Department of Medical and Health Sciences, Linköping University, SE 581 85 Linköping, Sweden.
Med Phys. 2012 Feb;39(2):1133-40. doi: 10.1118/1.3675401.
Experimental radiation dosimetry with thermoluminescent dosimeters (TLDs), calibrated in a (60)Co or megavoltage (MV) photon beam, is recommended by AAPM TG-43U1for verification of Monte Carlo calculated absorbed doses around brachytherapy sources. However, it has been shown by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] that for TLDs of LiF:Mg,Ti, detector response was 4% higher in a (137)Cs beam than in a (60)Co one. The aim of this work was to investigate if similar over-response exists when measuring absorbed dose to water around (192)Ir sources, using LiF:Mg,Ti dosimeters calibrated in a 6 MV photon beam.
LiF dosimeters were calibrated to measure absorbed dose to water in a 6 MV photon beam and used to measure absorbed dose to water at distances of 3, 5, and 7 cm from a clinical high dose rate (HDR) (192)Ir source in a polymethylmethacrylate (PMMA) phantom. Measured values were compared to values of absorbed dose to water calculated using a treatment planning system (TPS) including corrections for the difference in energy absorption properties between calibration quality and the quality in the users' (192)Ir beam and for the use of a PMMA phantom instead of the water phantom underlying dose calculations in the TPS.
Measured absorbed doses to water around the (192)Ir source were overestimated by 5% compared to those calculated by the TPS. Corresponding absorbed doses to water measured in a previous work with lithium formate electron paramagnetic resonance (EPR) dosimeters by Antonovic et al. [Med. Phys. 36, 2236-2247 (2009)], using the same irradiation setup and calibration procedure as in this work, were 2% lower than those calculated by the TPS. The results obtained in the measurements in this work and those obtained using the EPR lithium formate dosimeters were, within the expanded (k = 2) uncertainty, in agreement with the values derived by the TPS. The discrepancy between the results using LiF:Mg,Ti TLDs and the EPR lithium formate dosimeters was, however, statistically significant and in agreement with the difference in relative detector responses found for the two detector systems by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] and by Adolfsson et al. [Med. Phys. 37, 4946-4959 (2010)].
When calibrated in (60)Co or MV photon beams, correction for the linear energy transfer (LET) dependence of LiF:Mg,Ti detector response will be needed as to measure absorbed doses to water in a (192)Ir beam with highest accuracy. Such corrections will depend on the manufacturing process (MTS-N Poland or Harshaw TLD-100) and details of the annealing and read-out schemes used.
AAPM TG-43U1 建议使用热释光剂量计（TLD）进行实验辐射剂量学测量，该剂量计在（60）Co 或兆伏（MV）光子束中进行校准，以验证近距离放射治疗源周围蒙特卡罗计算的吸收剂量。然而，Carlsson Tedgren 等人[Med. Phys. 38, 5539-5550（2011）]表明，对于 LiF:Mg，Ti TLD，在（137）Cs 束中的探测器响应比在（60）Co 束中高 4%。本工作的目的是研究当使用在 6 MV 光子束中校准的 LiF:Mg，Ti 剂量计测量（192）Ir 源周围水中的吸收剂量时，是否存在类似的过度响应，方法：LiF 剂量计在 6 MV 光子束中进行校准，以测量水中的吸收剂量，并用于测量在 PMMA 模体中距离临床高剂量率（HDR）（192）Ir 源 3、5 和 7 cm 处水中的吸收剂量。将测量值与使用治疗计划系统（TPS）计算的水中吸收剂量值进行比较，TPS 包括对校准质量与用户（192）Ir 束中质量之间能量吸收特性差异的校正，以及对在 TPS 中剂量计算中使用 PMMA 模体而不是水模体的校正。结果：与 TPS 计算的吸收剂量相比，（192）Ir 源周围测量的水中吸收剂量高估了 5%。Antonovic 等人[Med. Phys. 36, 2236-2247（2009）]在之前的工作中使用甲酸锂电子顺磁共振（EPR）剂量计测量的水中吸收剂量低 2%，使用相同的辐照设置和校准程序与本工作相同，比 TPS 计算的低 2%。本工作中的测量结果和使用 EPR 甲酸锂剂量计获得的结果，在扩展的（k=2）不确定度内，与 TPS 得出的值一致。然而，LiF:Mg，Ti TLD 与 EPR 甲酸锂剂量计之间的结果差异具有统计学意义，与 Carlsson Tedgren 等人[Med. Phys. 38, 5539-5550（2011）]和 Adolfsson 等人发现的两个探测器系统的相对探测器响应差异一致。[Med. Phys. 37, 4946-4959（2010）]。结论：当在（60）Co 或 MV 光子束中进行校准时，需要进行线性能量转移（LET）对 LiF:Mg，Ti 探测器响应的依赖性校正，以最高精度测量（192）Ir 束中的水中吸收剂量。这种校正将取决于制造工艺（MTS-N 波兰或 Harshaw TLD-100）以及使用的退火和读出方案的细节。
