Saur Sigrun, Frengen Jomar
Department of Physics, The Norwegian University of Science and Technology, N-7491 Trondheim, Norway.
Med Phys. 2008 Jul;35(7):3094-101. doi: 10.1118/1.2938522.
Film dosimetry using radiochromic EBT film in combination with a flatbed charge coupled device scanner is a useful method both for two-dimensional verification of intensity-modulated radiation treatment plans and for general quality assurance of treatment planning systems and linear accelerators. Unfortunately, the response over the scanner area is nonuniform, and when not corrected for, this results in a systematic error in the measured dose which is both dose and position dependent. In this study a novel method for background correction is presented. The method is based on the subtraction of a correction matrix, a matrix that is based on scans of films that are irradiated to nine dose levels in the range 0.08-2.93 Gy. Because the response of the film is dependent on the film's orientation with respect to the scanner, correction matrices for both landscape oriented and portrait oriented scans were made. In addition to the background correction method, a full dose uncertainty analysis of the film dosimetry procedure was performed. This analysis takes into account the fit uncertainty of the calibration curve, the variation in response for different film sheets, the nonuniformity after background correction, and the noise in the scanned films. The film analysis was performed for film pieces of size 16 x 16 cm, all with the same lot number, and all irradiations were done perpendicular onto the films. The results show that the 2-sigma dose uncertainty at 2 Gy is about 5% and 3.5% for landscape and portrait scans, respectively. The uncertainty gradually increases as the dose decreases, but at 1 Gy the 2-sigma dose uncertainty is still as good as 6% and 4% for landscape and portrait scans, respectively. The study shows that film dosimetry using GafChromic EBT film, an Epson Expression 1680 Professional scanner and a dedicated background correction technique gives precise and accurate results. For the purpose of dosimetric verification, the calculated dose distribution can be compared with the film-measured dose distribution using a dose constraint of 4% (relative to the measured dose) for doses between 1 and 3 Gy. At lower doses, the dose constraint must be relaxed.
使用放射变色EBT胶片结合平板电荷耦合器件扫描仪进行胶片剂量测定，对于调强放射治疗计划的二维验证以及治疗计划系统和直线加速器的一般质量保证而言，都是一种有用的方法。不幸的是，扫描仪区域的响应是不均匀的，若不进行校正，这会导致测量剂量出现系统误差，该误差既与剂量有关，也与位置有关。在本研究中，提出了一种新的背景校正方法。该方法基于减去一个校正矩阵，这个矩阵是基于对在0.08 - 2.93 Gy范围内九个剂量水平进行照射的胶片扫描得到的。由于胶片的响应取决于胶片相对于扫描仪的方向，因此制作了横向扫描和纵向扫描的校正矩阵。除了背景校正方法外，还对胶片剂量测定程序进行了完整的剂量不确定度分析。该分析考虑了校准曲线的拟合不确定度、不同胶片片的响应变化、背景校正后的不均匀性以及扫描胶片中的噪声。对尺寸为16×16 cm、批号相同的胶片进行分析，所有照射均垂直于胶片进行。结果表明，对于横向和纵向扫描，2 Gy处的2σ剂量不确定度分别约为5%和3.5%。随着剂量降低，不确定度逐渐增加，但在1 Gy时，横向和纵向扫描的2σ剂量不确定度分别仍高达6%和4%。该研究表明，使用GafChromic EBT胶片、爱普生Expression 1680专业扫描仪和专用背景校正技术进行胶片剂量测定可得到精确准确的结果。为了进行剂量验证，对于1至3 Gy之间的剂量，可使用4%（相对于测量剂量）的剂量约束，将计算得到的剂量分布与胶片测量的剂量分布进行比较。在较低剂量下，剂量约束必须放宽。
