Department of Medical and Health Sciences, Linköping University, Linköping, Sweden.
Med Phys. 2011 Oct;38(10):5539-50. doi: 10.1118/1.3633892.
High energy photon beams are used in calibrating dosimeters for use in brachytherapy since absorbed dose to water can be determined accurately and with traceability to primary standards in such beams, using calibrated ion chambers and standard dosimetry protocols. For use in brachytherapy, beam quality correction factors are needed, which include corrections for differences in mass energy absorption properties between water and detector as well as variations in detector response (intrinsic efficiency) with radiation quality, caused by variations in the density of ionization (linear energy transfer (LET) -distributions) along the secondary electron tracks. The aim of this work was to investigate experimentally the detector response of LiF:Mg,Ti thermoluminescent dosimeters (TLD) for photon energies below 1 MeV relative to (60)Co and to address discrepancies between the results found in recent publications of detector response.
LiF:Mg,Ti dosimeters of formulation MTS-N Poland were irradiated to known values of air kerma free-in-air in x-ray beams at tube voltages 25-250 kV, in (137)Cs- and (60)Co-beams at the Swedish Secondary Standards Dosimetry Laboratory. Conversions from air kerma free-in-air into values of mean absorbed dose in the dosimeters in the actual irradiation geometries were made using EGSnrc Monte Carlo simulations. X-ray energy spectra were measured or calculated for the actual beams. Detector response relative to that for (60)Co was determined at each beam quality.
An increase in relative response was seen for all beam qualities ranging from 8% at tube voltage 25 kV (effective energy 13 keV) to 3%-4% at 250 kV (122 keV effective energy) and (137)Cs with a minimum at 80 keV effective energy (tube voltage 180 kV). The variation with effective energy was similar to that reported by Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)] with our values being systematically lower by 2%-4%. Compared to the results by Nunn et al. [Med. Phys. 35, 1861-1869 (2008)], the relative detector response as a function of effective energy differed in both shape and magnitude. This could be explained by the higher maximum read-out temperature (350 °C) used by Nunn et al. [Med. Phys. 35, 1861-1869 (2008)], allowing light emitted from high-temperature peaks with a strong LET dependence to be registered. Use of TLD-100 by Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)] with a stronger super-linear dose response compared to MTS-N was identified as causing the lower relative detector response in this work.
Both careful dosimetry and strict protocols for handling the TLDs are required to reach solid experimental data on relative detector response. This work confirms older findings that an over-response relative to (60)Co exists for photon energies below 200-300 keV. Comparison with the results from the literature indicates that using similar protocols for annealing and read-out, dosimeters of different makes (TLD-100, MTS-N) differ in relative detector response. Though universality of the results has not been proven and further investigation is needed, it is anticipated that with the use of strict protocols for annealing and read-out, it will be possible to determine correction factors that can be used to reduce uncertainties in dose measurements around brachytherapy sources at photon energies where primary standards for absorbed dose to water are not available.
在用于腔内治疗的剂量计校准中使用高能光子束，因为可以使用校准的电离室和标准剂量学协议，在这些束中准确地确定水的吸收剂量，并具有与初级标准的可追溯性，使用校准的电离室和标准剂量学协议。对于腔内治疗，需要质量校正因子，包括由于电离密度（线性能量传递（LET）分布）沿次级电子轨迹的变化，水和探测器之间的质量能量吸收特性以及探测器响应（固有效率）随辐射质量的变化而引起的差异的校正。这项工作的目的是实验研究 LiF：Mg，Ti 热释光剂量计（TLD）在光子能量低于 1 MeV 时相对于（60）Co 的探测器响应，并解决最近出版物中发现的探测器响应差异。
在瑞典二级标准剂量学实验室，在 X 射线管电压为 25-250 kV、（137）Cs 和（60）Co 束中，将 MTS-N 波兰配方的 LiF：Mg，Ti 剂量计照射到已知的空气比释动能值。使用 EGSnrc 蒙特卡罗模拟将空气比释动能自由空气转换为实际照射几何中剂量计内的平均吸收剂量值。为实际光束测量或计算 X 射线能谱。在每个束质下，确定相对于（60）Co 的探测器响应。
在所有束质下，都观察到相对响应增加，从管电压 25 kV（有效能量 13 keV）的 8%到 250 kV（有效能量 122 keV）和（137）Cs 的 3%-4%，有效能量为 80 keV（管电压 180 kV）时达到最小值。与 Davis 等人报道的结果相似。[放射防护剂量学 106，33-43（2003 年）]，我们的数值系统低 2%-4%。与 Nunn 等人的结果相比。[医学物理 35，1861-1869（2008 年）]，有效能量的相对探测器响应在形状和幅度上都有所不同。这可以通过 Nunn 等人使用的更高最大读出温度（350°C）来解释。[医学物理 35，1861-1869（2008 年）]，允许与 LET 依赖性强的高温峰发出的光进行登记。在这项工作中，与 MTS-N 相比，Davis 等人使用的 TLD-100 具有更强的超线性剂量响应，被认为是导致相对探测器响应较低的原因。[放射防护剂量学 106，33-43（2003 年）]。
为了获得相对探测器响应的可靠实验数据，需要进行仔细的剂量学和严格的 TLD 处理协议。这项工作证实了旧的发现，即在 200-300 keV 以下的光子能量下，存在相对于（60）Co 的过度响应。与文献结果的比较表明，使用类似的退火和读出协议，不同制造商（TLD-100、MTS-N）的剂量计在相对探测器响应方面存在差异。虽然结果的普遍性尚未得到证明，需要进一步调查，但预计使用严格的退火和读出协议，将有可能确定可用于降低在水的吸收剂量的初级标准不可用的光子能量下腔内治疗源周围剂量测量的不确定性的校正因子。
