Yuen K, McParland B J, Kornelsen R O
Med Phys. 1984 Jan-Feb;11(1):15-21. doi: 10.1118/1.595469.
For a uniform slab of inhomogeneity in a supervoltage beam, correction factors can be calculated from the Batho equation. In this report, we present a method for calculating the effect of an annular inhomogeneity, concentric about the beam axis, upon the dose at a point on the axis and below the annulus. A derivation of the equation required in the calculation for supervoltage radiation is given. Results from measurements made in 60Co beams for polystyrene foam, cedar, and aluminum annuli, all having 3.0 x 2.0 cm2 in cross section but with different inside diameters, are compared with correction values calculated by the method. For situations where the annulus is just submerged in the phantom, measured and calculated values are in good agreement. For a general situation, two calculation types are proposed and the data show that in general the measured scatter perturbation lies between the calculated values of the two types. Application of our technique predicts a sign reversal in the scatter perturbation due to an inhomogeneity. This reversal has previously been observed and reported and is also demonstrated in our measurements.
对于超高压射束中不均匀性的均匀平板，校正因子可根据巴索方程计算得出。在本报告中，我们提出了一种计算围绕射束轴同心的环形不均匀性对轴上且在环下方某点剂量影响的方法。给出了超高压辐射计算中所需方程的推导。将在60Co射束中对聚苯乙烯泡沫、雪松和铝环进行测量的结果进行了比较，所有这些环的横截面均为3.0×2.0平方厘米，但内径不同，与用该方法计算的校正值进行了比较。对于环刚好浸没在体模中的情况，测量值和计算值吻合良好。对于一般情况，提出了两种计算类型，数据表明，一般来说，测量的散射扰动介于两种类型的计算值之间。我们技术的应用预测了由于不均匀性导致的散射扰动的符号反转。这种反转此前已被观察到并报道过，在我们的测量中也得到了证明。
