Study of the optimal thresholds of gamma passing rate in VMAT plan verification for cervical cancer / 中华放射肿瘤学杂志

ABSTRACT

Objective To investigate the optimal thresholds of the passing rate with different gamma measurement criteria (percent dose difference/DTA) based on the Delta 4 three-dimensional dosimetric verification system in the verification of volumetric modulated arc-therapy (VMAT) plan for cervical cancer.Methods Thirty clinically-approved dual-arc VMAT plans using the RapidArcTM (Varian Medical Systems Inc.) for cervical cancer were randomly selected.The gamma analysis and dose-volume histogram (DVH) evaluation were performed using Delta 4.All the plans were classified according to the following two criteria1.If the absolute percentage dose errors of all specific dosimetry indices on the DVH were less than 5％,the plan was regarded as clinically acceptable.2.If the gamma passing rate was 90％ or 95％ under the criteria of 2％/2 mm and 3％/3 mm,the plan was regarded as acceptable.The sensitivity and specificity analyses were conducted based on the classification results and the receiver operating characteristic (ROC) curve was plotted.By calculating the Youden Index,the optimal thresholds under different Gamma criteria (global and local 2％/2 mm and 3％/3 mm) were investigated.Finally,the ability of distinguishing the plan was clinically acceptable or not between the conventional and optimal thresholds was quantitatively compared according to the sensitivity and specificity analyses.Results The optimal thresholds under the global 3％/3 mm and 2％/2 mm criteria were 98.3％ and 87.05％;and 97.55％ 、86.05％ for the local gamma analysis.Compared with the conventional thresholds,the sensitivity of the optimal thresholds was 0.93 by using the global and local gamma analyses under the 3％/3 mm criterion.Under the 2％/2 mm criterion,the sensitivity of the optimal thresholds was 0.65 and the specificity was 0.49 by using the global gamma analysis.The sensitivity was 0.7 and the specificity was 0.46 by using the local gamma analysis,suggesting that the sensitivity and the specificity were more balanced under the 2％/2 mm criterion.Conclusions Application of the optimal thresholds in the verification of VMAT plans can maintain the balance between the sensitivity and specificity,prevent the harm of clinically unacceptable plans to patients to certain extent and reduce the probability of increasing the daily work load for physicists due to the misjudgement of clinically acceptable plans.