Massillon-J L G, Cabrera-Santiago A, Minniti R, O'Brien M, Soares C G
Instituto de Física, Universidad Nacional Autónoma de México, 01000 Distrito Federal, México.
Phys Med Biol. 2014 Aug 7;59(15):4149-66. doi: 10.1088/0031-9155/59/15/4149. Epub 2014 Jul 8.
LiF:Mg,Ti, are widely used to estimate absorbed-dose received by patients during diagnostic or medical treatment. Conveniently, measurements are usually made in plastic phantoms. However, experimental conditions vary from one group to another and consequently, a lack of consensus data exists for the energy dependence of thermoluminescent (TL) response. This work investigated the energy dependence of TLD-100 TL-response and the effect of irradiating the dosimeters in different phantom materials for a broad range of energy photons in an attempt to understand the parameters that affect the discrepancies reported by various research groups. TLD-100s were exposed to 20-300 kV narrow x-ray spectra, (137)Cs and (60)Co photons. Measurements were performed in air, PMMA, wt1, polystyrene and TLDS as surrounding material. Total air-kerma values delivered were between 50 and 150 mGy for x-rays and 50 mGy for (137)Cs and (60)Co beams; each dosimeter was irradiated individually. Relative response, R, defined as the TL-response per air-kerma and relative efficiency, RE, described as the TL-response per absorbed-dose (obtained through Monte Carlo (MC) and analytically) were used to describe the TL-response. Both R and RE are normalized to the responses in a (60)Co beam. The results indicate that the use of different phantom materials affects the TL-response and this response varies with energy and material type. MC simulations reproduced qualitatively the experimental data: a) R increases, reaches a maximum at ~25 keV and decreases; b) RE decreases, down to a minimum at ~60 keV, increases to a maximum at ~150 keV and after decreases. Independent of the phantom materials, RE strongly depends on how the absorbed dose is evaluated and the discrepancies between RE evaluated analytically and by MC simulation are around 4% and 18%, dependent on the photon energy. The comparison between our results and that reported in the literature suggests that the discrepancy observed between different research groups appears to be most likely related to supralinearity effect, phantom materials, difference on the energy-spectra and geometry conditions during each experiment rather than parameters such as heating-rate or annealing procedure, which was supported by MC simulation. From the results obtained in this work and the strict analysis performed, we can conclude that for clinical applications of TLD-100, special attention must be taken when published data are used to convert TL calibration curve from (60)Co to low-energy photons. Otherwise, this can lead to incorrect results when later used to measure absorbed dose in human tissue.
氟化锂镁钛（LiF:Mg,Ti）被广泛用于估算患者在诊断或医疗过程中所接受的吸收剂量。方便的是，测量通常在塑料模体中进行。然而，不同研究组的实验条件各不相同，因此，关于热释光（TL）响应的能量依赖性缺乏一致的数据。本研究调查了TLD - 100 TL响应的能量依赖性以及在不同模体材料中对剂量计进行辐照对宽范围能量光子的影响，试图了解影响各研究组所报告差异的参数。TLD - 100被暴露于20 - 300 kV窄X射线谱、铯 - 137（¹³⁷Cs）和钴 - 60（⁶⁰Co）光子。测量在空气、聚甲基丙烯酸甲酯（PMMA）、wt1、聚苯乙烯和热释光剂量计系统（TLDS）作为周围材料的条件下进行。对于X射线，传递的总空气比释动能值在50至150 mGy之间，对于¹³⁷Cs和⁶⁰Co射线为50 mGy；每个剂量计单独进行辐照。相对响应R定义为每空气比释动能的TL响应，相对效率RE描述为每吸收剂量的TL响应（通过蒙特卡罗（MC）方法和解析方法获得），用于描述TL响应。R和RE均以⁶⁰Co射线中的响应进行归一化。结果表明，使用不同的模体材料会影响TL响应，并且这种响应随能量和材料类型而变化。MC模拟定性地再现了实验数据：a）R增加，在约25 keV处达到最大值然后减小；b）RE减小，在约60 keV处降至最小值，在约150 keV处增加到最大值然后又减小。与模体材料无关，RE强烈依赖于吸收剂量的评估方式，通过解析方法和MC模拟评估的RE之间的差异约为4%和18%，这取决于光子能量。我们的结果与文献报道结果的比较表明，不同研究组之间观察到的差异似乎最有可能与超线性效应、模体材料、每次实验期间能谱和几何条件的差异有关，而不是加热速率或退火程序等参数，这得到了MC模拟的支持。从本工作获得的结果以及所进行的严格分析中，我们可以得出结论，对于TLD - 100的临床应用，当使用已发表的数据将TL校准曲线从⁶⁰Co转换为低能光子时，必须特别注意。否则，当后来用于测量人体组织中的吸收剂量时，这可能会导致错误的结果。
