加速器运行误差对宫颈癌容积旋转调强放疗的剂量学影响

The dosimetric impacts of accelerator operation error on the volumetric modulated arc therapy for cervical cancer

目的研究加速器机架旋转角度、准直器到位和多叶光栅（MLC）叶片到位等误差对宫颈癌容积旋转调强放疗（VMAT）的剂量学影响。方法选取10例已行VMAT的宫颈癌计划，提取Pinnacle^3 V9．2计划系统（美国Philips公司）中每个临床计划的plan．Trail文件，使用Matlab编写的程序读取并修改每个控制点运行参数，从而模拟加速器运行误差。通过各引入误差的计划与原计划的剂量比较，评估加速器各参数运行误差对VMAT的剂量学影响及主要的影响因素。结果在引入机架旋转角度误差、准直器到位误差和两侧MLC叶片同向偏移误差中，PTV各剂量限值的最大变化分别为0．16％、0．46％和0．57％，危及器官（OAR）各剂量限值的最大变化分别为0．38％、-1．32％和-0．44％。引入两侧MLC叶片反向或相向运动误差，其幅度为±0．5、4^-1和±2mm时，导致PTV各剂量限值变化的最大值分别为2．11％、3．04％和6．03％，各OAR平均剂量变化的最大值分别为2．17％、3．92％和7．97％，且PTV和OAR各剂量限值与MLC反向或相向运动误差呈强线性相关（t=21．201～90．562，P〈0．05）。引入各参数实际执行误差值时，PTV和OAR各剂量限值的最大变化分别为0．16％和1．30％，适形指数（CI）和均匀性指数（HI）基本不变。结论执行宫颈癌VMAT计划时，两侧MLC叶片反向或同向运动误差相比机架旋转角度误差、准直器到位误差和MLC叶片同向偏移误差对VMAT剂量学影响更加显著，因此，应加强对加速器质控尤其MLC叶片到位误差的质量控制以提高放疗的精度。

Objective To investigate the dosimetric effect of accelerator gantry rotation angle errors, collimator and multileaf collimator （MLC） leaf position errors on volumetric-modulated arc therapy （VMAT） for cervical cancer. Methods A total of 10 patients with cervical cancer were selected. The plan. Trail file of each clinical plan was extracted from the Pinnacle3 V9.2 planning system of USA Philips, then the operating parameters of tach control point were read and modified by Matlab programs, and thus the operating error of the accelerator was simulated. Results In this paper, it was discovered that systematic accelerator gantry rotation angle errors, systematic collimator position errors and systematic MLC shift errors which led to the maximum changes of the PTV dose limit were 0. 16% , 0. 46% and 0.57% , respectively, and the maximum changes of the dose limit of organs at risk （OAR） were 0. 38% , - 1.32%and - 0.44% , respectively. When the systematic MLC gap width errors were α± 0.5, ± 1 and ± 2 mm, respectively, the maximum changes of PTV dose were 2.11% , 3.04% and 6.03% , respectively, while the maximum changes of the OAR average dose were 2. 17%, 3.92% and 7.97%, respectively. Furthermore, the dose limits of PTV and OAR showed a strong linear correlation with MLC open or close errors（ t = 21. 201～90. 562 ,P 〈 0.05 ）. If actual errors of each parameter of accelerator were introduced, the maximum changes of PTV and OAR dose limits were 0. 16% and 1.30% , respectively, and conformity index （CI） and homogeneity index （HI） were barely changed. Conelusions No significant effect was found for systematic accelerator gantry rotation angle errors, systematic collimator position errors and systematic MLC shift errors for ceivical cancer VMAT patients. However, there is a high sensitivity to dose distribution for MLC open or close errors. Therefore, it is necessary to pay more attention on the quality control of the accelerator running in particular MLC position errors to ensure the therapeutic accuracy.