Study On Dosimetric Verification System Based On Amorphous Silicon Flat Panel Detector

Electronic portal imaging devices (EPIDs) were originally developed for thepurpose of patients setup verification. With the continuous development of flat paneldetector, amorphous silicon electronic portal imaging device (a-Si EPID) has nowbecome the most advanced portal image tool with characteristic of fast imaging, largesensitive area, high resolution, digital format storage, tolerance to high dose, gettingthe two dimensional dose information quickly and so on which bring great advantagesin quality control of accelerator radiation field and dose verification.The research is mainly constitute of amorphous silicon plate panel detector fordose verification and the testing of the quality control of accelerator radiation field. Asoftware tool, TiGRT DVS, was developed in the Visual C#integrated environment toassist the study.As to the application of quality control of accelerator radiation field, the physicaldose quality of amorphous silicon plate panel detector is studied, besides that, thebeam non-uniformity brought by the accelerator flattening filter is corrected usingDose Mode. Off axis ratio profile of different field size can be acquired afterconverting pixels to relative dose. The beam flatness, symmetry, and the penumbra arecompared with the3dimensional water tank. The consistency of radiation field andlight field is validated by calculating the distance between the beam center to50%ofradiation dose line, which is almost the same as the film measurement results.After acquiring the continuous images, dose characteristics of this detector arestudied, analyzing the practical significance of the various parameters and related acting factors, such as stability, dose linear response, scattering effects of differentradiation field size, frame time, electric capacity. A dose response function is fitted outaccording to dose-response relation and then the gray image is converted into doseimage. After correction of lateral scatter in the EPID, by introducing the mathematicscatter model, the portal dose image can be calculated which can be used to calculatedfor any field size and shape. The accuracy of our approach was tested by settingregular and irregular fields. The RT dose image as the reference image is read andcompared with the real dose distribution tested by EPID. By using Gamma analysis,distance agreement analysis and relative dose difference method, the dose distributioncalculated by TPS and the real dose distribution tested by a-Si EPID can be evaluated.