Oita Masataka, Watanabe Yoshiharu, Fujita Katsuhisa, Furuya Teruo, Nankumo Junya, Oda Makoto
Department of Radiology, Hokkaido University Hospital.
Nihon Hoshasen Gijutsu Gakkai Zasshi. 2004 Nov;60(11):1604-10. doi: 10.6009/jjrt.kj00003326585.
The output factor of high-energy X-ray machines varies with collimation. According to Khan's theory, collimator and phantom scatter factors contribute to total scatter factor. For precise X-ray irradiation, the two factors need to be taken into consideration. To obtain proper factors, we made two original polystyrene cylindrical mini-phantoms. These phantoms are both 4 cm in diameter and have a pinpoint ion chamber placed at a depth of 5 cm and 10 cm, respectively. Using a 6 MV X-ray machine, collimator scatter factors were calculated for various field arrangements (i.e., field sizes ranging from 4 cm x 4 cm to 40 cm x 40 cm at isocenter). To determine if calculated values were appropriate, we measured point doses of 20 X-ray irradiation patterns using a Farmer-type ion chamber with a water equivalent phantom at depths of 5 cm and 10 cm, respectively. Two hundred MUs were irradiated to the above-mentioned depths for each field. Based on the measured doses, variations were obtained for four calculation methods. Accounting for 1) secondary collimator (jaw) setting, 2) blocked field (multi-leaf collimator) setting, 3) Khan's theory using a 5 cm mini-phantom, and 4) Khan's theory using a 10 cm mini-phantom. Dose variations in each method of calculation were as follows: 1) +0.3 to +10.2% (mean, +2.0 to +3.2%) , 2) -2.3 to 0.0% (mean, -0.8 to -0.6%), 3) 0.0 to +1.5% (mean, +0.1 to +0.3%), 4) 0.0 to +1.4% (mean, -0.1 to +0.1%).
高能X射线机的输出因子随准直而变化。根据汗氏理论，准直器和体模散射因子构成总散射因子。对于精确的X射线照射，需要考虑这两个因素。为了获得合适的因子，我们制作了两个原始的聚苯乙烯圆柱形小型体模。这些体模的直径均为4 cm，分别在深度5 cm和10 cm处放置了一个针点电离室。使用一台6 MV X射线机，针对各种射野布置（即等中心处射野尺寸范围为4 cm×4 cm至40 cm×40 cm）计算准直器散射因子。为了确定计算值是否合适，我们分别使用一个水等效体模的 Farmer 型电离室，测量了20种X射线照射模式在深度5 cm和10 cm处的点剂量。对每个射野在上述深度照射200 MU。根据测量剂量，获得了四种计算方法的变化情况。分别考虑1）二级准直器（光阑）设置，2）遮挡射野（多叶准直器）设置，3）使用5 cm小型体模的汗氏理论，以及4）使用10 cm小型体模的汗氏理论。每种计算方法的剂量变化如下：1）+0.3%至+10.2%（平均值，+2.0%至+3.2%），2）-2.3%至0.0%（平均值，-0.8%至-0.6%），3）0.0%至+1.5%（平均值，+0.1%至+0.3%），4）0.0%至+1.4%（平均值，-0.1%至+0.1%）。
