Department of Radiation Oncology, Dongguk University Gyeongju Hospital, Gyeongju, South Korea.
Department of Radiation Oncology, Daegu Fatima Hospital, Daegu, South Korea.
Radiat Oncol. 2019 Apr 25;14(1):70. doi: 10.1186/s13014-019-1274-4.
In this study, an external 8 mm thick aluminum target was installed on the upper accessory tray mount of a medical linear accelerator head. The purpose of this study was to determine the effects of the external aluminum target beam (Al-target beam) on the portal image quality by analyzing the spatial and contrast resolutions. In addition, the image resolutions with the Al-target beams were compared with those of conventional 6 megavoltage (MV) images.
The optimized Al-target beam was calculated using Monte Carlo simulations. To validate the simulations, the percentage depth dose and lateral profiles were measured and compared with the modeled dose distributions. A PTW resolution phantom was used for imaging to assess the image resolution. The spatial resolution was quantified by determining the modulation transfer function. The contrast resolution was determined by a fine contrast difference between the 27 measurement areas. The spatial and contrast resolutions were compared with the those of conventional portal images.
The measured and calculated percentage depth dose of the Al-target beam were consistent within 1.6%. The correspondence of measured and modelled profiles was evaluated by gamma analysis (3%, 3 mm) and all gamma values inside the field were less than one. The critical spatial frequencies (f) of the images obtained with the Al-target beam and conventional imaging beam were 0.745 lp/mm and 0.451 lp/mm, respectively. The limiting spatial frequencies (f) for the Al-target beam image and the conventional portal image were 2.39 lp/mm and 1.82 lp/mm, respectively. The Al-target beam resolved the smaller and lower contrast objects better than that of the MV photon beam.
The Al-target beams generated by the simple target installation method provided better spatial and contrast resolutions than those of the conventional 6 MV imaging beam.
本研究在医用直线加速器机头的上附件托盘上安装了 8 毫米厚的外部铝靶。本研究的目的是通过分析空间分辨率和对比度分辨率来确定外部铝靶射线（Al-target beam）对射野影像质量的影响。此外，还比较了带有 Al-target 射线的图像分辨率与传统的 6 兆伏（MV）图像的分辨率。
使用蒙特卡罗模拟计算优化的 Al-target 射线。为了验证模拟，测量了百分深度剂量和侧向轮廓，并与建模的剂量分布进行了比较。使用 PTW 分辨率体模进行成像以评估图像分辨率。通过确定调制传递函数来量化空间分辨率。通过 27 个测量区域之间的细微对比度差异来确定对比度分辨率。将空间分辨率和对比度分辨率与传统的射野影像进行比较。
Al-target 射线的实测和计算的百分深度剂量在 1.6%以内一致。通过伽马分析（3%，3mm）和场内所有伽马值均小于 1 来评估测量和模型化的轮廓的一致性。带有 Al-target 射线和传统成像射线的图像的临界空间频率（f）分别为 0.745lp/mm 和 0.451lp/mm。Al-target 射线图像的极限空间频率（f）和传统射野影像的极限空间频率（f）分别为 2.39lp/mm 和 1.82lp/mm。Al-target 射线比 MV 光子射线更好地分辨更小和更低对比度的物体。
通过简单的靶安装方法生成的 Al-target 射线提供了比传统的 6MV 成像射线更好的空间分辨率和对比度分辨率。
