Department of Therapeutic Radiology, University of Minnesota, Minneapolis, MN 55455, USA.
J Appl Clin Med Phys. 2012 Sep 6;13(5):3930. doi: 10.1120/jacmp.v13i5.3930.
Some dosimeters require a relationship between detector signal and delivered dose. The relationship (characteristic curve or calibration equation) usually depends on the environment under which the dosimeters are manufactured or stored. To compensate for the difference in radiation response among different batches of dosimeters, the measured dose can be scaled by normalizing the measured dose to a specific dose. Such a procedure, often called "relative dosimetry", allows us to skip the time-consuming production of a calibration curve for each irradiation. In this study, the magnitudes of errors due to the dose scaling procedure were evaluated by using the characteristic curves of BANG3 polymer gel dosimeter, radiographic EDR2 films, and GAFCHROMIC EBT2 films. Several sets of calibration data were obtained for each type of dosimeters, and a calibration equation of one set of data was used to estimate doses of the other dosimeters from different batches. The scaled doses were then compared with expected doses, which were obtained by using the true calibration equation specific to each batch. In general, the magnitude of errors increased with increasing deviation of the dose scaling factor from unity. Also, the errors strongly depended on the difference in the shape of the true and reference calibration curves. For example, for the BANG3 polymer gel, of which the characteristic curve can be approximated with a linear equation, the error for a batch requiring a dose scaling factor of 0.87 was larger than the errors for other batches requiring smaller magnitudes of dose scaling, or scaling factors of 0.93 or 1.02. The characteristic curves of EDR2 and EBT2 films required nonlinear equations. With those dosimeters, errors larger than 5% were commonly observed in the dose ranges of below 50% and above 150% of the normalization dose. In conclusion, the dose scaling for relative dosimetry introduces large errors in the measured doses when a large dose scaling is applied, and this procedure should be applied with special care.
一些剂量计需要探测器信号与传递剂量之间的关系。该关系（特征曲线或校准方程）通常取决于剂量计制造或存储的环境。为了补偿不同批次剂量计之间辐射响应的差异，可以通过将测量剂量归一化为特定剂量来缩放测量剂量。这种程序通常称为“相对剂量测量”，可以省去为每次辐照制作校准曲线的耗时过程。在这项研究中，使用 BANG3 聚合物凝胶剂量计、射线照相 EDR2 胶片和 GAFCHROMIC EBT2 胶片的特征曲线来评估剂量缩放程序引起的误差幅度。为每种类型的剂量计获得了几组校准数据，并使用一组数据的校准方程来估计来自不同批次的其他剂量计的剂量。然后将缩放后的剂量与预期剂量进行比较，预期剂量是使用针对每个批次的真实校准方程获得的。通常，误差幅度随着剂量缩放因子与 1 的偏差增大而增大。此外，误差强烈依赖于真实和参考校准曲线形状的差异。例如，对于特征曲线可以用线性方程近似的 BANG3 聚合物凝胶，需要剂量缩放因子为 0.87 的批次的误差大于需要较小剂量缩放幅度的其他批次的误差，或者缩放因子为 0.93 或 1.02。EDR2 和 EBT2 胶片的特征曲线需要非线性方程。对于这些剂量计，在归一化剂量的 50%以下和 150%以上的剂量范围内，通常会观察到大于 5%的误差。总之，当应用较大的剂量缩放时，相对剂量测量的剂量缩放会在测量剂量中引入较大的误差，因此应特别小心地应用此程序。
