切伦科夫效应的光子束投影成像。
Projection imaging of photon beams by the Čerenkov effect.
机构信息
Thayer School of Engineering, Dartmouth College, Hanover, NH, USA.
出版信息
Med Phys. 2013 Jan;40(1):012101. doi: 10.1118/1.4770286.
PURPOSE
A novel technique for beam profiling of megavoltage photon beams was investigated for the first time by capturing images of the induced Čerenkov emission in water, as a potential surrogate for the imparted dose in irradiated media.
METHODS
A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire 2D projection images of Čerenkov emission from a 4 × 4 cm(2) 6 MV linear accelerator (LINAC) x-ray photon beam operating at a dose rate of 400 MU∕min incident on a water tank with transparent walls. The ICCD acquisition was gated to the LINAC sync pulse to reduce background light artifacts, and the measurement quality was investigated by evaluating the signal to noise ratio and measurement repeatability as a function of delivered dose. Monte Carlo simulations were used to derive a calibration factor for differences between the optical images and deposited dose arising from the anisotropic angular dependence of Čerenkov emission. Finally, Čerenkov-based beam profiles were compared to a percent depth dose (PDD) and lateral dose profile at a depth of d(max) from a reference dose distribution generated from the clinical Varian ECLIPSE treatment planning system (TPS).
RESULTS
The signal to noise ratio was found to be 20 at a delivered dose of 66.6 cGy, and proportional to the square root of the delivered dose as expected from Poisson photon counting statistics. A 2.1% mean standard deviation and 5.6% maximum variation in successive measurements were observed, and the Monte Carlo derived calibration factor resulted in Čerenkov emission images which were directly correlated to deposited dose, with some spatial issues. The dose difference between the TPS and PDD predicted by Čerenkov measurements was within 20% in the buildup region with a distance to agreement (DTA) of 1.5-2 mm and ±3% at depths beyond d(max). In the lateral profile, the dose difference at the beam penumbra was within ±13% with a DTA of 0-2 mm, ±5% in the central beam region, and 2%-3% in the beam umbra.
CONCLUSIONS
The results from this initial study demonstrate the first documented use of Čerenkov emission imaging to profile x-ray photon LINAC beams in water. The proposed modality has several potential advantages over alternative methods, and upon future refinement may prove to be a robust and novel dosimetry method.
目的
首次研究了一种用于测量兆伏级光子束射束分布的新方法，即通过捕捉水诱导切伦科夫辐射的二维投影图像来测量束流分布，作为辐照介质中吸收剂量的替代测量手段。
方法
采用高灵敏度、增强型 CCD 相机（ICCD）采集在剂量率为 400 MU/min 下运行的 4×4cm²6MV 直线加速器（LINAC）X 射线光子束在具有透明壁的水箱中产生的切伦科夫辐射的二维投影图像。ICCD 采集与 LINAC 同步脉冲门控，以减少背景光伪影，通过评估信噪比和测量重复性作为剂量函数，研究测量质量。使用蒙特卡罗模拟得出光学图像与沉积剂量之间的校准因子，该因子源于切伦科夫辐射的各向异性角度依赖性。最后，将基于切伦科夫的射束分布与从临床瓦里安 ECLIPSE 治疗计划系统（TPS）生成的参考剂量分布得出的百分深度剂量（PDD）和侧向剂量分布在 d（max）深度处进行比较。
结果
在 66.6cGy 的剂量下，信噪比达到 20，符合泊松光子计数统计的预期，即与剂量的平方根成正比。观察到连续测量的平均值标准偏差为 2.1%，最大变化为 5.6%，蒙特卡罗模拟得出的校准因子使切伦科夫辐射图像与沉积剂量直接相关，存在一些空间问题。TPS 预测的 PDD 和切伦科夫测量得到的剂量差在 1.5-2mm 的距离符合度（DTA）范围内，在 d（max）以上的深度处为 2%-3%。在侧向分布中，在射束半影区的剂量差在 13%以内，DTA 为 0-2mm，在中央射束区域为 5%，在射束暗区为 2%-3%。
结论
这项初步研究的结果首次证明了使用切伦科夫辐射成像来测量水介质中的 X 射线光子 LINAC 射束的分布。与其他方法相比，该方法具有几个潜在的优势，在进一步改进后，可能成为一种强大而新颖的剂量测量方法。
Zotero 插件