Medical Physics Unit, McGill University Health Center, Montréal, Québec, Canada.
Med Phys. 2007 Jul;34(7):2896-906. doi: 10.1118/1.2745243.
Since taken with megavoltage, forward-directed bremsstrahlung beams, the image quality of current portal images is inferior to that of diagnostic quality images produced by kilovoltage beams. In this paper, the beam quality of orthogonal bremsstrahlung beams defined as the 90 degrees component of the bremsstrahlung distribution produced from megavoltage electron pencil beams striking various targets is presented, and the suitability of their use for improved radiotherapy imaging is evaluated. A 10 MeV electron beam emerging through the research port of a Varian Clinac-18 linac was made to strike targets of carbon, aluminum, and copper. PDD and attenuation measurements of both the forward and orthogonal beams were carried out, and the results were also used to estimate the effective and mean energy of the beams. The mean energy of a spectrum produced by a carbon target dropped by 83% from 1296 keV in the forward direction to 217 keV in the orthogonal direction, while for an aluminum target it dropped by 77% to 412 keV, and for a copper target by 65% to 793 keV. An in-depth Monte Carlo study of photon yield and electron contamination was also performed. Photon yield and effective energy are lower for orthogonal beams than for forward beams, and the differences are more pronounced for targets of lower atomic number. Using their relatively low effective energy, orthogonal bremsstrahlung beams produced by megavoltage electrons striking low atomic number targets yield images with a higher contrast in comparison with forward bremsstrahlung beams.
由于采用兆伏级、正向定向韧致辐射束，当前的射野影像质量不如千伏级射线产生的诊断质量影像。本文介绍了由兆伏电子束垂直入射各种靶材产生的韧致辐射分布的 90 度分量定义的正交韧致辐射束的束质，并评估了其用于改进放射治疗成像的适用性。从瓦里安 Clinac-18 直线加速器的研究端口引出 10MeV 电子束，使其垂直入射碳、铝和铜靶材。进行了正向和正交束的 PDD 和衰减测量，结果还用于估计束的有效和平均能量。碳靶材产生的能谱在正向方向上从 1296keV 下降到 83%，在正交方向上下降到 217keV，而铝靶材下降到 77%，至 412keV，铜靶材下降到 65%，至 793keV。还对光子产额和电子污染进行了深入的蒙特卡罗研究。对于正交束，光子产额和有效能量低于正向束，对于原子序数较低的靶材，差异更为明显。利用相对较低的有效能量，兆伏级电子垂直入射低原子序数靶材产生的正交韧致辐射束产生的图像与正向韧致辐射束相比具有更高的对比度。
