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

在放射治疗领域，需要对复杂的剂量分布进行评估，这就要求剂量评估具有较高的精度。辐射变色胶片作为进行二维剂量分布评估的辐射计，近年来得到了广泛的关注。胶片密度的不均匀性以及胶片与密度测量仪器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密度的影响，并考虑了这种影响的校正方法。首先，对由用于X射线胶片的1710型激光密度计和Gafchromic胶片MD-55-2组成的剂量测定系统，进行密度测量精度的检查，并在1-60戈伊下观察密度数据的二维图像的形成所引起的伪影的存在。因此，在需要高度详细的像素级的测量精度的情况下，例如在3D-CRT或IMRT中执行复杂的剂量分布评估，认为有必要进一步添加由双曝光技术表示的浓度不均匀性的校正处理。此外，结果是照射后的样本像素值也随着照射前的样本像素值变高而变高。此外，由于在两个样本像素值之间的关系中线性近似是可能的，所以采用了在每次照射之前针对每个样本像素值创建吸收剂量到像素值的转换曲线并且在照射之后校正样本像素值的过程。通过该过程，可以获得比仅利用用于精确度的吸收剂量到像素值的传统转换曲线执行的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.