Application of Electronic Portal Imaging Device in Quality Control for Multileaf Collimator Position Accuracy of Volumetric-modulated Arc Therapy

Objective To design a method for testing the accuracy of multileaf collimator(MLC) positions during the practice of the volumetric-modulated arc therapy (VMAT) plans with an electronic portal imaging device (EPID). Methods Eight VMAT plans for nasopharynx cancer were selected randomly for the analysis, Dose verification was carried out based on 2 Dion chamber array matrix to obtain the pass rate of dose verification. HIS(heimann imaging software) and EPID loaded in the Synergy Accelerator provided by Elekta were used to obtain the position information of MLC during the practice of VMAT. The actual positions of MLC were calculated with gradient algorithm and compared with MLC positions specified in VMAT plans, to find the position errors and calculate the passing rates of the plans. Results The pass rate of dose verification of 8 VMAT plans for nasopharynx cancer was (94.8±2.1)% with the distance-to-agreement criteria of 3%/3 mm; when the allowance error of MLC position accuracy was 1 mm, the pass rate of leaf's position verification was (91.1±4.0)%. Conclusion All 8 VMAT plans have passed the dose verification, but there are still MLC position errors of different degrees. So only Dose verification of patient-specific VMAT plan was not sensitive enough to detect the tiny MLC positon errors, so additional MLC QA is needed. The detection of MLC positions accuracy with EPID could provide more detailed and underlying quality control information, thus, to provide more guarantee for the application of VMAT.