Joint Department of Physics, The Royal Marsden NHS Foundation Trust, Sutton, Surrey, UK.
Med Phys. 2012 Mar;39(3):1218-26. doi: 10.1118/1.3681011.
In this paper, the effect on image quality of significantly reducing the primary electron energy of a radiotherapy accelerator is investigated using a novel waveguide test piece. The waveguide contains a novel variable coupling device (rotovane), allowing for a wide continuously variable energy range of between 1.4 and 9 MeV suitable for both imaging and therapy.
Imaging at linac accelerating potentials close to 1 MV was investigated experimentally and via Monte Carlo simulations. An imaging beam line was designed, and planar and cone beam computed tomography images were obtained to enable qualitative and quantitative comparisons with kilovoltage and megavoltage imaging systems. The imaging beam had an electron energy of 1.4 MeV, which was incident on a water cooled electron window consisting of stainless steel, a 5 mm carbon electron absorber and 2.5 mm aluminium filtration. Images were acquired with an amorphous silicon detector sensitive to diagnostic x-ray energies.
The x-ray beam had an average energy of 220 keV and half value layer of 5.9 mm of copper. Cone beam CT images with the same contrast to noise ratio as a gantry mounted kilovoltage imaging system were obtained with doses as low as 2 cGy. This dose is equivalent to a single 6 MV portal image. While 12 times higher than a 100 kVp CBCT system (Elekta XVI), this dose is 140 times lower than a 6 MV cone beam imaging system and 6 times lower than previously published LowZ imaging beams operating at higher (4-5 MeV) energies.
The novel coupling device provides for a wide range of electron energies that are suitable for kilovoltage quality imaging and therapy. The imaging system provides high contrast images from the therapy portal at low dose, approaching that of gantry mounted kilovoltage x-ray systems. Additionally, the system provides low dose imaging directly from the therapy portal, potentially allowing for target tracking during radiotherapy treatment. There is the scope with such a tuneable system for further energy reduction and subsequent improvement in image quality.
本文使用新型波导测试件研究了显著降低放射治疗加速器初级电子能量对图像质量的影响。该波导包含一种新型可变耦合装置（rotovane），可在 1.4 至 9 MeV 之间实现宽连续可变的能量范围，适用于成像和治疗。
通过实验和蒙特卡罗模拟研究了接近 1 MV 的直线加速器加速势下的成像。设计了一个成像束流线，并获得了平面和锥形束计算机断层扫描图像，以便与千伏和兆伏成像系统进行定性和定量比较。成像束的电子能量为 1.4 MeV，入射到由不锈钢、5 毫米碳电子吸收器和 2.5 毫米铝过滤组成的水冷电子窗上。使用对诊断 X 射线能量敏感的非晶硅探测器获取图像。
X 射线束的平均能量为 220 keV，半值层为 5.9 毫米铜。使用剂量低至 2 cGy 获得了与龙门架安装的千伏成像系统具有相同对比度噪声比的锥形束 CT 图像。该剂量相当于单个 6 MV 门户图像。虽然比 100 kVp CBCT 系统（Elekta XVI）高 12 倍，但比 6 MV 锥形束成像系统低 140 倍，比以前发表的在更高（4-5 MeV）能量下运行的 LowZ 成像束低 6 倍。
新型耦合装置提供了适用于千伏级质量成像和治疗的宽范围的电子能量。该成像系统在低剂量下从治疗门户提供高对比度的图像，接近龙门架安装的千伏 X 射线系统。此外，该系统可直接从治疗门户进行低剂量成像，有可能在放射治疗过程中实现靶区跟踪。在这样一个可调谐系统中，有进一步降低能量和随后提高图像质量的空间。
