MLC系统误差对不同复杂度宫颈癌VMAT计划剂量分布的影响

Effect of MLC systematic error on planned dose distribution of VMAT in cervical cancer of different complexity

目的:研究多叶准直器(MLC)误差对宫颈癌VMAT计划剂量的影响,定量分析计划复杂度与剂量变化量之间的相关性。方法:回顾性选取14例宫颈癌VMAT计划。通过Python对每个计划的MLC引入偏移误差(Type 1)、单边叶片偏移误差(Type 2)以及双边叶片异向整体偏移误差(Type 3)形成模拟计划与原计划进行比较,分析两者间剂量差异。采用小子野得分(small aperture score,SAS)衡量计划复杂度,线性回归分析SAS与计划剂量偏差之间的相关性。结果:当MLC误差为2 mm时,Type 1对计划剂量影响较小,误差导致剂量参数变化<2%;Type 3对剂量影响最明显,PTV D95%、Dmean、Dmax和gEUD的变化量分别为5.31%、5.52%、9.37%和5.48%,小肠、直肠、股骨头和膀胱的Dmean变化量分别为7.96%、6.68%、8.85%和7.41%;Type 2造成的影响几乎为Type 3的一半。当MLC误差类型为Type 2、Type 3时,靶区与危及器官Dmean的平均剂量偏差与SAS(5 cm)呈强线性相关性(R2≥0.63)。结论:Type 2和Type 3 MLC叶片系统误差对计划剂量参数影响明显,为保证MLC系统误差导致靶区剂量变化<2%,质控时MLC叶片整体误差需小于0.4 mm每侧或0.9 mm单侧,SAS值可作为评价指标估算MLC误差对宫颈癌计划剂量影响的大小。

Objective:To study the effect of multi-leaf collimator(MLC)systematic error on the dose of cervical cancer VMAT planning,and quantitatively analyze the correlation between planning complexity and target dose variation.Methods:A retrospective selection was perfored in 14 cases of cervical cancer VMAT plan.The simulation plan was formed by artificially introducing the shifts error(Type 1),the single-sided shifts error(Type 2),and bilateral reverse shifts error(Type 3)for each planned MLC in Python,and the dosimetric differences between the original plan and the simulation plan were compared.The plan complexity was measured by the small aperture score(SAS),and the correlation between the complexity and the amount of dose change in the target area was analyzed by linear regression.Results:When the MLC error was 2 mm,Type 1 has little effect on planned dosimetry,error results in a<2%change in dose parameters.Type 3 had the most obvious effect on dose,with changes of PTV D_(95%),D_(mean),D_(max)and gEUD of 5.31%,5.52%,9.37%and 5.48%,respectively.The D_(mean)changes in the small intestine,rectum,femoral head and bladder were 7.96%,6.68%,8.85%and 7.41%,respectively.Type 2 had almost half the impact of Type 3.In addition,when the MLC error type was Type 2,Type 3,the mean dose deviation between the target and the organ at rise D_(mean)was strongly and linearly correlated with SAS(5 cm)(R~2≥0.63).Conclusion:Type 2 and Type 3 MLC leaf systematic errors have a significant effect on the planned dose parameters.In order to ensure that the MLC systematic error leads to a<2%change in the target dose,during quality control,the overall error of MLC leafs should be less than 0.4 mm on each side or 0.9 mm on one side,and the SAS value can be used as an evaluation index to estimate the impact of MLC error on the planned dose of cervical cancer.