Georg D, Garibaldi C, Dutreix A
Radiotherapy Department, University Hospital Gasthuisberg, Leuven, Belgium.
Phys Med Biol. 1997 Sep;42(9):1821-31. doi: 10.1088/0031-9155/42/9/012.
Basic dosimetric quantities necessary to specify wedged beans (beam quality, wedge factors, output ratios) are obtained by measurements performed in a narrow coaxial mini-phantom for 6, 18 and 15 MV photon beams. To express beam quality, an attenuation coefficient mu is derived from measurements in a mini-phantom at 20 and 10 cm depth. Wedge factors and output ratios are measured as a function of field size at 10 cm water-equivalent depth. In open beams one observes beam softening with increasing distance from the collimator axis for all energies. With an inserted wedge a beam hardening is observed at 6 MV. This beam hardening decreases at 18 MV while at 25 MV a slight beam softening is detected. Larger variations of output ratios with field sizes are observed with a wedge than without a wedge. An equivalent square formula for head-scatter factors can be used with a good accuracy for rectangular wedged fields. For irregular wedged fields a method is proposed to calculate the product of the output ratio and the wedge factor. Measurements and calculations agree within 1% for all irregular wedged fields checked.
用于确定楔形射束（射束质、楔形因子、输出剂量率）所需的基本剂量学量，是通过在窄同轴微型体模中对6、18和15 MV光子射束进行测量获得的。为了表示射束质，衰减系数μ是从在微型体模中20和10 cm深度处的测量得出的。楔形因子和输出剂量率是在10 cm水等效深度处作为射野大小的函数进行测量的。在开放射束中，对于所有能量，人们观察到离准直器轴距离增加时射束变软。插入楔形板后，在6 MV时观察到射束变硬。这种射束变硬在18 MV时减小，而在25 MV时检测到轻微的射束变软。与没有楔形板相比，有楔形板时观察到输出剂量率随射野大小的变化更大。头部散射因子的等效方公式可很好地用于矩形楔形射野。对于不规则楔形射野，提出了一种计算输出剂量率与楔形因子乘积的方法。对于所有检查的不规则楔形射野，测量值与计算值在1%以内相符。
