Comparison Of Detection Sensitivity Of MLC Errors In RapidArc Dose Verification With ArcCHECK And EPID

Rapid Arc is a new technology of Varian’s Volumetric Modulated Arc Therapy.By changing the shape of multi-leaf collimators(MLCs),the radiation dose rate,and the rotation speed of the linear accelerator treatment head,RapidArc gives the best dose distribution and reduces the patient’s treatment time.The RapidArc treatment plans require dose verification before clinical treatment.ArcCHECK and EPID are commonly used as dose verification tools for RapidArc.This study aims to investigate the sensitivity of ArcCHECK and EPID to MLC position errors.Five head and neck clinical treatment cases were considered and the DICOM-RT plan files were modified by writing an external program to introduce the MLC gap "open" errors.Using the Eclipse treatment planning system,doses on ArcCHECK and EPID for all plans were calculated.Then,the calculated doses of original plans and the calculated doses of MLC modified plans were compared using the gamma analysis(2%/2 mm).Before all plans had be actual measured,multiple static measurements and New-setup measurements on the same plan were performed 7 times to understand the repeatability of ArcCHECK and EPID.The gamma analysis(2%/2 mm and 3%/3 mm)was performed on measured doses of original plans with calculated doses of original plans,and measured doses of the MLC modified plans with calculated doses of original plans.Reproducibility results show that EPID deviations both are ±0.01,ArcCHECK deviations are ±0.01 and ±0.02,respectively.Comparison results of calculated doses show that,when the MLC gap change is less than 2 mm,the gamma pass rates of ArcCHECK and EPID are the same;for gap changes of more than 2 mm,however,ArcCHECK shows better sensitivity than EPID.In clinical practice,the results show that the sensitivity of the two verification tools for the MLC gap is approximately the same,but ArcCHECK’s lower gamma pass rates for calculated doses indicate a better response to MLC errors.Therefore EPID has a better repeatability for dose verification,but for detecting MLC errors,the theoretical calculation and clinical application of ArcCHECK are more sensitive to changes in the MLC gap when compared with the EPID.