Zhu T C, Bjärngard B E
Department of Radiation Oncology, Roger Williams Medical Center, Brown University, Providence, RI 02908-4735, USA.
Phys Med Biol. 1995 Jun;40(6):1127-34. doi: 10.1088/0031-9155/40/6/011.
The output factor in air for a high-energy x-ray beam varies with the collimator setting. Collimator backscatter and obscuring of the source in the target contribute to this variation, but the main component is photon scatter in structures in the accelerator head. To determine the scatter-to-primary ratio, SPR, between the air kerma from such scattered photons and from those that originate directly from the source, it is essential to know the shape of this function SPR(c) of the square collimator opening c, especially for small c. To determine this, simulated head scatter was generated in lead blocks inside the head. By taking the ratio of the output factors for two such blocks of slightly different thickness, the effects of source obscuring and collimator backscatter were eliminated. Using the measured difference in transmission through the two lead blocks, the limiting value for c-->0 could be determined and hence the scatter-to-primary ratios SPR(c). The results could be fitted well with an error function, which then was applied to data measured for clinical beams of 6 and 25 MV. For c = 20 cm, SPR for one 6 MV beam was determined to be 0.016 without a flattening filter, 0.06 for the open beam, and 0.16 with the built-in wedge. At 25 MV, the SPR was higher, 0.09 and 0.20 for the open and wedged beams, respectively.
高能X射线束在空气中的输出因子随准直器设置而变化。准直器的反向散射以及靶中源的遮挡会导致这种变化，但主要因素是加速器头部结构中的光子散射。为了确定此类散射光子与直接源自源的光子的空气比释动能之间的散射与原发射线比率（SPR），了解方形准直器开口c的此函数SPR(c)的形状至关重要，特别是对于小c值。为了确定这一点，在头部内部的铅块中生成了模拟头部散射。通过获取两个厚度略有不同的此类铅块的输出因子之比，消除了源遮挡和准直器反向散射的影响。利用测量得到的通过两个铅块的透射率差异，可以确定c趋近于0时的极限值，从而确定散射与原发射线比率SPR(c)。结果可以很好地用误差函数拟合，然后将该误差函数应用于6和25 MV临床射束的测量数据。对于c = 20 cm，一束6 MV射束在无均整滤过器时的SPR确定为0.016，开放射束的SPR为0.06，内置楔形板时的SPR为0.16。在25 MV时，SPR更高，开放射束和楔形射束的SPR分别为0.09和0.20。
