Varian加速器不同的射野形成方式对射野剂量学参数的影响

目的 探讨Varian加速器不同射野形成方式对射野剂量学参数的影响，为治疗计划系统（TPS）数据建模提供理论依据。方法 在准直器（JAW）、多叶光栅（MLC）和准直器跟随多叶光栅（JAW+MLC）3种射野的形成方式下，分别测量百分深度剂量（PDD）、射野离轴量（OAR）及射野总散射因子（Scp），并对实测数据进行分析比较。结果 3种射野形成方式对中心轴的百分深度剂量影响很小；在加速器的左右方向和枪靶方向，MLC形成的射野均较JAW形成射野大，在左右方向最大可达2.9 mm。在枪靶方向，最大可达1.7 mm。在左右方向MLC形成的射野测量曲线的半影较在相同射野大小JAW形成射野的半影大。在枪靶方向MLC形成的射野测量曲线的半影较在相同射野大小JAW形成射野的半影小。在两个方向 JAW+MLC形成射野与JAW形成射野大小与半影均无明显差异。结论 射野的不同形成方式对射野大小、半影、总散射因子有影响，建议做调强放射治疗（IMRT）时，在TPS数据建模过程中，应对MLC射野的剂量参数进行关注。

Influences of various field-defining methods of Varian accelerator on radiation dosimetry parameters

Objective To evaluate the influences of various field-defining method of Varian accelerator on radiation dosimetry parameters, and provide theoretical basis for the beam modeling of the treatment planning system (TPS).Methods The percentage depth dose (PDD), the off-axis ratio (OAR) and the total scattering factors (Scp) of radiation fields were measured in three different conditions, including collimators (JAW), multileaf collimator(MLC) and JAW+MLC. The measured data was analyzed and compared with each other.Results The PDD of central axis was marginally influenced by three field-defining method. In both directions, the MLC-defined field sizes were larger by a maximum of 2.9 mm (left-right) or 1.7 mm (gun-target) than the JAW-defined ones. In the left-right direction, the width of field penumbra shaped by MLC was larger than the field of the same size as defined by JAW. The result of gun-target direction was to the contrary. In both directions, the field penumbra and size displayed no significant differences as defined by JAW+MLC or JAW.Conclusions The field size, penumbra width, and the total scattering factors were influenced by field-defining method, which indicates that special attention should be paid to the dosimetric parameters of MLC during TPS beam modeling for IMRT planning.