Poirier Yannick, Kuznetsova Svetlana, Villarreal-Barajas Jose Eduardo
Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, 21201, USA.
Department of Physics and Astronomy, University of Calgary, Calgary, AB, T2N 1N4, Canada.
Med Phys. 2018 Jan;45(1):402-413. doi: 10.1002/mp.12691. Epub 2017 Dec 22.
To investigate empirically the energy dependence of the detector response of two in vivo luminescence detectors, LiF:Mg,Cu,P (MCP-N) high-sensitivity TLDs and Al O :C OSLDs, in the 40-300-kVp energy range in the context of in vivo surface dose measurement. As these detectors become more prevalent in clinical and preclinical in vivo measurements, knowledge of the variation in the empirical dependence of the measured response of these detectors across a wide spectrum of beam qualities is important.
We characterized a large range of beam qualities of three different kilovoltage x-ray units: an Xstrahl 300 Orthovoltage unit, a Precision x-Ray X-RAD 320ix biological irradiator, and a Varian On-Board Imaging x-ray unit. The dose to water was measured in air according to the AAPM's Task Group 61 protocol. The OSLDs and TLDs were irradiated under reference conditions on the surface of a water phantom to provide full backscatter conditions. To assess the change in sensitivity in the long term, we separated the in vivo dosimeters of each type into an experimental and a reference group. The experimental dosimeters were irradiated using the kilovoltage x-ray units at each beam quality used in this investigation, while the reference group received a constant 10 cGy irradiation at 6 MV from a Varian clinical linear accelerator. The individual calibration of each detector was verified in cycles where both groups received a 10 cGy irradiation at 6 MV.
The nanoDot OSLDs were highly reproducible, with ±1.5% variation in response following >40 measurement cycles. The TLDs lost 20% of their signal sensitivity over the course of the study. The relative light output per unit dose to water of the MCP-N TLDs did not vary with beam quality for beam qualities with effective energies <50 keV (150 kVp/6 mm Al). At higher energies, they showed a reduced (~75-85%) light output per unit dose relative to 6 MV x rays. The nanoDot OSLDs exhibited a very strong (120-408%) dependency of the light output relative to 6 MV x rays. Variations up to 15% between different x-ray units with equivalent effective energies were also observed.
While convenient for clinical use, nanoDot OSLDs exhibit a strong variation in their measured light output per unit dose relative to 6 MV in the 40-300 kV x-ray range. This variability differs unit-to-unit, limiting their effective use for in vivo dosimetry applications in the kilovoltage x-ray energy range. MCP-N TLDs offer a much more stable response, but suffer from variations in sensitivity over time dependent on radiation history, which requires careful experimental handling.
在体内表面剂量测量的背景下，通过实验研究两种体内发光探测器——氟化锂镁铜磷（MCP-N）高灵敏度热释光剂量计（TLD）和铝酸锶碳光激励发光剂量计（OSLD）在40 - 300 kVp能量范围内探测器响应的能量依赖性。随着这些探测器在临床和临床前体内测量中越来越普遍，了解这些探测器在广泛的射束质量范围内测量响应的经验依赖性变化非常重要。
我们对三种不同千伏级X射线装置的大量射束质量进行了表征：一台Xstrahl 300千伏级X射线装置、一台Precision x-Ray X-RAD 320ix生物辐照仪和一台Varian机载成像X射线装置。根据美国医学物理师协会任务组61协议在空气中测量水的剂量。将OSLD和TLD在参考条件下照射水模表面以提供全反向散射条件。为了评估长期灵敏度的变化，我们将每种类型的体内剂量计分为实验组和参考组。使用本研究中每种射束质量的千伏级X射线装置照射实验组剂量计，而参考组则接受来自Varian临床直线加速器的6 MV下恒定的10 cGy照射。在两组均接受6 MV下10 cGy照射的周期中验证每个探测器的单独校准。
nanoDot OSLD具有高度可重复性，在超过40个测量周期后响应变化±1.5%。在研究过程中，TLD失去了约20%的信号灵敏度。对于有效能量<50 keV（150 kVp/6 mm铝）的射束质量，MCP-N TLD每单位水剂量的相对光输出不随射束质量变化。在更高能量下，相对于6 MV X射线，它们每单位剂量的光输出降低（75 - 85%）。nanoDot OSLD相对于6 MV X射线的光输出表现出非常强的（120 - 408%）依赖性。在具有等效有效能量的不同X射线装置之间也观察到高达15%的变化。
虽然nanoDot OSLD便于临床使用，但在千伏级X射线范围内，相对于6 MV，其每单位剂量测量的光输出表现出强烈变化。这种变异性因装置而异，限制了它们在千伏级X射线能量范围内用于体内剂量测定应用的有效性。MCP-N TLD提供了更稳定的响应，但灵敏度会随辐射历史随时间变化，这需要仔细的实验操作。
