Biggs P J, Goitein M, Russell M D
Int J Radiat Oncol Biol Phys. 1985 Mar;11(3):635-43. doi: 10.1016/0360-3016(85)90201-9.
The quality of portal films taken at megavoltage energies is frequently poor, despite the use of various techniques such as lead screens to improve the image. When critical structures, such as the spinal cord, are to be blocked out of the treatment field, it is frequently difficult to determine whether the block is correctly placed. To solve this problem, a diagnostic X ray source has been mounted on the side of a 10 MV linear accelerator to provide accurate verification of patient positioning and location of shielding blocks. The coincidence between the mechanical, 10 MV and diagnostic X ray isocenters is about 1 mm. The system has been designed so that procedures to monitor this coincidence, and adjustment procedures to maintain it, are easily performed.
尽管使用了各种技术（如铅屏）来改善图像，但在兆伏能量下拍摄的射野片质量常常很差。当诸如脊髓等关键结构要被挡在治疗野之外时，常常难以确定挡块是否放置正确。为了解决这个问题，在一台10兆伏直线加速器的侧面安装了一个诊断用X射线源，以提供对患者体位和屏蔽挡块位置的精确验证。机械等中心、10兆伏等中心和诊断用X射线等中心之间的重合度约为1毫米。该系统的设计使得监测这种重合度的程序以及维持重合度的调整程序都易于执行。
