Chu J C, Galvin J M, Lockett P, Bloch P
Med Phys. 1981 Jul-Aug;8(4):520-2. doi: 10.1118/1.595003.
A general method for calculating the dose distribution in an irradiated volume is to evaluate the primary and scatter components separately according to the method described by Clarkson and Cunningham. It was found, however, that for a 6 MV Siemens accelerator the calculated dose overestimated the peripheral dose at depths beyond 10 cm by 3%-6%. The difference was attributed to the varying beam quality across the field. This beam quality variation was decreased by hardening the beam with a permanently installed 1/3 in. tungsten filter inserted between the beam flattening filter and the mirror base assembly. The tungsten filter had a more pronounced beam hardening effect at the beam edge than along the central ray. For example, the dose rate in air at the beam edge for a 30 x 30 cm2 field was 13% higher than along the central ray without the tungsten filter. The addition of the 1/8 in. filter decreased this horn to 6%. The beam quality along the central ray also increased. The tissue-air ratio for zero field size along the central ray increased by 2% with the addition of the tungsten filter. The scatter-air ratio, however, did not change with the added filter. Agreement within 1%-2% was achieved between the calculated and measured beam profiles at all depths in a phantom when the tungsten filter was added.
一种计算受照体积内剂量分布的通用方法是根据克拉克森和坎宁安描述的方法分别评估原射线和散射成分。然而，研究发现，对于一台6兆伏西门子加速器，在深度超过10厘米处，计算得到的剂量比实测周边剂量高估了3% - 6%。这种差异归因于射野内射线质的变化。通过在均整滤过器和平整滤过器之间永久安装一块1/3英寸厚的钨滤过器来硬化射线，可减小这种射线质的变化。钨滤过器在射野边缘比在中心轴线上具有更显著的射线硬化效果。例如，对于30×30平方厘米的射野，在无钨滤过器时，射野边缘空气中的剂量率比中心轴线上高13%。添加1/8英寸厚的滤过器后，这种差异减小到6%。中心轴线上的射线质也有所提高。添加钨滤过器后，中心轴线上零射野尺寸时的组织空气比增加了2%。然而，添加滤过器后散射空气比没有变化。添加钨滤过器后，在模体中所有深度处计算得到的和实测的射野剖面之间的一致性达到了1% - 2%。
