电子射野影像板（EBT2）校准曲线随辐照后时间的依赖性。
Dependency of EBT2 film calibration curve on postirradiation time.
机构信息
Department of Medical Imaging and Radiological Sciences, I-Shou University, Kaohsiung 82445, Taiwan.
Department of Radiation Oncology, Chi Mei Medical Center, Liouying, Tainan 73657, Taiwan.
出版信息
Med Phys. 2014 Feb;41(2):021726. doi: 10.1118/1.4862511.
PURPOSE
The Ashland Inc. product EBT2 film model is a widely used quality assurance tool, especially for verification of 2-dimensional dose distributions. In general, the calibration film and the dose measurement film are irradiated, scanned, and calibrated at the same postirradiation time (PIT), 1-2 days after the films are irradiated. However, for a busy clinic or in some special situations, the PIT for the dose measurement film may be different from that of the calibration film. In this case, the measured dose will be incorrect. This paper proposed a film calibration method that includes the effect of PIT.
METHODS
The dose versus film optical density was fitted to a power function with three parameters. One of these parameters was PIT dependent, while the other two were found to be almost constant with a standard deviation of the mean less than 4%. The PIT-dependent parameter was fitted to another power function of PIT. The EBT2 film model was calibrated using the PDD method with 14 different PITs ranging from 1 h to 2 months. Ten of the fourteen PITs were used for finding the fitting parameters, and the other four were used for testing the model.
RESULTS
The verification test shows that the differences between the delivered doses and the film doses calculated with this modeling were mainly within 2% for delivered doses above 60 cGy, and the total uncertainties were generally under 5%. The errors and total uncertainties of film dose calculation were independent of the PIT using the proposed calibration procedure. However, the fitting uncertainty increased with decreasing dose or PIT, but stayed below 1.3% for this study.
CONCLUSIONS
The EBT2 film dose can be modeled as a function of PIT. For the ease of routine calibration, five PITs were suggested to be used. It is recommended that two PITs be located in the fast developing period (1 ∼ 6 h), one in 1 ∼ 2 days, one around a week, and one around a month.
目的
Ashland Inc. 的 EBT2 膜模型是一种广泛使用的质量保证工具，特别是用于验证二维剂量分布。通常，校准膜和剂量测量膜在相同的后照射时间（PIT）下进行照射、扫描和校准，即在胶片照射后 1-2 天。然而，对于繁忙的诊所或在某些特殊情况下，剂量测量膜的 PIT 可能与校准膜不同。在这种情况下，测量的剂量将是不正确的。本文提出了一种包含 PIT 效应的胶片校准方法。
方法
将剂量与胶片光密度拟合为具有三个参数的幂函数。其中一个参数依赖于 PIT，而另外两个参数几乎保持不变，平均值的标准偏差小于 4%。将 PIT 相关参数拟合为另一个 PIT 的幂函数。使用 14 个不同的 PIT（范围从 1 小时到 2 个月），通过 PDD 方法对 EBT2 膜模型进行校准。其中 10 个 PIT 用于寻找拟合参数，另外 4 个用于测试模型。
结果
验证测试表明，使用该建模方法计算的剂量与胶片剂量之间的差异主要在 60 cGy 以上的剂量下小于 2%，总不确定度通常在 5%以下。使用建议的校准程序，胶片剂量计算的误差和总不确定度与 PIT 无关。然而，拟合不确定度随着剂量或 PIT 的降低而增加，但在本研究中仍保持在 1.3%以下。
结论
EBT2 胶片剂量可以建模为 PIT 的函数。为了便于常规校准，建议使用五个 PIT。建议选择两个 PIT 位于快速发展期（1～6 小时），一个位于 1～2 天，一个位于一周左右，一个位于一个月左右。
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