Kim S, Liu C, Chen C, Palta J R
Department of Radiation Oncology, University of Florida College of Medicine, Gainesville 32610, USA.
Med Phys. 1999 Jun;26(6):949-55. doi: 10.1118/1.598487.
A simple algorithm was developed for calculation of the in-air output at various source-to-detector distances (SDDs) on the central axis for wedged fields. In the algorithm we dealt independently with two effective sources, one for head scatter and the other for wedge scatter. Varian 2100C with 18 and 8 MV photon beams was used to examine this algorithm. The effective source position for head scatter for wedged fields was assumed to be the same as that for open fields, and the effective source position for wedge scatter was assumed to be a certain distance upstream from the physical location of the wedge. The shift of the effective source for wedge scatter, w, was found to be independent of field size. Moreover, we observed no systematic dependency of w on wedge angle or beam energy. One value, w = 5.5 cm, provided less than 1% difference in in-air outputs through the whole experimental range, i.e., 6 x 6 to 20 x 20 cm2 field size (15 x 20 cm2 for 60 degrees wedge), 15 degrees-60 degrees wedge angle, 80-130 cm SDD, and both 18 and 8 MV photon beams. This algorithm can handle the case in which use of a tertiary collimator with an external wedge makes the field size for the determination of wedge scatter different from that for head scatter. In this case, without the two-effective-source method, the maximum of 4.7% and 2.6% difference can be given by the inverse square method and one-effective-source method in a 45 degrees wedged field with 18 MV. Differences can be larger for thicker wedges. Enhanced dynamic wedge (EDW) fields were also examined. It was found that no second effective source is required for EDW fields.
开发了一种简单算法，用于计算楔形野中心轴上不同源轴距（SDD）时的空气中输出剂量。在该算法中，我们分别处理两个有效源，一个用于头部散射，另一个用于楔形散射。使用配备18和8MV光子束的Varian 2100C直线加速器来验证此算法。楔形野头部散射的有效源位置假定与开放野相同，楔形散射的有效源位置假定在楔形物理位置上游一定距离处。发现楔形散射有效源的偏移量w与射野大小无关。此外，我们观察到w与楔形角或射束能量没有系统相关性。一个值w = 5.5 cm，在整个实验范围内，即6×6至20×20 cm2射野大小（60度楔形为15×20 cm2）、15度 - 60度楔形角、80 - 130 cm SDD以及18和8MV光子束条件下，空气中输出剂量的差异小于1%。该算法可以处理使用带有外部楔形的三级准直器使确定楔形散射的射野大小与头部散射不同的情况。在这种情况下，如果不采用双有效源方法，对于18MV的45度楔形野，反平方法和单有效源法给出的最大差异分别为4.7%和2.6%。对于更厚的楔形，差异可能更大。还对增强动态楔形（EDW）野进行了验证。发现EDW野不需要第二个有效源。
