Study On The Sensitivity Of MLC Positioning Error Of Nasopharyngeal Carcinoma VMAT Plan By Delta~4,ArcCHECK And EPID

Background:With the rapid development of computer technology and treatment planning systems,radiotherapy technology is changing with each passing day,three-dimensional conformal radiation therapy（3D-CRT）,intensity-modulated radiation therapy（IMRT）and volumetric modulated arc radiation therapy（VMAT）and other radiotherapy techniques.The IMRT dose verification method has also evolved from a single point dose verification to 3D dose verification,and the verification object has also evolved from a phantom to a patient.VMAT uses single or mult-arc to produce a highly conformable target dose distribution with the advantages of short treatment time,low machine monitor units,and more uniform dose distribution.However,during the VMAT irradiation process,the position of the MLC,the beam dose rate and the rotational speed of the gantry are dynamically changing while the gantry rotates.The position of the MLC determines each control point of the VMAT plan.The size of the field,the excessive positioning errors of the MLC leads to the inaccurate projection of the planned dose,which directly affects the correct distribution during the treatment,and ultimately brings unpredictable danger to the patient.Therefore,the detection and quality control of the error accuracy of the MLC position is an important part of quality assurance.Delta⁴,Compass,and ArcCHECK are the most commonly used VMAT 3D dose verification tools.These 3D verification devices are expensive and relatively inconvenient to use.At the same time,dose verification based on uniform phantom does not fully reflect the patient’s implementation of IMRT.As the accelerator’s own electronic portal imaging device（EPID）,it is widely used in pre-treatment and in-vivo 3D dose verification,due to its better dosimetry,high resolution,convenient to use,etc..Objective:To validate the VMAT original plan and the plan with MLC position error in patients with nasopharyngeal carcinoma using Delta^4,ArcCHECK and EPID-based three-dimensional dose verification system,and analyze the feasibility and effectiveness of different verification systems in VMAT plan dose verification.Methods:Ten VMAT plans for nasopharyngeal carcinoma were selected,and the position error of 0.5-4.0 mm was introduced into the MLC leaves at each control point in the original file,so that the area of the control point was expand,contract or shifted to one side.Three different types of positional errors that may occur in MLC.The 3D dose verification system of Delta⁴,ArcCHECK and EPID was used to verify the measurement.Under differentγanalysis standards,the verification passing rate of different 3D dose verification systems was obtained,and theγpassing rate of the planned system and paired t-test for theγpassing rate of the measurement results were compared.Results:When the evaluation standard was 3mm/3%,theγpassing rate of all patients of the three detectors was greater than 95%.Delta⁴,ArcCHECK and EPID-based verification systems can detect MLC expand,contract and shift error is 1.5mm,1.0mm,2.0mm;3.0mm,1.0mm,3.0mm and 2mm,1mm,2mm;and when taking the 2mm/2%evaluation standard,the patient originally planned to verify that theγpassing rate is relatively large.The MLC expand,contract and shift MLC errors that can be detected by Delta⁴,ArcCHECK and EPID-based verification systems are 1.0mm,1.0mm,2.0mm,respectively;1.5mm,0.5mm,2.0mm and 1.5mm,0.5 mm and 1.5mm.Conclusion:Delta⁴,ArcCHECK and EPID-based 3D dose verification systems can be used for 3D dose verification of the VMAT plan for nasopharyngeal carcinoma.All detectors can be used to detect MLC position errors of different types and sizes,but at2mm/2%γanalysis standard is more sensitive to the MLC positioning error than the3mm/3%,and the detection sensitivity of the three is slightly different.Because the three detectors have insufficient sensitivity for MLC positioning error detection under the complex VMAT plan of 1mm or less,in order to ensure the accurate execution of the VMAT plan,it is necessary to strengthen the quality control of MLC position error and other related contents in daily work.