Helt-Hansen Jakob, Rosendal Flemming, Kofoed Inger Matilde, Andersen Claus Erik
Risø-DTU, Risø High Dose Reference Laboratory, Technical University of Denmark, Roskilde, Denmark.
Acta Oncol. 2009;48(2):216-22. doi: 10.1080/02841860802279725.
Electron spin resonance (EPR) is used to determine the absorbed dose of alanine dosimeters exposed to clinical photon beams in a solid-water phantom. Alanine is potentially suitable for medical reference dosimetry, because of its near water equivalence over a wide energy spectrum, low signal fading, non-destructive measurement and small dosimeter size.
A Bruker EMX-micro EPR spectrometer with a rectangular cavity and a measurement time of two minutes per dosimeter was used for reading of irradiated alanine dosimeters. Under these conditions a new algorithm based on scaling of known spectra was developed to extract the alanine signal.
The dose accuracy, including calibration uncertainty, is less than 2% (k=1) above 4 Gy (n=4). The measurement uncertainty is fairly constant in absolute terms (approximately 30 mGy) and the relative uncertainty therefore rises for dose measurements below 4 Gy. Typical reproducibility is <1% (k=1) above 10 Gy and <2% between 4 and 10 Gy. Below 4 Gy the uncertainty is higher. A depth dose curve measurement was performed in a solid-water phantom irradiated to a dose of 20 Gy at the maximum dose point (d(max)) in 6 and 18 MV photon beams. The typical difference between the dose measured with alanine in solid water and the dose measured with an ion chamber in a water tank was about 1%. A difference of 2% between 6 and 18 MV was found, possibly due to non-water equivalence of the applied phantom.
Compared to previously published methods the proposed algorithm can be applied without normalisation of phase shifts caused by changes in the g-value of the cavity. The study shows that alanine dosimetry is a suitable candidate for medical reference dosimetry especially for quality control applications.
电子自旋共振（EPR）用于确定在固体水模体中暴露于临床光子束的丙氨酸剂量计的吸收剂量。丙氨酸因其在宽能谱范围内接近水等效性、低信号衰减、无损测量和剂量计尺寸小等特点，有可能适用于医学参考剂量测定。
使用配备矩形腔且每个剂量计测量时间为两分钟的布鲁克EMX-微型EPR光谱仪读取辐照后的丙氨酸剂量计。在这些条件下，开发了一种基于已知光谱缩放的新算法来提取丙氨酸信号。
包括校准不确定度在内的剂量准确度在4 Gy以上时小于2%（k = 1）（n = 4）。测量不确定度在绝对值上相当恒定（约30 mGy），因此对于低于4 Gy的剂量测量，相对不确定度会升高。典型的重复性在10 Gy以上时<1%（k = 1），在4至10 Gy之间时<2%。低于4 Gy时，不确定度更高。在6和18 MV光子束中，在最大剂量点（d(max)）处辐照至20 Gy的固体水模体中进行了深度剂量曲线测量。在固体水中用丙氨酸测量的剂量与在水箱中用电离室测量的剂量之间的典型差异约为1%。发现6和18 MV之间存在2%的差异，这可能是由于所用模体的非水等效性所致。
与先前发表的方法相比，所提出的算法无需对由腔的g值变化引起的相移进行归一化即可应用。该研究表明，丙氨酸剂量测定是医学参考剂量测定的合适候选方法，尤其适用于质量控制应用。
