Kurup R G, Hogstrom K R, Hwang C C
Department of Radiation Physics, University of Texas, M. D. Anderson Hospital and Tumor Institute, Houston 77030.
Med Phys. 1987 Nov-Dec;14(6):1053-5. doi: 10.1118/1.595985.
Computer algorithms for rotational therapy beams, in most cases, perform dose calculations by summing stored fixed beam data at finite angular steps. Such an algorithm, based on the Bentley beam model, was evaluated by comparing calculations with measured data for an 18-MV x-ray beam. Measurements were made in a specially constructed cylindrical water phantom of 15-cm radius using a 0.1-cm3 ionization chamber for an arc of 180 degrees and for a field size of 7.2 X 7.2 cm2 at 100-cm source-axis distance. This study revealed that the Bentley beam model, with fixed beams summed every 10 degrees, predicts the dose in the treatment volume, centered about the isocenter, with an accuracy of approximately 2%. However, dose at depths between the phantom surface and the treatment volume could be underestimated by as much as 10% (3% of isocenter). This was shown to be partially due to the truncated tails of the off-axis profiles in the Bentley model, which extend only 8 mm outside the edge of the radiation field, and the large angular increment of integration (10 degrees). Using beam profiles extending to 4 cm outside the edge of the radiation field and angular steps of 5 degrees or less for summation of fixed beams reduced errors to less than 5%. Therefore, extended beam profiles and smaller angular steps for summing fixed beams are recommended for photon rotation calculation when increased accuracy is required.
用于旋转治疗束的计算机算法，在大多数情况下，通过在有限的角度步长上对存储的固定束数据求和来进行剂量计算。基于本特利束模型的这样一种算法，通过将计算结果与18兆伏X射线束的测量数据进行比较来进行评估。测量是在一个专门构建的半径为15厘米的圆柱形水模体中进行的，使用一个0.1立方厘米的电离室，在100厘米源轴距下，对180度的弧和7.2×7.2平方厘米的射野尺寸进行测量。这项研究表明，本特利束模型，每10度对固定束求和一次，预测等中心周围治疗体积内的剂量，精度约为2%。然而，模体表面和治疗体积之间深度处的剂量可能被低估多达10%（等中心剂量的3%）。结果表明，部分原因是本特利模型中离轴轮廓的截断尾部，其仅在辐射野边缘外延伸8毫米，以及积分的大角度增量（10度）。使用延伸到辐射野边缘外4厘米的束轮廓以及5度或更小的角度步长来对固定束求和，可将误差降低到5%以下。因此，当需要提高精度时，建议在光子旋转计算中使用延伸的束轮廓和更小的角度步长来对固定束求和。
