Objective By studying the effect of X-ray dosimetry of three accelerators, including Elekta Precise, Elekta Synergy, and Varian 21CX, this paper aimed to set up optimized parameters for treatment plan system.Methods On different level monitoring unit (MU), the research measured three basic parameters to describer X-ray characteristics: linearity of output dose, ifeld uniformity, and hardness. Results When 1≤MU<5, the dosage difference of each monitoring unit was less than 1.9%; when MU≥5, the dosage difference of each monitoring unit was less than 0.8%; when MU≥3, the errors of the uniformity, symmetry, and hardness were all within the allowable range.Conclusion Different types of occelerators had different parameters, the effects of low MU on accelerators varied. Thus it was necessary to measure basic characteristics of different accelerators at regular intervals.%目的：通过研究低MU对医科达Precise、医科达Synergy、瓦里安21CX三台加速器X射线剂量学的影响，设置计划系统的优化参数。方法测量三台加速器在不同MU下输出剂量的线性、剂量分布均匀度和射线质3种描述射线束基本特性的参数。结果在1≤MU<5时，每个MU的剂量差别≤1.9%，当MU≥5时，每个MU的剂量差别≤0.8%。MU≥3时三台加速器的平坦度、对称性、射线质误差均在允许范围内。结论不同型号加速器参数不同，低MU对加速器的影响大小是不同的，因此要定期测量加速器的启动特性。
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