Jarry G, Verhaegen F
Medical Physics Unit, McGill University, Montreal General Hospital, 1650 Cedar avenue, Montreal, Quebec H3G 1A4, Canada.
Phys Med Biol. 2005 Nov 7;50(21):4977-94. doi: 10.1088/0031-9155/50/21/002. Epub 2005 Oct 12.
Electron beam treatments may benefit from techniques to verify patient positioning and dose delivery. This is particularly so for complex techniques such as mixed photon and electron beam radiotherapy and electron beam modulated therapy. This study demonstrates that it is possible to use the bremsstrahlung photons in an electron beam from a dual scattering foil linear accelerator to obtain portal images of electron beam treatments. The possibility of using Monte Carlo (MC) simulations to predict the electron beam treatment portal images was explored. The MC code EGSnrc was used to model a Varian CL21EX linear accelerator (linac) and to characterize the bremsstrahlung photon production in the linac head. It was found that the main sources of photons in the electron beam are the scattering foils, the applicator and the beam-shaping cut-out. Images were acquired using the Varian CL21EX linac and the Varian aS500 electronic portal imager (EPI); four electron energies (6, 9, 12, 16 MeV), and different applicator and cut-out sizes were used. It was possible to acquire images with as little as 10.7 MU per image. The contrast, the contrast-to-noise ratio (CNR), the signal-to-noise ratio (SNR), the resolution and an estimate of the modulated transfer function (MTF) of the electron beam portal images were computed using a quality assurance (QA) phantom and were found to be comparable to those of a 6 MV photon beam. Images were also acquired using a Rando anthropomorphic phantom. MC simulations were used to model the aS500 EPID and to obtain predicted portal images of the QA and Rando phantom. The contrast in simulated and measured portal images agrees within +/-5% for both the QA and the Rando phantom. The measured and simulated images allow for a verification of the phantom positioning by making sure that the structure edges are well aligned. This study suggests that the Varian aS500 portal imager can be used to obtain patient portal images of electron beams in the scattering foil linacs.
电子束治疗可能会受益于用于验证患者体位和剂量输送的技术。对于诸如混合光子和电子束放射治疗以及电子束调制治疗等复杂技术而言尤其如此。本研究表明，利用来自双散射箔直线加速器的电子束中的轫致辐射光子来获取电子束治疗的射野图像是可行的。探讨了使用蒙特卡罗（MC）模拟来预测电子束治疗射野图像的可能性。MC代码EGSnrc被用于对瓦里安CL21EX直线加速器（直线加速器）进行建模，并表征直线加速器机头中轫致辐射光子的产生情况。结果发现，电子束中光子的主要来源是散射箔、施源器和射野成形限束装置。使用瓦里安CL21EX直线加速器和瓦里安aS500电子射野成像器（EPI）采集图像；使用了四种电子能量（6、9、12、16兆电子伏）以及不同的施源器和限束装置尺寸。每张图像仅用低至10.7MU就有可能采集到图像。使用质量保证（QA）体模计算了电子束射野图像的对比度、对比噪声比（CNR）、信噪比（SNR）、分辨率以及调制传递函数（MTF）的估计值，发现它们与6兆伏光子束的相当。还使用了兰多人体模型采集图像。MC模拟被用于对aS500电子射野成像探测器（EPID）进行建模，并获得QA体模和兰多人体模型的预测射野图像。对于QA体模和兰多人体模型，模拟和测量的射野图像中的对比度在±5%范围内相符。通过确保结构边缘良好对齐，测量和模拟图像可用于验证体模的体位。本研究表明，瓦里安aS500射野成像器可用于获取散射箔直线加速器中电子束的患者射野图像。
