Zhu T C, Bjärngard B E, Vadash P
Department of Radiation Oncology, Shands Cancer Center, University of Florida, Gainesville 32610-0385, USA.
Med Phys. 1995 Aug;22(8):1339-42. doi: 10.1118/1.597517.
The presence of a wedge increases the fraction of "head-scattered" photons in a high-energy x-ray beam. We have compared internal and external wedges for x-ray beam energies between 6 and 25 MV by determining their SPRw, i.e., the ratio of the dose contribution from photons scattered by the wedge and photons either coming directly from the target or scattered by other structures, including the flattening filter. Marked differences were observed. First, SPRw for the thickest external wedge (60 degrees) was 1.8% at a field size of 10 x 10 cm2 and mildly dependent on the photon energy, while SPRw for internal wedges for this field size varied between 4.4% and 5.4% depending mostly on the location and size of the wedge and marginally on the photon energy. Second, the variation of SPRw with the collimator setting c x c was different for the internal and external wedges. SPRw for the internal wedge approached a limiting value at large c and could be fitted with an error function, while SPRw for the external wedge increased quadratically with c. As a result, the difference in SPRw (c) for internal and external wedges is reduced for large fields.
楔形板的存在增加了高能X射线束中“头部散射”光子的比例。我们通过测定内部和外部楔形板的SPrw（即楔形板散射光子与直接来自靶区或被其他结构（包括均整器）散射光子的剂量贡献比），比较了能量在6至25MV之间的X射线束的内部和外部楔形板。观察到显著差异。首先，最厚的外部楔形板（60度）在10×10cm2射野尺寸下的SPrw为1.8%，且对光子能量的依赖性较弱，而该射野尺寸下内部楔形板的SPrw在4.4%至5.4%之间变化，主要取决于楔形板的位置和尺寸，对光子能量的依赖性较小。其次，内部和外部楔形板的SPrw随准直器设置c×c的变化不同。内部楔形板的SPrw在大c值时接近一个极限值，可用误差函数拟合，而外部楔形板的SPrw随c呈二次方增加。因此，对于大射野，内部和外部楔形板的SPrw（c）差异减小。
