Partridge M, Symonds-Tayler J R, Evans P M
Joint Department of Physics, Institute of Cancer Research/Royal Marsden NHS Trust, Sutton, UK.
Phys Med Biol. 1999 Jan;44(1):271-9. doi: 10.1088/0031-9155/44/1/019.
Methods of removing the effects of linear accelerator (linac) output fluctuation from electronic portal images are described and compared. The output of the linac is measured using a specially constructed large-area ionization chamber during imaging and recorded with the image. The use of a dose-rate signal directly from the linac monitor chamber is discussed. Various versions of a quadratic thickness calibration scheme are tested, incorporating linac output data measured by the ionization chamber. Experimental results are presented showing that the incorporation of data from the ionization chamber gives improved absolute calibration accuracy and flatness. Immediately after calibration, the mean systematic thickness error in calibration of a uniform 136.8 mm water-equivalent slab was shown to be no more than 0.6 mm with a thickness variation within each image also of no more than +/-0.8 mm. This was true even when imaging with an unstable linac beam giving mean thickness errors between images of 8.8 mm and variations within each image of +/-4.9 mm without the ionization chamber correction. Up to one month after calibration, use of the ionization chamber to remove short-term linac fluctuations is shown to still keep mean thickness errors to less than 1.6 mm with variations within each image of no more than +/-1.4 mm.
描述并比较了从电子射野影像中消除直线加速器（linac）输出波动影响的方法。在成像过程中，使用特制的大面积电离室测量直线加速器的输出，并与影像一起记录。讨论了直接使用来自直线加速器监测电离室的剂量率信号。测试了各种版本的二次厚度校准方案，纳入了由电离室测量的直线加速器输出数据。给出的实验结果表明，纳入电离室数据可提高绝对校准精度和平坦度。校准后立即显示，均匀的136.8毫米水等效模体校准中的平均系统厚度误差不超过0.6毫米，每个影像内的厚度变化也不超过±0.8毫米。即使使用不稳定的直线加速器束成像，在没有电离室校正时影像间平均厚度误差为8.8毫米且每个影像内变化为±4.9毫米，情况也是如此。校准后长达一个月，使用电离室消除直线加速器的短期波动仍可使平均厚度误差保持在小于1.6毫米，每个影像内变化不超过±1.4毫米。
