VMAT加速器日常运行分析及过程质量控制

Quality control of linac in daily volumetric-modulated arc therapy

目的 利用医科达加速器日志文件监测VMAT中加速器日常运行精度,并通过SPC技术实现加速器PQC.方法 提取加速器上每日治疗的日志文件,自动分析加速器执行VMAT弧形野的各参数精度并生成评估报告.分析加速器运行的稳定性和不同病种治疗精度的差异性.通过SPC技术,基于Johnson转换计算各参数的控制限,并绘制控制图实现加速器PQC.结果 连续16 d中VMAT患者76例（主要包括鼻咽癌、宫颈癌、直肠癌、喉癌）,共执行VMAT弧形射野2446个.加速器机架角、y和x钨门、MCL叶片到位和剂量输出的平均误差分别为0.49°、0.09 mm和0.38 mm、0.31mm和0.05 MU.病种差异引起剂量输出和机架角误差的变异系数分别为8.10％和4.54％.由SPC技术分析,加速器各参数均处于受控状态,但存在误差大于上控制限的异常点,各参数中机架角处于失控状态的比率（0.45％）最高.结论 利用加速器日志文件能监测加速器运行精度,通过SPC技术绘制控制图,能实现VMAT中加速器日常运行的PQC,是一种经济、有效的日常QA方法.

Objective To verify the daily delivery accuracy of volumetric-modulated arc therapy （VMAT） and achieve the process quality control （PQC） of linac using statistical process control （SPC）technology.Methods The log files of all treatments were taken out from the linac system.An in-house software which created by Matlab 7.14 was used to analyze the daily parameters accuracy and the mechanism and dose delivery accuracy of the linac for each VMAT,and an daily evaluation report was automatically created for the linac.The stability of the linac and the deviation of the delivery accuracy for various cancer sites were also analyzed.To achieve the process quality control of hnac using SPC technology,the control limit of each parameter was calculated by Johnson alternation and the single value control charts were drawn.Results 76 patients mainly with nasopharyngeal carcinoma,cervical carcinoma,rectal cancer and laryngeal cancer,got the treatment daily with VMAT technology,and a total of 2 446 arcs were delivered by the linac in continuous 16 days.The gantry angle error,y and x collimator position error,MLC leaf position error and dose delivery error were 0.49°,0.09 mm,0.38 mm,0.31 mm,0.05 MU,respectively.Compare to the nasopharynx,uteri,rectal and larynx cancer,it can be seen that the variation factors of dose delivery and gantry angle errors for various cancer sites were higher,which was 8.10% and 4.54%,respectively.Through the process quality control of the linac,it was found that all the parameters were in control.While some abnormal points arose （the error was greater than UCL）,and the ratio of the gantry angle which was out of control （0.45%） was the highest in all parameters.Conclusions linac delivery accuracy could be monitored and verified during the whole VMAT treatment for each patient,and the daily running condition of linac could be monitored.Through the introduction of SPC technology and the control charts,the process quality control of linac in VMAT could be achieved.It is a financial and valid method for the daily quality assurance of linac.