Crop F, Van Rompaye B, Paelinck L, Vakaet L, Thierens H, De Wagter C
Department of Medical Physics, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium.
Phys Med Biol. 2008 Jul 21;53(14):3971-84. doi: 10.1088/0031-9155/53/14/015. Epub 2008 Jul 2.
The purpose of this study was both putting forward a statistically correct model for film calibration and the optimization of this process. A reliable calibration is needed in order to perform accurate reference dosimetry with radiographic (Gafchromic) film. Sometimes, an ordinary least squares simple linear (in the parameters) regression is applied to the dose-optical-density (OD) curve with the dose as a function of OD (inverse regression) or sometimes OD as a function of dose (inverse prediction). The application of a simple linear regression fit is an invalid method because heteroscedasticity of the data is not taken into account. This could lead to erroneous results originating from the calibration process itself and thus to a lower accuracy. In this work, we compare the ordinary least squares (OLS) inverse regression method with the correct weighted least squares (WLS) inverse prediction method to create calibration curves. We found that the OLS inverse regression method could lead to a prediction bias of up to 7.3 cGy at 300 cGy and total prediction errors of 3% or more for Gafchromic EBT film. Application of the WLS inverse prediction method resulted in a maximum prediction bias of 1.4 cGy and total prediction errors below 2% in a 0-400 cGy range. We developed a Monte-Carlo-based process to optimize calibrations, depending on the needs of the experiment. This type of thorough analysis can lead to a higher accuracy for film dosimetry.
本研究的目的是提出一种统计上正确的胶片校准模型并优化此过程。为了使用放射成像（Gafchromic）胶片进行准确的参考剂量测定，需要进行可靠的校准。有时，普通最小二乘简单线性（参数方面）回归被应用于剂量 - 光密度（OD）曲线，将剂量作为OD的函数（反向回归），或者有时将OD作为剂量的函数（反向预测）。应用简单线性回归拟合是一种无效方法，因为未考虑数据的异方差性。这可能导致源自校准过程本身的错误结果，从而降低准确性。在这项工作中，我们将普通最小二乘（OLS）反向回归方法与正确的加权最小二乘（WLS）反向预测方法进行比较以创建校准曲线。我们发现，对于Gafchromic EBT胶片，OLS反向回归方法在300 cGy时可能导致高达7.3 cGy的预测偏差，总预测误差达到3%或更高。应用WLS反向预测方法在0 - 400 cGy范围内导致的最大预测偏差为1.4 cGy，总预测误差低于2%。我们根据实验需求开发了一种基于蒙特卡罗的过程来优化校准。这种全面的分析可以提高胶片剂量测定的准确性。
