Objective To explore whether there is error in data results after the movement of electronic portal imaging device(EPID)plate,and study the factors that cause errors,and explore the feasibility of the long-range target plan verification mea-surement method.Methods From September 2021 to November 2022,20 patients received radiotherapy were enrolled,which included 7 males and 13 females,aged 39-79 years old with mean age of 57 years old.There were 11 cases of rec-tal cancer and 9 of cervical cancer.The length of coronal target was 17.96-31.86 cm,with mean length of coronal target of 22.6 cm.The target volume was 898.8-2 139.2 cm3,with mean target volume of 1 299.5 cm3.Eleven cases were treated with volumetric modulated arc therapy,and 9 were treated with intensity modulated radiotherapy.All of them were treated with 6 MV radiations at dose rate of 600 MU/min.The anisotropic analysis algorithm and portal dosimetry(PD)image prediction algorithm were used to calculate volume dose and predicted image,respectively.The open field test of movement EPID plate and collimator test of zeroing and non-zeroing collimator angles were designed and carried out,and MatriXX was used to verify the plan,which as the standard to verify the influence of zeroing collimator angle on planned pass rate,and the data and planned Gamma pass rate were analyzed.Results The results showed that there was a significant error in data after EPID plate shifted,and correction process was incompletely consistent with predicted Profile,but there was a certain degree of simi-larity in curve trend,and with high degree of consistency at intersection of key data curve.There was a strong correlation(Corr=0.994,P＜0.05)between Gamma passing rate of PD plan verification,and the difference was no statistically significant(t=-1.63,P=0.119)for collimator zeroing and non-zeroing.At the same time,the Bland-Altman consistency analysis results showed that the plan pass rates of PD plan verification and MatriXX plan verification were in good agreement regardless of whether the collimator was zeroed or not.Conclusion It is demonstrated that the application of long-range target plan for EPID verification measurement should ensure that EPID plate is at the calibration zero position.Significant errors may occur in the re-sults obtained if the EPID board is moved during the execution of the plan verification.In determining the verification plan will not irradiate and damage the EPID circuit board,the EPID plate position can be fixed.If there is a risk of radiation damage to EPID circuit board,the collimator angle should be zeroed to perform PD plan verification,and the obtained plan pass rate can approxi-mately replace that of non-zeroing collimator angle plan.
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