Heukelom S, Lanson J H, Mijnheer B J
Academic Hospital Vrije Universiteit, Amsterdam, The Netherlands.
Med Phys. 1997 Dec;24(12):1986-91. doi: 10.1118/1.598112.
The head scatter dose contribution to the output of a treatment machine has been determined for an open and wedged 60Co gamma-ray beam and for open and wedged x-ray beams of 4, 8, and 16 MV. From those data wedge factor values "in air" have been deduced, expressed as the ratio of the dose to water, measured in air, for the situation with and without wedge, for the same number of monitor units (or treatment time for 60Co). The measurements have been performed using a polymethyl-metacrylate (PMMA) and a graphite-walled ionization chamber inserted in a brass build-up cap and in a PMMA mini-phantom, respectively. Absolute wedge factor values deduced with both detector systems and based on the ratio of ionization chamber readings, differ for the investigated photon beams, up to 3.5% for the 4 MV x-ray beam. The deviations results from the difference in composition between the detector materials and water and can be taken into account by conversion of the ionization chamber readings for both the open and wedged photon beams to the absorbed dose to water. For the brass build-up cap detector system the ratio of the conversion factors for the wedged and open beam changes the ratio of the ionization chamber readings up to about 3.6% for the 4 MV x-ray beam. For the mini-phantom the conversion factors for the wedged and open beam are almost equal for all photon beams. Consequently, for that system wedge factors based on ionization chamber readings or dose values are the same. With respect to the wedge factor variation with field size a somewhat larger increase has been determined for the 60Co and 4 MV photon beam using the brass build-up cap: about 1% for field sizes varying between 5 cm x 5 cm and 15 cm x 15 cm. This effect has to be related to an apparent more pronounced variation of the head scatter dose contribution with field size for the wedged photon beams if the brass build-up cap detection system is used. It can be concluded that determination of wedge factors "in air" under reference irradiation conditions, performed with both the mini-phantom and brass build-up cap yields within 0.5% the same result if the wedge factors are based on a dose to water ratio. However, by using high-Z build-up materials the determination is more complicated because appropriate conversion factors are then required, while similar conversion factors can be ignored if more water equivalent build-up materials such as PMMA are applied.
已针对开放和楔形的钴 - 60γ射线束以及4兆伏、8兆伏和16兆伏的开放和楔形X射线束，确定了头部散射剂量对治疗机输出剂量的贡献。根据这些数据推导出了“空气中”的楔形因子值，其表示为在相同监测单位数量（或钴 - 60的治疗时间）下，有楔形和无楔形情况下在空气中测量的水吸收剂量之比。测量分别使用插入黄铜建成帽中的聚甲基丙烯酸甲酯（PMMA）和插入PMMA微型体模中的石墨壁电离室进行。基于电离室读数之比，用两种探测器系统推导出的绝对楔形因子值，对于所研究的光子束有所不同，对于4兆伏X射线束，差异高达3.5%。这些偏差源于探测器材料与水的成分差异，可通过将开放和楔形光子束的电离室读数转换为水吸收剂量来考虑。对于黄铜建成帽探测器系统，楔形和开放束的转换因子之比使电离室读数之比对于4兆伏X射线束变化高达约3.6%。对于微型体模，所有光子束的楔形和开放束转换因子几乎相等。因此，对于该系统，基于电离室读数或剂量值的楔形因子是相同的。关于楔形因子随射野大小的变化，使用黄铜建成帽对钴 - 60和4兆伏光子束确定了稍大的增加：对于射野大小在5厘米×5厘米至15厘米×15厘米之间变化的情况，约为1%。如果使用黄铜建成帽探测系统，这种效应与楔形光子束头部散射剂量贡献随射野大小更明显的变化有关。可以得出结论，如果楔形因子基于水吸收剂量比，在参考照射条件下用微型体模和黄铜建成帽进行“空气中”楔形因子的测定，结果相差在0.5%以内。然而，使用高原子序数的建成材料时测定会更复杂，因为此时需要适当的转换因子，而如果应用更多水等效的建成材料如PMMA，则可以忽略类似的转换因子。
