A Study Of Dosimetric Quality Control Based On EPID

Previously electronic portal imaging device(EPID)is mainly used for the verification of position deviation,but it is actually a kind of radiation detector,which can be used for radiotherapy dosimetry by the correct calibration.The most recent research indicates that EPID has a significant advantage in pre-treatment and in-vivo dose verification.The process of in-vivo dosimetry will obtain the transit real-time portal image and then reconstruct the dose in patient or phantom,so as to realize the comparison between the reconstructed dose and the actual delivered dose.Dynalog files record the operating parameters in the process of dose delivery.The method of reconstructing dose on CT images using Dynalog files can be a supplementary to experimental measurement for dosimetric quality control.The dosimetric quality control based on EPID has been further studied in this paper.Firstly,the physics modelling,dose calculation algorithm and dose calculation process of EPIbeam based on EPID and Mobius 3D combined with treatment planning system(TPS)were studied.Secondly,the pre-treatment verification for 28 VMAT pelvic plans was carried on through the two systems.Thirdly,this article further studied the physics modelling,dose calculation algorithm and dose calculation process of EPIgray based on EPID and Mobius FX based on Dynalog files.Finally,the in-vivo dosimetry quality control for 30 VMAT plans(15 pelvic plans and 15 head and neck plans)was carried on.At the same time,the point dose in the center of each VMAT plan(the center of the effective sensitive volume of ionization chamber)was measured by the ionization chamber.The principle of EPIbeam relies on the comparison of two images in terms of absolute dose which was converted to the planar dose distribution underwater 5 cm.A RT_plan defined with the TPS was used to generate with the treatment unit an actual portal in open EPID conditions.And the same RT_plan was used to compute a theoretical portal image in the same conditions.Mobius 3D used the Collapsed Cone Convolution/Superposition algorithm to calculate dose based the original treatment parameters from TPS.In our study,the y passing rate of EPIbeam calculating planar dose distribution results is greater than 95%.Mobius 3D shows the three-dimensional y passing rate.There are two cases that y passing rate is between 90%and 95%,but others are higher than 95%.The dose reconstruction formalism of EPIgray is adopted"fTMR" to transform EPI to dose of interested points.Like Mobius 3D,Mobius FX uses CCC/S to calculate dose distribution based Dynalog files.The measurements by ionization chamber and reconstructed doses of EPID-based and "Dynalog files" for 15 pelvic plans and 15 chest plans are(2.34±0.28)Gy,(2.31±0.26)Gy,(2.35±0.29)Gy and(1.58±0.12)Gy,(1.561±0.12)Gy,(1.59±0.10)Gy.The relative deviation percentage between the reconstructed doses of EPID-based or the "Dynalogs file"-based with the measurement results by the ionization chamber is(-1.11±2.42)%and(0.82 ±1.73)%,(-1.27±2.48)%and(0.56±1.91)%.The difference between the reconstructed doses of EPID-based or the "Dynalogs file"-based with the measurement results by the ionization chamber was not statistically significant(t=1.91、-1.83,P>0.05;t=2.01、-0.88,P>0.05).In conclusion,it is a viable method for the dose verification of advanced radiotherapy technology used EPID which does not affect the treatment and is easy to popularize.The dosimetry based EPID can not only improve efficiency but also has a significant advantage in real-time in-vivo dose verification.Besides,it is beneficial for examining the accuracy of radiotherapy dose.The method of dose reconstruction based on accurate Dynalog files is easy to used and simple to implement.And it can also provide reference combined with measurements for the formation of a rapid,simple method of monitoring dose.