外挂式多叶准直器对准直器散射因子的影响

Influence of tertiary add-on mult-leaf collimator over the collimator-scatter factor

目的 用电离室测量外挂式多叶准直器 (MLC)对准直器散射因子 (Sc)的影响 ,并用双源模型对结果进行分析。方法 测量MLC形成的 2个不规则射野序列 ,并与等效方野的测量值进行了比较 ,应用双源模型得出MLC对Sc产生影响时叶片所处的位置公式。结果 当MLC叶片位置离中心足够近时 ,叶片将对准直器散射因子产生影响 ;产生影响时叶片位置计算值与测量结果相符。结论 基于双源模型的MLC位置公式较好地描述了当外挂式准直器 (MLC或铅块 )形成的射野小于公式给出值时 ,准直器散射因子将受其影响。

Objective To investigate the influence of add-on mult-leaf collimator (MLC or cerrobend block) on the collimator-scatter factors. Methods Collimator-scatter factors were measured for two series of irregular MLC fields defined by X and Y jaws and blocked by a multileaf collimator with a 6 MV photon beam from a Varian 600CD accelerator. The measured data from irregular MLC fields were compared with those obtained with the equivalent field method. A formula based on the two-component source model was given. Results Collimator-scatter factors obtained with the equivalent field method were not always in accordance with the measured data. The closer the MLC was to the central beam axis, the more influence it would have on the scatter factors. The threshold location of MLC given by a formula was in accordance with the measured data. Conclusion When the MLC is beyond the location given by a formula, the collimator-scatter factors will be influenced by the tertiary collimator .