Study On Dose Verification Of EPID And Octavius 4D In Volume Modulated Arc Therapy

Objective:By measuring the response of EPID and Octavius 4D in terms of field size,dose rate and cumulative dose,and analyzing the changes in the pass rate of different positive and negative doses in the head,chest and pelvic of the VMAT program validation,discussing the advantages and disadvantages of the two devices in the dose response and the program verification in the VMAT.Method:This experiment adopts the varian ix linear accelerator,with 6MV energy and the Octavius 4D center location,which is set in the accelerator and other centers.When the ionization chamber is used for measurement,the constant large solid water with ionization chamber jack is replaced by the 729 ionization chamber matrix.When EPID was measured,SDD=100cm,and when compared with the ionization chamber,the effective measurement point of the ionization chamber was placed under solid water1cm.(1)EPID,Octavius 4D and 0.6cc ionization Chambers were used at the same location for field dependence measurement:the field size was 3cm x 3cm,5cm x 5cm,10cm x 10cm,15cm x 15cm,20cm x 20cm,25m x 25cm,28cm x 28cm,and the dose was 100MU.Record the measurement results and average the three times.All data were normalized to 10cm x 10cm radiation dose and output factors were calculated.Cumulative dose linear measurement:under the condition of standard beam(10×10cm~2),frame angle and the collimator angle for 0°,measuring the dose at different MU.Dose range is 10MU-600MU.EPID,Octavius 4D and 0.6cc ionization Chambers were used for measurement,and the average value was obtained three times and the measurement results were recorded.Dose rate dependent measurement:EPID and Octavius 4D at the set position,the output dose of the accelerator was100MU,the dose rate was 100MU/min-600mu/min.The average value was obtained for three times of measurement,and the measured value when the dose was normalized to 400MU/min.The QA program for 15 head and neck cases,17 chest cases and 13 pelvic cases was validated with EPID and Octavius 4D,respectively.Dose in each case consists of a dose free error plan and a positive and negative dose error plan,dose error was+/-1%,+/-2%,+/-3%,+/-4%,+/-5,+/-7%,+/-10%,respectively.After data collection,the database was analyzed separately using Verisoft and Portal Dosemitry dose-analysis software.Analysis was performed using the 3mm,3%standard.Then,the pass-through rate of non-dose error and the pass-through rate of positive and negative dose error were statistically analyzed in accordance with the sequence of error from low to high,until there is a significant difference in the results of the group,the equipment can be resolved to the minimum dose error as?.Results:The EPID and 0.6 cc standard ionization chamber measurement result output factor are increased with the radiation field gradually increase,when the field is less than 10cm x 10cm,EPID decreases compared to 0.6cc ionization chamber.Decreased from 0.937 of the ionization chamber to 0.865 at 3cm x 3cm.When the field was larger than 10cm x 10cm,EPID increased compared to 0.6cc ionization chamber,and at 28cm x 28cm,EPID increased from 1.015 to 1.083.The output factors of the measured results of Octavius 4D and 0.6cc standard ionization chamber gradually increased with the increase of the irradiation field.The emission factor of Octavius on4D was reduced when the concentration was less than 5cm x 5cm.At 3cm x 3cm,the ionization chamber decreased from 0.79 to 0.77.(2)The EIPD and 0.6cc ionization Chambers are basically linear in cumulative dose.The two curves under 500MU were basically consistent,and the EPID measurement results were slightly higher than that of the ionization chamber between above 500MU and 600MU.At 600MU,it increased from 5.985 of the ionization chamber to 6.064,increasing by 1.3%.Both the Octavius4D and 0.6cc ionization Chambers were basically linear in terms of cumulative dose,and the two curves were highly consistent between 100MU and 600MU.(3)The response of EIPD at each dose rate was within 0.6%,and the minimum value of 99.4appeared at 600mu/min.Octavius 4D has a good dose-rate response in the100-600mu/min range.(4)Head case:the minimum dose error of Octavius 4D in the positive dose error was 5%(P=0.011).The minimum dose error that can be resolved under negative dose error is-1%(P=0.003).The minimum dose error of EPID can be resolved under positive dose error is 1%(P=0.003).The minimum dose error that can be resolved under negative dose error is-1%(P=0.001).Chest case:Octavius 4D has a resolved minimum dose error of 1%under positive dose error(P=0.032).The minimum dose error that can be resolved under negative dose error is-1%(P=0.001).The minimum dose error of EPID in positive dose error was 1%(P=0.001).The minimum dose error that can be resolved under negative dose error is-1%(P=0.001).pelvic cases:The minimum dose error of Octavius 4D is 3%(P=0.005)in the positive dose error.The minimum dose error that can be resolved under a negative dose error is-2%(P=0.002).The minimum dose error of EPID under positive dose error was 1%(P=0.016).The minimum dose error that can be resolved under negative dose error is-1%(P=0.001).Conclusion:(1)EPID has obvious radiation dependence at a certain dose,and Octavius 4D has a larger radiation dependence than 0.6cc ionization chamber when the exposure field is less than 5cm x 5cm;(2)The dose response of EPID and Octavius 4D is linear between 100MU and 600MU;(3)EPID and Octavius 4D have good dose-rate response in the range of 100mu/min-600mu/min;(4)In QA planned dose validation in VMAT,EPID is more sensitive to dose change than Octavius 4D.