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

在放射治疗领域，必须评估复杂的剂量分布，并要求剂量评估具有较高的精度。放射致色膜作为一种进行二维剂量分布评估的辐射计，近年来得到了广泛的发展和关注。薄膜的密度不均匀性以及薄膜与密度测量仪的匹配直接影响测量误差。然后，在研究薄膜密度不均匀性的同时，分析了辐照前光密度对辐照后光密度的影响，并考虑了这种影响的校正过程。首先，对由1710型激光密度计和MD-55-2型Gafchromic胶片组成的剂量测量系统进行了密度测量精度的检验，并通过在1-60 Gy范围内形成密度数据的二维图像观察到是否存在伪影。因此，当需要高度精细像素级的测量精度时，例如在3D-CRT或IMRT中进行复杂的剂量分布评估时，我们认为有必要进一步增加以双曝光技术为代表的密度不均匀性校正过程。此外，辐照后的样品像素值也随着辐照前的样品像素值变高而变高。此外，由于两个样本像素值之间的关系可以线性近似，因此采用每次照射前每个样本像素值建立吸收剂量到像素值的转换曲线，照射后对样本像素值进行校正的程序。通过该方法，可以获得比仅使用传统的吸收剂量到像素值的转换曲线进行精度测量更高的测量结果。

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.