To investigate the γ pass rate limit of plan verification equipment for volumetric modulated arc therapy (VMAT) plan verification and its sensitivity on the opening and closing errors of multi-leaf collimator (MLC), 50 cases of nasopharyngeal carcinoma VMAT plan with clockwise and counterclockwise full arcs were randomly selected. Eight kinds of MLC opening and closing errors were introduced in 10 cases of them, and 80 plans with errors were generated. Firstly, the plan verification was conducted in the form of field-by-field measurement and true composite measurement. The γ analysis with the criteria of 3% dose difference, distance to agreement of 2 mm, 10% dose threshold, and absolute dose global normalized conditions were performed for these fields. Then gradient analysis was used to investigate the sensitivity of field-by-field measurement and true composite measurement on MLC opening and closing errors, and the receiver operating characteristic curve (ROC) was used to investigate the optimal threshold of γ pass rate for identifying errors. Tolerance limits and action limits for γ pass rates were calculated using statistical process control (SPC) method for another 40 cases. The error identification ability using the tolerance limit calculated by SPC method and the universal tolerance limit (95%) were compared with using the optimal threshold of ROC. The results show that for the true composite measurement, the clockwise arc and the counterclockwise arc, the descent gradients of the γ passing rate with per millimeter MLC opening error are 10.61%, 7.62% and 6.66%, respectively, and the descent gradients with per millimeter MLC closing error are 9.75%, 7.36% and 6.37%, respectively. The optimal thresholds obtained by the ROC method are 99.35%, 97.95% and 98.25%, respectively, and the tolerance limits obtained by the SPC method are 98.98%, 97.74% and 98.62%, respectively. The tolerance limit calculated by SPC method is close to the optimal threshold of ROC, both of which could identify all errors of ±2 mm, while the universal tolerance limit can only partially identify them, indicating that the universal tolerance limit is not sensitive on some large errors. Therefore, considering the factors such as ease of use and accuracy, it is suggested to use the true composite measurement in clinical practice, and to formulate tolerance limits and action limits suitable for the actual process of the institution based on the SPC method. In conclusion, it is expected that the results of this study can provide some references for institutions to optimize the radiotherapy plan verification process, set appropriate pass rate limit, and promote the standardization of plan verification.
为探讨计划验证设备用于容积旋转调强放疗（VMAT）计划验证的 γ通过率限值及其对多叶准直器（MLC）开合误差的敏感性，本研究选取50例含顺时针和逆时针两个全弧的鼻咽癌VMAT计划，对其中10例引入8种大小的MLC开合误差，生成80例含误差的计划。首先，对单野和合成野使用计划验证设备进行验证并执行剂量差异3%、距离差异2 mm、10%剂量阈值、绝对剂量全局归一条件下的 γ分析，再使用梯度分析研究合成野和单野对MLC开合误差的敏感性，并使用受试者工作特征曲线（ROC）研究识别误差的最佳通过率阈值。然后，对另外40例应用统计过程控制（SPC）方法计算 γ通过率的容差限值和干预限值，将SPC容差限值、通用容差限值（95%）对误差的识别能力与ROC最佳阈值对误差的识别能力进行比较。研究结果显示，对于合成野、顺时针弧及逆时针弧， γ通过率随每毫米MLC打开误差下降的梯度分别为10.61%、7.62%及6.66%，随每毫米MLC闭合误差下降的梯度分别为9.75%、7.36%及6.37%。ROC方法得到的最佳阈值分别为99.35%、97.95%及98.25%；SPC方法得到的容差限值分别为98.98%、97.74%及98.62%。SPC容差限值与ROC最佳阈值较为接近，两者均能识别全部±2 mm的错误，而通用容差限值只能部分识别，说明通用容差限值对部分较大的误差并不敏感。因此综合考虑易用性、精确性等因素，在临床实践中建议可使用实际射野角度合成测量的方法，并根据SPC方法制定与机构自身流程相适宜的通过率限值。综上，期待本文研究结果可为各机构优化放疗计划验证流程、设置合适的通过率限值、促进放疗计划验证规范化提供一定的参考。.
Keywords: Errors of multi-leaf collimator; Limit value of γ pass rate; Plan verification; Receiver operating characteristic curve; Statistical process control; Volumetric modulated arc therapy.