Yu M K, Sloboda R
Cross Cancer Institute, Edmonton, Alberta, Canada.
Med Phys. 1996 Jun;23(6):973-84. doi: 10.1118/1.597827.
An analytic representation proposed for the relative intensity distribution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a closed-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15-MV photon beams from Varian 600C and 2300CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.
在双组分X射线源模型中，为焦外源的相对强度分布提出的一种解析表示法用作计算的基础。根据这种表示法，通过在从测量点可见的焦外源平面区域上进行积分，推导出了在中心光束轴上定义的头部散射因子的封闭形式表达式。所得表达式适用于来自不同瓦里安加速器和不同光子能量的光子束，并且包括使用铈基合金挡块或多叶准直器（MLC）进行光束整形所产生的影响。对于来自瓦里安600C和2300CD加速器（瓦里安肿瘤系统公司，加利福尼亚州帕洛阿尔托）的6兆伏和15兆伏光子束，将由X和Y光阑、MLC和Y光阑或仅由MLC形成的方形、矩形、不对称和任意形状射野的361个测量头部散射因子与模型计算结果进行了比较。结果表明，93.4%的计算值与相应实验点的匹配度在0.5%以内，平均偏差为0.23%，最大偏差为0.9%。因此，这项工作定量描述了X光阑、Y光阑和MLC对头部散射因子的不同影响。特别是，结果表明，在径向对称散射的情况下，准直器交换效应是由于X和Y光阑到焦点的距离不同而产生的。
