtime image guided radiotherapy. The design of a new linac-MR system with the in-line 6 MV linac generating x-rays along the symmetry axis of an open MR imager is outlined. This new design allows for a greater MR field strength to achieve better quality images while reducing hot and cold spots in treatment planning. An investigation of linac's performance in the longitudinal fringe magnetic fields of the MR imager is given.

METHODS

The open MR imager fringe magnetic field was modeled using the analytic solution of the magnetic field generated from current carrying loops. The derived solution was matched to the magnetic fringe field isolines provided for a 0.5 T open MR imager through Monte Carlo optimization. The optimized field solution was then added to the previously validated 6 MV linac simulation to quantify linac's performance in the fringe magnetic field of a 0.5 T MR imager. To further the investigation, linac's performance in large fringe fields expected from other imagers was investigated through the addition of homogeneous longitudinal fields.

RESULTS

The Monte Carlo optimization of the analytic current loop solution provided good agreement with the magnetic fringe field isolines supplied by the manufacturer. The range of magnetic fields the linac is expected to experience when coupled to the 0.5 T MR imager was determined to be from 0.0022 to 0.011 T (as calculated at the electron gun cathode). The effect of the longitudinal magnetic field on the electron beam was observed to be only in the electron gun. The longitudinal field changed the electron gun optics, affecting beam characteristics, such as a slight increase in the injection current and beam diameter, and an increasingly nonlaminar transverse phase space. Although the target phase space showed little change in its energy spectrum from the altered injection phase space, a reduction in the target current and spatial distribution peak intensity was observed. Despite these changes, the target phase space had little effect on the depth dose curves or dose profiles calculated for a 40 x 40 cm2 field at 1.5 cm depth. At longitudinal fields larger than 0.012 T, a drastic reduction in the injection current from the electron gun was observed due to a large fraction of electrons striking the anode. This further reduced the target current, which reached a minimum of 28 +/- 2 mA at 0.06 T. A slow increase in the injection and target currents was observed at fields larger than 0.06 T due to greater beam collimation in the anode beam tube.

CONCLUSIONS

In an effort to achieve higher quality images and a reduction in hot and cold spots in the treatment plan, a parallel configuration linac-MR system is presented. The longitudinal magnetic fields of the MR imager caused large beam losses within the electron gun. These losses may be eliminated through a redesign of the electron gun optics incorporating a longitudinal magnetic field, or through magnetic shielding, which has already been proven successful for the transverse configuration.

摘要

目的

为了实现实时图像引导放射治疗，已经提出了直线加速器磁共振（MR）系统。概述了一种带有在线 6 MV 直线加速器的新型直线加速器-MR 系统的设计，该直线加速器沿着开放式 MR 成像仪的对称轴产生 X 射线。这种新设计允许更大的 MR 场强，以获得更好的图像质量，同时减少治疗计划中的热点和冷点。本文研究了直线加速器在 MR 成像仪纵向条纹磁场中的性能。

方法

采用载流环产生的磁场的解析解对开放式 MR 成像仪的条纹磁场进行建模。通过蒙特卡罗优化，将推导的解与为 0.5 T 开放式 MR 成像仪提供的磁场条纹场等位面进行匹配。然后，将优化后的场解添加到先前经过验证的 6 MV 直线加速器模拟中，以量化直线加速器在 0.5 T MR 成像仪的条纹磁场中的性能。为了进一步研究，通过添加均匀纵向场，研究了其他成像仪预期的大条纹场对直线加速器的性能影响。

结果

对解析电流环解的蒙特卡罗优化与制造商提供的磁场条纹场等位面很好地吻合。确定了直线加速器在与 0.5 T MR 成像仪耦合时预期会遇到的磁场范围为 0.0022 至 0.011 T（在电子枪阴极处计算）。观察到纵向磁场对电子束的影响仅在电子枪中。纵向磁场改变了电子枪的光学性能，影响了束特性，例如注入电流和束直径略有增加，以及横向相位空间越来越不层流。尽管目标相位空间在从改变的注入相位空间到改变的目标相位空间的能量谱中几乎没有变化，但观察到靶电流和空间分布峰值强度降低。尽管发生了这些变化，但在 1.5 厘米深度处对 40×40 厘米 2 场进行计算时，目标相位空间对深度剂量曲线或剂量分布几乎没有影响。在纵向场大于 0.012 T 时，由于大量电子撞击阳极，电子枪的注入电流大幅下降。这进一步降低了靶电流，在 0.06 T 时达到最小值 28±2 mA。在大于 0.06 T 的场中，观察到注入电流和靶电流缓慢增加，这是由于阳极束管中的束更准直。