Dong L, Shiu A, Tung S, Hogstrom K
Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston 77030, USA.
Med Phys. 1998 Jun;25(6):841-50. doi: 10.1118/1.598294.
A computer-controlled miniature multileaf collimator (MMLC) with 4 mm leaf width and a maximum field size of 6 cm X 6 cm has been designed as a tertiary beam-shaping device for linac-based stereotactic radiosurgery. The purpose of this study is to develop an accurate and efficient dose calculation model for use with the MMLC. A pencil-beam based algorithm using a sum of three Gaussian kernels was developed to model the off-axis ratio of MMLC fields. Because the kernel integration over a rectangular field can be solved in closed form, dose to any point from an arbitrary MMLC field can be calculated efficiently by summing dose contribution from a set of rectangular apertures and transmission blocks that model individual leaf openings or leaf transmissions. The model uses an effective rectangular field and equivalent square method for determination of depth dose and dose output. Results showed that the calculated percentage depth dose was within 1% and output factor was within 1.5% of measured data. The parameters of the pencil beam kernels were extracted by fitting measured off-axis profiles for a few field sizes at a few depths. The accuracy of the calculated off-axis ratio was tested by comparison with measured data for a number of MMLC fields. The algorithm was shown to be accurate to within 1.5% or 1 mm for off-axis ratios. The algorithm computes at a speed of 34,600 data points per second on a DEC Alpha server model 2000/433 (Digital Equipment Corp., Maynard, MA), which is about 15 times faster than a Clarkson-type summation method.
一种计算机控制的微型多叶准直器（MMLC）已被设计出来，其叶片宽度为4毫米，最大射野尺寸为6厘米×6厘米，用作基于直线加速器的立体定向放射外科的三级射野整形装置。本研究的目的是开发一种准确且高效的剂量计算模型，用于该MMLC。开发了一种基于笔形束的算法，该算法使用三个高斯核的总和来模拟MMLC射野的离轴比。由于矩形射野上的核积分可以以封闭形式求解，所以通过对一组模拟单个叶片开口或叶片透射的矩形孔径和透射块的剂量贡献求和，可以有效地计算任意MMLC射野到任意点的剂量。该模型使用有效矩形射野和等效方形方法来确定深度剂量和剂量输出。结果表明，计算得到的百分深度剂量在测量数据的1%以内，输出因子在测量数据的1.5%以内。笔形束核的参数通过拟合几个深度处几种射野尺寸的测量离轴轮廓来提取。通过与多个MMLC射野的测量数据进行比较，测试了计算离轴比的准确性。结果表明，该算法对于离轴比的计算精度在1.5%或1毫米以内。该算法在DEC Alpha服务器型号2000/433（数字设备公司，马萨诸塞州梅纳德）上的计算速度为每秒34600个数据点，比克拉克森型求和方法快约15倍。
