Study On Construction And Clinical Application Of Intensity-modulated 3D Dose Verification Physical Model Based On EPID

The execution dose deviation of intensity-modulated radiotherapy treatment(IMRT) applicable to the anatomical structure cannot be reflected completely by the dose verification basing on homogeneous phantom. Electronic Portal Imaging Device(EPID), which has the advantages of high resolution, no angular response and simplicity of measurement, is supposed to be applied as dose verification more and more widely in clinical. The current study is one preliminary research to explore a novel physical model constructed basing on the EPID imagines for intensity-modulated radiotherapy treatment plan dose verification according to the EPID three-dimensional Dose measurement system.The study was to conduct the following researches: 1) to analyze the dosimetry characteristics and mechanical position deviation of the EPID; 2) to establish a physical model of dose verification system basing on the EPID images collected from different equivalent square field, and to determine all the parameters of the EPID three-dimensional Dose measurement system physical model. 3) to test the preclinical dose calculation accuracy of the physical model about the EPID three-dimensional Dose measurement system. Including: to analyze the point dose differences between the results of the EPID reconstruction and the results measured by the ionization chamber or calculated by the treatment planning system(TPS) basing on homogeneous phantom; to analyze the Gamma passing rates using the Gamma-evaluation method by comparing the EPID reconstruction, the film measurement and the TPS values inside the homogeneous and inhomogeneous phantom, respectively. 4) to analyze the corresponding Gamma(5%/3mm),(3%/3mm),(2%/2mm)passing rates and the dose deviations of the clinical targets and the organs at risk(OARs) in the intensity-modulated radiotherapy treatment plan three-dimensional dose verification basing on the EPID reconstruction in different site of tumors(10 cases of head and neck, 11 cases of chest, and 10 cases of bowl and abdominal).Results: 1) The measurement indicated the long-term stability and dose-linear relationship of the EPID was less than 1%; 2) A three-dimensional dose verification physical model basing on EPID images and the various parameters of the model was created. 3) In the homogeneous phantom, the average deviation between the point dose measured by the ionization chamber or calculated by the TPS and those reconstructed by EPID was of < 0.5% in the square fields and the combined fields, respectively, and those of iso-center was of < 2% in IMRT plans. In the homogeneous or inhomogeneous phantom, the average passing rates of Gamma(5% / 3 mm),(3% / 3 mm) were > 95% whether between the planar dimensional evaluated by the EPID reconstruction and the film measurement, or between the planar- or three-dimensional doses reconstructed by the EPID and calculated by the TPS through the test cases. 4) The Gamma(5% / 3 mm),(3% / 3mm),(2% / 2mm)passing rates of three-dimensional dose verification basing on EPID were 98.16%,98.23%,99.56% and 98.16%,95.06%,97.80% and79.94%,81.32%, 87.96%in the three different parts of tumor treated with IMRT, respectively, and the D98, D95, Dmean of the clinical targets and doses deviation of the OARs were provided.The dosimetry was verified for the EPID three-dimensional dose verification system in the thesis, and the preliminary clinical application was researched basing on the EPID for three-dimensional dose verification of intensity-modulated radiotherapy treatment plan. The results showed that the method of the three-dimensional dose verification method based on EPID was accurate and reliable, and it could provide a more simple and convenient method and more sufficient dose information for the dose verification in clinical.