Gamal I El, Cojocaru C, Ross C, Marchington D, McEwen M
Ionizing Radiation Standards Group, National Research Council, Ottawa.
Carleton University.
Med Phys. 2012 Jul;39(7Part4):4645. doi: 10.1118/1.4740210.
By measuring the dose to water directly a metrology standard, independent of air kerma, can be developed to make the basis of HDR brachytherapy dosimetry consistent with current dosimetry methods for external radiation beams. The Fricke dosimeter system, a liquid chemical dosimeter, provides a means of measuring the absorbed dose rate to water directly by measuring the radiation-induced change in absorption of the Fricke solution. In an attempt to measure the absorbed dose to water directly for a Ir HDR brachytherapy source a ring shaped Fricke holder was constructed from PMMA, essentially following the work of Austerlitz et al. (Med. Phys. 2008). Benchmark measurements conducted in a Co beam yielded a standard uncertainty in the absorption reading of 0.16 %, comparable with previous results in the literature. Measurements of the standard uncertainty of the control (unirradiated) solution using the holder yielded 0.2 %, indicating good process control and minimal contamination from the holder itself. However, it was found that the holder sealing method (to allow measurements in a water phantom) significantly contaminated the Fricke solution, resulting in an excessive background reading. Irradiations were therefore conducted in air to determine the feasibility of the procedure. Irradiations with a 17 GBq source gave a standard uncertainty of approximately 0.5 %, indicating that the target uncertainty of 1.5% for the measurement of absorbed dose to water using a Fricke-based primary standard is achievable. This would be comparable with calorimeter-based systems currently being developed.
通过直接测量水的剂量，可以开发一种独立于空气比释动能的计量标准，以使高剂量率近距离放射治疗剂量测定的基础与当前外照射束的剂量测定方法保持一致。弗里克剂量计系统是一种液体化学剂量计，它通过测量弗里克溶液吸收的辐射诱导变化，提供了一种直接测量水吸收剂量率的方法。为了直接测量铱高剂量率近距离放射治疗源对水的吸收剂量，基本上按照奥斯特利茨等人（《医学物理》，2008年）的工作，用聚甲基丙烯酸甲酯制作了一个环形弗里克支架。在钴束中进行的基准测量得出吸收读数的标准不确定度为0.16%，与文献中先前的结果相当。使用该支架对对照（未辐照）溶液的标准不确定度测量结果为0.2%，表明过程控制良好且支架本身的污染最小。然而，发现支架密封方法（以便在水模体中进行测量）严重污染了弗里克溶液，导致背景读数过高。因此在空气中进行辐照以确定该程序的可行性。用17吉贝可的源进行辐照得到的标准不确定度约为0.5%，这表明使用基于弗里克的一级标准测量水吸收剂量时目标不确定度为1.5%是可以实现的。这将与目前正在开发的基于量热计的系统相当。
