Varian Medical Systems Inc., Palo Alto, CA, 94304, USA.
Med Phys. 2018 Nov;45(11):5080-5093. doi: 10.1002/mp.13196. Epub 2018 Oct 12.
The shape, size, and location of the x-ray beam spot (where the electron beam strikes the target) in a linac-based radiation therapy machine are of potential clinical significance. Established techniques to measure the beam spot parameters involve specialized hardware and typically assess size and shape of the beam spot or its position, but not both. A simple apparatus and algorithm for measuring all beam spot parameters simultaneously is proposed here.
The apparatus is composed of two partially transmitting edge plates mounted at different vertical positions. The mount for the apparatus slides into the accessory tray of the linac treatment head so that it rotates with the collimator, and it is imaged by the existing electronic portal imaging device (EPID) over multiple collimator angles. A software algorithm takes the acquired images and uses a parallel-beam CT reconstruction technique to compute beam spot size, shape, and position in one computation. In addition, the wobble of the collimator assembly can be estimated. The overall method was validated with both Monte Carlo simulation and with comparison to in-house spot camera measurements on a radiation therapy system.
The algorithm correctly predicted the beam spot parameters used for the Monte Carlo simulation to better than 50 μm accuracy in all cases. Furthermore, results from the dual edge method matched spot camera results with 30 μm accuracy for beam spot size and shape, with 80 μm average accuracy for beam spot position, and better than 200 μm accuracy for collimator assembly wobble.
We have developed a combination dual edge apparatus and image processing algorithm that, when used on a radiotherapy linac with an EPID, can accurately determine the size and shape of the electron beam spot, its position relative to collimator rotation axis, and the wobble of the collimator assembly.
直线加速器放射治疗机中 X 射线束斑（电子束撞击靶的位置）的形状、大小和位置具有潜在的临床意义。现有的测量束斑参数的技术涉及专用硬件，通常评估束斑的大小和形状或其位置，但不能同时评估两者。本文提出了一种简单的测量所有束斑参数的装置和算法。
该装置由两个安装在不同垂直位置的部分透光边缘板组成。该装置的安装座滑入直线加速器治疗头的附件托盘，使其随准直器旋转，并通过现有的电子射野影像装置（EPID）在多个准直器角度进行成像。软件算法采用获取的图像，并使用平行束 CT 重建技术在一次计算中计算束斑的大小、形状和位置。此外，还可以估计准直器组件的摆动。该方法整体通过蒙特卡罗模拟和与放射治疗系统上的内部点片相机测量结果进行比较进行了验证。
该算法正确预测了用于蒙特卡罗模拟的束斑参数，在所有情况下精度均优于 50 μm。此外，双边缘法的结果与点片相机的结果相匹配，束斑大小和形状的精度为 30 μm，束斑位置的平均精度为 80 μm，准直器组件摆动的精度优于 200 μm。
我们开发了一种组合的双边缘装置和图像处理算法，当与配备 EPID 的放射治疗直线加速器一起使用时，可以准确确定电子束斑的大小和形状、相对于准直器旋转轴的位置以及准直器组件的摆动。
