Antonovic Laura, Gustafsson Håkan, Carlsson Gudrun Alm, Carlsson Tedgren Asa
Department of Medical and Health Sciences, Radiation Physics, Faculty of Health Sciences, Linköping University, SE-581 85 Linköping, Sweden.
Med Phys. 2009 Jun;36(6):2236-47. doi: 10.1118/1.3110068.
A dosimetry system using lithium formate monohydrate (HCO2Li x H2O) as detector material and electron paramagnetic resonance (EPR) spectroscopy for readout has been used to measure absorbed dose distributions around clinical 192Ir sources. Cylindrical tablets with diameter of 4.5 mm, height of 4.8 mm, and density of 1.26 g/cm3 were manufactured. Homogeneity test and calibration of the dosimeters were performed in a 6 MV photon beam. 192Ir irradiations were performed in a PMMA phantom using two different source models, the GammaMed Plus HDR and the microSelectron PDR-v1 model. Measured absorbed doses to water in the PMMA phantom were converted to the corresponding absorbed doses to water in water phantoms of dimensions used by the treatment planning systems (TPSs) using correction factors explicitly derived for this experiment. Experimentally determined absorbed doses agreed with the absorbed doses to water calculated by the TPS to within +/-2.9%. Relative standard uncertainties in the experimentally determined absorbed doses were estimated to be within the range of 1.7%-1.3% depending on the radial distance from the source, the type of source (HDR or PDR), and the particular absorbed doses used. This work shows that a lithium formate dosimetry system is well suited for measurements of absorbed dose to water around clinical HDR and PDR 192Ir sources. Being less energy dependent than the commonly used thermoluminescent lithium fluoride (LiF) dosimeters, lithium formate monohydrate dosimeters are well suited to measure absorbed doses in situations where the energy dependence cannot easily be accounted for such as in multiple-source irradiations to verify treatment plans. Their wide dynamic range and linear dose response over the dose interval of 0.2-1000 Gy make them suitable for measurements on sources of the strengths used in clinical applications. The dosimeter size needs, however, to be reduced for application to single-source dosimetry.
一种使用一水甲酸锂（HCO2Li·H2O）作为探测器材料并通过电子顺磁共振（EPR）光谱进行读数的剂量测定系统，已被用于测量临床192Ir源周围的吸收剂量分布。制造了直径为4.5毫米、高度为4.8毫米、密度为1.26克/立方厘米的圆柱形片剂。剂量计的均匀性测试和校准在6兆伏光子束中进行。使用两种不同的源模型，即GammaMed Plus HDR和microSelectron PDR-v1模型，在聚甲基丙烯酸甲酯（PMMA）体模中进行192Ir照射。使用为此实验明确推导的校正因子，将PMMA体模中测得的水吸收剂量转换为治疗计划系统（TPS）所使用尺寸的水体模中的相应水吸收剂量。实验确定的吸收剂量与TPS计算的水吸收剂量在±2.9%范围内一致。根据离源的径向距离、源的类型（高剂量率或脉冲剂量率）以及所使用的特定吸收剂量，实验确定的吸收剂量的相对标准不确定度估计在1.7% - 1.3%范围内。这项工作表明，甲酸锂剂量测定系统非常适合测量临床高剂量率和脉冲剂量率192Ir源周围的水吸收剂量。与常用的热释光氟化锂（LiF）剂量计相比，一水甲酸锂剂量计对能量的依赖性较小，非常适合在能量依赖性难以轻易考虑的情况下测量吸收剂量，例如在多源照射以验证治疗计划时。它们在0.2 - 1000戈瑞的剂量区间内具有宽动态范围和线性剂量响应，使其适用于临床应用中所用强度源的测量。然而，为了应用于单源剂量测定，剂量计的尺寸需要减小。
