Study on the dose distribution measurement precision in the radiotherapy using GAFchromic film

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In the radiotherapy field, a complicated dose distribution must be evaluated and it is required that dose assessment should have the high precision. As the radiometer that performs dose distribution assessment of 2-dimensional one, Radiochromic film is developed and attracts attention. The density ununiformity of the film and the matching with the film and a density-measuring instrument are directly involved in a measurement error directly. Then, while investigating about the ununiformity of the density of the film in this study, the influence, which the optical density before irradiation has on the density after irradiation, was analyzed, and the correction procedure of he influence was considered. First, to dosimetry system, which consists of Laser Densitometer Model 1710 for an x-ray film and Gafchromic film MD-55-2, the check of density measurement precision was performed and the existence of the artifact by the formation, of a 2-dimensional picture of density data was observed in 1-60 Gy. Consequently, in case where the measurement precision in highly detailed pixel level is required, such as performing complicated dose distribution assessment in 3D-CRT or IMRT, it was thought to be necessary to add further the correction process of density ununiformity that is represented by double exposure technique. Furthermore, it turned out that the sample pixel value after irradiation also becomes high as the sample pixel value before irradiation became high. Moreover, since the linear approximation, was possible in the relation between both sample pixel values, the procedure of creating conversion curve of an absorbed dose to a pixel value for every sample pixel value before each irradiation, and correcting the sample pixel value after irradiation was adopted. By this procedure, the measurement result higher than the measurement performed only with the conventional conversion curve of an absorbed dose to a pixel value for precision could be obtained.