Department of Physics, University of Zurich, Zurich, Switzerland; Radiotherapy Hirslanden, Hirslanden Medical Center, Aarau, Switzerland.
Department of Physics, University of Zurich, Zurich, Switzerland; Radiotherapy Hirslanden, Hirslanden Medical Center, Aarau, Switzerland.
Z Med Phys. 2018 Apr;28(2):96-109. doi: 10.1016/j.zemedi.2017.07.002. Epub 2017 Aug 12.
Long-term survivors of cancer who were treated with radiotherapy are at risk of a radiation-induced tumor. Hence, it is important to model the out-of-field dose resulting from a cancer treatment. These models have to be verified with measurements, due to the small size, the high sensitivity to ionizing radiation and the tissue-equivalent composition, LiF thermoluminescence dosimeters (TLD) are well-suited for out-of-field dose measurements. However, the photon energy variation of the stray dose leads to systematic dose errors caused by the variation in response with radiation energy of the TLDs. We present a dosimeter which automatically corrects for the energy variation of the measured photons by combining LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD100H) chips.
The response with radiation energy of TLD100 and TLD100H compared to Co was taken from the literature. For the measurement, a TLD100H was placed on top of a TLD100 chip. The dose ratio between the TLD100 and TLD100H, combined with the ratio of the response curves was used to determine the mean energy. With the energy, the individual correction factors for TLD100 and TLD100H could be found. The accuracy in determining the in- and out-of-field dose for a nominal beam energy of 6MV using the double-TLD unit was evaluated by an end-to-end measurement. Furthermore, published Monte Carlo (M.C.) simulations of the mean photon energy for brachytherapy sources, stray radiation of a treatment machine and cone beam CT (CBCT) were compared to the measured mean energies. Finally, the photon energy distribution in an Alderson phantom was measured for different treatment techniques applied with a linear accelerator. Additionally, a treatment plan was measured with a cobalt machine combined with an MRI.
For external radiotherapy, the presented double-TLD unit showed a relative type A uncertainty in doses of -1%±2% at the two standard deviation level compared to an ionization chamber. The type A uncertainty in dose was in agreement with the theoretically calculated type B uncertainty. The measured energies for brachytherapy sources, stray radiation of a treatment machine and CBCT imaging were in agreement with M.C. simulations. A shift in energy with increasing distance to the isocenter was noticed for the various treatment plans measured with the Alderson phantom. The calculated type B uncertainties in energy were in line with the experimentally evaluated type A uncertainties.
The double-TLD unit is able to predict the photon energy of scatter radiation in external radiotherapy, X-ray imagine and brachytherapy sources. For external radiotherapy, the individual energy correction factors enabled a more accurate dose determination compared to conventional TLD measurements.
接受放射治疗的癌症长期幸存者有罹患放射性肿瘤的风险。因此，对癌症治疗的场外剂量进行建模非常重要。由于尺寸小、对电离辐射高度敏感以及组织等效组成，LiF 热释光剂量计 (TLD) 非常适合场外剂量测量。然而，杂散剂量的光子能量变化会导致 TLD 辐射能量响应变化引起的系统剂量误差。我们提出了一种通过组合 LiF:Mg,Ti (TLD100) 和 LiF:Mg,Cu,P (TLD100H) 芯片自动校正测量光子能量变化的剂量计。
从文献中获取 TLD100 和 TLD100H 的辐射能量响应与 Co 的比较。在测量中，将 TLD100H 放置在 TLD100 芯片的顶部。TLD100 和 TLD100H 的剂量比，结合响应曲线的比值，用于确定平均能量。利用该能量，可以找到 TLD100 和 TLD100H 的单独校正因子。使用双 TLD 单元对名义束能量为 6MV 的场内和场外剂量的确定精度进行了端到端测量评估。此外，还比较了发表的用于近距离放射治疗源、治疗机散射辐射和锥形束 CT (CBCT) 的平均光子能的蒙特卡罗 (M.C.) 模拟与测量的平均能量。最后，测量了不同治疗技术应用于直线加速器时的 Alderson 体模中的光子能分布。此外，还结合 MRI 对钴机治疗计划进行了测量。
对于外部放射治疗，与电离室相比，所提出的双 TLD 单元在两个标准差水平下的相对 A 型不确定度在剂量上显示为 -1%±2%。剂量的 A 型不确定度与理论计算的 B 型不确定度一致。近距离放射治疗源、治疗机散射辐射和 CBCT 成像的测量能量与 M.C. 模拟一致。对于用 Alderson 体模测量的各种治疗计划，注意到能量随离等中心距离的增加而发生偏移。计算得出的能量 B 型不确定度与实验评估的 A 型不确定度一致。
双 TLD 单元能够预测外放射治疗、X 射线成像和近距离放射治疗源中的散射线光子能量。对于外放射治疗，与传统 TLD 测量相比，个别能量校正因子能够更准确地确定剂量。
