瓦里安加速器扩充型动态楔形板楔形因子特性研究

Characteristics of wedge factor of an enhanced dynamic wedge on Varian 600C accelerator

目的 探讨瓦里安扩充型动态楔形板 (EDW )楔形因子的特性。方法 对瓦里安 6 0 0C直线加速器 6MVX射线 ,用砝玛 2 5 70剂量仪和 2 5 71指形电离室对不同的挡块设置情况下EDW的楔形因子进行测量 ,同时应用文献中给出的方程进行计算 ,将二者的结果进行对比分析。结果 同一EDW下对称照射野的楔形因子随照射野的增大而平滑减小 ,对于同一照射野 ,楔形因子随楔形板角度的增大而减小 ,减小幅度随照射野的增大而增大。楔形因子的离轴分布与物理楔形板的相似 ,呈近似指数变化。对于仅由上叶独立准直器非对称而形成的离轴方野 ,6 0°动态楔形野中心轴线上楔形因子随照射野中心位置坐标增大而增大 ,而其余的则都减小 ;同一EDW下 ,同一位置的楔形因子随照射野的增大而减小。对于固定的测量点 ,EDW的楔形因子与运动挡块的初始位置无关。楔形因子与下挡块开放大小无关。除楔形因子的离轴分布的计算值与实测值差异略大外 ,对称和离轴方野中心轴线上的楔形因子的符合性较好。结论 瓦里安加速器EDW楔形因子呈平滑变化 ;固定点的楔形因子只与固定挡块的位置有关 ,与运动挡块初始位置和下叶挡块开放大小无关 ;离轴方野楔形因子分布较为复杂 ,在临床使用中应谨慎 ,切忌与物理楔形板相混淆 。

ObjectiveTo study the characteristics of wedge factor of enhanced dynamic wedge (EDW) on Varian 600C linear accelerator . MethodsEnhanced dynamic wedge factors (EDWFs) of different jaw settings were measured on a Varian 600C linear accelerator with FARMER 2570 dosimeter and 2571 0.6 ml Thimble ion chamber. At the same time,equations were used to compute EDWFs of symmetric and asymmetric fields and off axis points. The same measurements were carried out on 30 degree physical wedge in contrast to EDW. The accelerator's jaw position was described according to IEC radiotherapy equipment coordinate standard. Results The EDWFs of symmetric field decreased smoothly while the field size increased. Under the same field,EDWFs decreased as the wedge angle increased and the increasing extent became apparent when the field became larger. In off axis fields formed by the upper independent jaws,only the EDWFs of 60 degree had the tendency to increase when the field center moved toward the toe of the wedge,the rest gave the opposite results. The EDWFs of fixed points varied only by 0.5 percent when the fixed jaw remained at the same place and the moving jaw and lower jaws stayed at different positions. But the WF of 30 degree physical wedge increased by 2.9 percent when the lower jaw settings increased from 4 cm to 40 cm. The EDWFs derived from analytic equations for symmetric and asymmetric fields coincided well with measured results. But at off-axis points,the differences between calculated values and measured data were apparent. ConclusionsThe curve of EDWF versus field size is very smooth. The EDWF of fixed point only depends on the position of the stationary upper jaw,and is affected little by the moving upper jaw and lower jaws. The EDWFs of off axis fields are much more complex than those of physical wedge; it shouldn't be confused in clinical application. The conversion method of rectangular field to square field is of no use when EDW field is confronted. Analytic methods are highly conformal with the calculation of EDWFs of symmetric and asymmetric fields.