Department of Health Sciences, Faculty of Life Sciences, Kumamoto University, 4-24-1 Kuhonji, Kumamoto 862-0976, Japan.
Med Phys. 2013 Sep;40(9):092101. doi: 10.1118/1.4816673.
In this study, a dedicated device for ion chamber measurements of absorbed dose-to-water for a Nucletron microSelectron-v2 HDR (192)Ir brachytherapy source is presented. The device uses two ionization chambers in a so-called sandwich assembly. Using this setup and by taking the average reading of the two chambers, any dose error due to difficulties in absolute positioning (centering) of the source in between the chambers is cancelled to first order. The method's accuracy was examined by comparing measurements with absorbed dose-to-water determination based on the AAPM TG-43 protocol.
The optimal source-to-chamber distance (SCD) for (192)Ir dosimetry was determined from ion chamber measurements in a water phantom. The (192)Ir source was sandwiched between two Exradin A1SL chambers (0.057 cm(3)) at the optimal SCD separation. The measured ionization was converted to the absorbed dose-to-water using a (60)Co calibration factor and a Monte Carlo-calculated beam quality conversion factor, kQ, for (60)Co to (192)Ir. An uncertainty estimate of the proposed method was determined based on reproducibility of measurements at different institutions for the same type of source.
The optimal distance for the A1SL chamber measurements was determined to be 5 cm from the (192)Ir source center, considering the depth dependency of kQ for (60)Co to (192)Ir and the chamber positioning. The absorbed dose to water measured at (5 cm, 90°) on the transverse axis was 1.3% lower than TG-43 values and its reproducibility and overall uncertainty were 0.8% and 1.7%, respectively. The measurement doses at anisotropic points agreed within 1.5% with TG-43 values.
The ion chamber measurement of absorbed dose-to-water with a sandwich method for the (192)Ir source provides a more accurate, direct, and reference dose compared to the dose-to-water determination based on air-kerma strength in the TG-43 protocol. Due to the simple but accurate assembly, the sandwich measurement method is useful for daily dose management of (192)Ir sources.
本研究介绍了一种用于 Nucletron 微选择器-v2 HDR（192）Ir 近距离放射治疗源的水中吸收剂量的离子室测量的专用设备。该设备在所谓的三明治组件中使用两个电离室。使用此设置并取两个腔室的平均读数，可以首先消除由于源在腔室之间的绝对定位（居中）困难而导致的任何剂量误差。通过将测量结果与基于 AAPM TG-43 协议的水吸收剂量确定进行比较，检查了该方法的准确性。
通过在水模体中的离子室测量确定（192）Ir 剂量学的最佳源-腔室距离（SCD）。将（192）Ir 源夹在两个 Exradin A1SL 腔室（0.057 cm³）之间，SCD 分离最佳。使用 60Co 校准因子和蒙特卡罗计算的束质转换因子 kQ 将测量的离子化转换为水吸收剂量，用于 60Co 到 192Ir。根据同一类型源在不同机构的测量结果的可重复性，确定了所提出方法的不确定度估计。
考虑到 60Co 到 192Ir 的 kQ 的深度依赖性和腔室定位，从（192）Ir 源中心测量 A1SL 腔室的最佳距离为 5 cm。在横向轴上的（5 cm，90°）处测量的水吸收剂量比 TG-43 值低 1.3%，其重复性和总不确定度分别为 0.8%和 1.7%。各向异性点的测量剂量与 TG-43 值相差在 1.5%以内。
与 TG-43 协议中基于空气比释动能强度的水吸收剂量确定相比，（192）Ir 源的三明治方法的水中吸收剂量的离子室测量提供了更准确、直接和参考剂量。由于组装简单但准确，三明治测量方法可用于（192）Ir 源的日常剂量管理。
