Jacobs F W, van Kleffens H J
Radiother Oncol. 1986 Dec;7(4):363-9. doi: 10.1016/s0167-8140(86)80067-6.
The "Ring" method provides a fast dose calculation and isodose presentation for photon beams with blocks. The method takes into account the change in scatter due to the blocks at each calculation point. Firstly, the dose in a point is calculated assuming that no blocks are present. Secondly, the scatter reduction caused by the blocks is calculated and subtracted. To determine the scatter reduction the irradiated surface is divided in concentric rings around a point at the surface at the intersection with a ray line between focus and calculation point. The scatter reduction caused by blocks for each ring is calculated. The effect of scatter for rings with an outer radius greater than 15 cm where the scatter contribution is less than 1.0% is neglected. Results of the method for 4 MV photons using eight rings are presented. Comparison of dose measurements with calculations in an arrow-shaped photon field showed maximum deviations of 4.0%, using the IRREG program of Cunningham, 6.5% using the BLKINP program of Schlegel, which is based on Clarkson's method, 5.0% using the method of Wrede and 2.2% using the "Ring" method. Contrary to the first two calculation programs, the programs using the last two calculation methods provide isodose lines dose values at points.
“环”方法可为带有挡块的光子束提供快速剂量计算和等剂量线显示。该方法考虑了每个计算点处因挡块导致的散射变化。首先，假设不存在挡块来计算某点处的剂量。其次，计算并减去由挡块引起的散射减少量。为确定散射减少量，将受照表面围绕表面上与焦点和计算点之间的射线相交处的一点划分为同心环。计算每个环因挡块引起的散射减少量。对于外半径大于15 cm且散射贡献小于1.0%的环，其散射效应可忽略不计。给出了使用八个环的4 MV光子该方法的结果。在一个箭头形光子野中，剂量测量值与计算值的比较显示，使用坎宁安的IRREG程序时最大偏差为4.0%，使用基于克拉克森方法的施莱格尔的BLKINP程序时为6.5%，使用弗雷德方法时为5.0%，使用“环”方法时为2.2%。与前两个计算程序不同，使用后两种计算方法的程序会在各点提供等剂量线剂量值。
