| At present,there are already many ways to verify the dose accuracy of IMRT.Dose-measurement based verification technique in phantom is the main dosimetric verification method for IMRT,but its time-consuming and labor-intensive is still an challenge for physicist,and it can not reflect the real condition for the patients.Presently,In vivo dosimetry(IVD)is the most direct and effective means of quality assurance.The EPID(Electronic Portal Image Device)has been used for in vivo dose measurement because of it's favorable characteristics.Accelerator log file recorded the accelerator execution plan for each control point corresponding to the gantry angle,multi-leaf collimator(MLC)leave position,cumulative machine monitor units(MU).The log file-based three-dimensional dose verification system will be able to read the content,and reconstruct the 3D dose distributions in patients by combining with the patients original CT image.Objective:Based on the modified position error of EPID in the process of radiation therapy with accelerator rotation,investigate the feasibility of the three-dimensional dose verification system based on EPID applied to volumetric modulated arc therapy and its clinical application;Investigate the feasibility of the three-dimensional dose verification system based on log file applied to volumetric modulated arc therapy and its clinical application,and detect the factors that affect the VMAT precision through comparative analysis of the clinical outcome of two groups.Methods:30 patients with VMAT,including head and neck(14 cases),abdomen(16 cases)were studied.Patients' EPID transmission images were acquired during the courses of treatment,after pre-processing,all these images are imported into the EDose system to reconstruct the dose distribution in the patients,and then compared with the dose calculated by planning system and the first fraction reconstruction in patients,respectively.The dose verification system was commissioned by extracting basic beam parameters for simple field geometries and its clinical use.After import the treatment plans of the same 30 patients into Mobius3D system.This system automatically performs highly accurate calculations using the full set of patient data(DICOM CT datasets,RT plans,RT structures,and RT dose).After 3D dose is recalculated,comparisons are performed with 3D Gamma calculation and DVH graphs for each structure.Results:Commission of the Mobius3D system:the result of square fields test showed that the PDD deviation<±0.7 between the calculated and the measured dose distribution;the infield profile deviation<0.9 between the calculated and the measured dose distribution except for small field of 2cmX2cm,analysis of the penumbra region yields results ranging from 64.58%points,meeting the DTA criteria to 100%of the points except for the large field of 40cmX40cm,the average out-of-field profile error for the system ranged from-0.37-1.39.Clinical exercise results shows that when comparing the reconstructed dose of 14 cases of head and neck by two different methods to the TPS,the average percentage pixels passing a 3%/3mm criteria based gamma analysis were 91.58±2.72 and 97.79±2.00,respectively.And the gamma(3%,3mm)passing rates of three-dimensional dose verification basing on EPID and log file were 90.96±2.58 and 98.28±1.51 for 16 cases of abdomen treated with VMAT,respectively,and the D98?D95?D50?D2 of the clinical targets and doses deviation of the organs at risk(OARs)were provided.Conclusions:After the EPID position error correction,the three-dimensional dose verification system based on EPID should be able to reconstruct dose adequately as a verification tool for the VMAT treatment plans.After the commission of the three-dimensional dose verification system based on log file,it should be able to reconstruct dose adequately as a verification tool for the VMAT treatment plans.It is revealed that the results of different reconstruct methods are in good correlation,this advantage was even more pronounced in the head and neck cancers. |
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