Schäfer Mattias, Münter Marc, Sterzing Florian, Häring Peter, Rhein Bernhard, Debus Jürgen
Clinical Cooperation Unit Radiotherapy, German Cancer Research Center (DKFZ), Heidelberg, Germany.
Strahlenther Onkol. 2005 Sep;181(9):587-94. doi: 10.1007/s00066-005-1289-7.
Although intensity-modulated radiotherapy (IMRT) has already shown its clinical benefit, there are some issues which are not yet fully understood. Among these is the question whether the protracted dose delivery due to the lowered dose rate has any radiobiological consequences. To investigate this question, an exact characterization of dose rate profiles in typical clinical plans is needed. Furthermore, such a characterization may lead to an increased knowledge how to improve IMRT technically.
A new IMRT phantom which allows precise measurement of up to nine points of interest simultaneously with pin-point ionization chambers was developed. To examine dose rates, a new software tool (GRAYHOUND) was developed which can measure doses in short time intervals of up to 0.5 s. 250 points in four clinical IMRT plans were examined. A set of parameters was defined to describe the dose rate profiles including the effective fraction time (eft, which is the percentage of the fraction time in which any dose is delivered to a specific point), and a quotient of the percentage of dose delivered in high dose pulses (> 0.01 Gy/s) divided by the percentage of fraction time needed to deliver this dose (d(HD)/t(HD)).
These quotients are excellent markers for the inhomogeneity of dose rate delivery in IMRT. In both parameters a wide variance in points of the same plan and between different plans was found. For example, eft ranged between 11.6% and 37.3% in high dose points and the time in which high dose rates are delivered to a single high dose point ranged between 3.6% and 10.1% of total fraction time.
These data show a great inhomogeneity of dose rates not only between different plans but also between different points in the same plan. Biological investigations are needed to quantify the relevance of these inhomogeneities. The parameters which are introduced in this work may be suitable to compare different optimization algorithms in IMRT.
尽管调强放射治疗（IMRT）已显示出其临床益处，但仍存在一些尚未完全理解的问题。其中之一是由于剂量率降低导致的延长剂量输送是否具有任何放射生物学后果。为了研究这个问题，需要对典型临床计划中的剂量率分布进行精确表征。此外，这种表征可能会增加我们对如何从技术上改进IMRT的了解。
开发了一种新的IMRT体模，它允许使用针点电离室同时精确测量多达九个感兴趣点的剂量率。为了检查剂量率，开发了一种新的软件工具（GRAYHOUND），它可以在长达0.5秒的短时间间隔内测量剂量。对四个临床IMRT计划中的250个点进行了检查。定义了一组参数来描述剂量率分布，包括有效分次时间（eft，即向特定点输送任何剂量的分次时间百分比），以及高剂量脉冲（>0.01 Gy/s）中输送的剂量百分比除以输送该剂量所需的分次时间百分比的商（d(HD)/t(HD)）。
这些商是IMRT中剂量率输送不均匀性的优秀指标。在这两个参数中，同一计划的不同点之间以及不同计划之间都发现了很大的差异。例如，高剂量点的eft在11.6%至37.3%之间，向单个高剂量点输送高剂量率的时间占总分次时间的3.6%至10.1%。
这些数据表明，不仅不同计划之间，而且同一计划的不同点之间，剂量率都存在很大的不均匀性。需要进行生物学研究来量化这些不均匀性的相关性。本文引入的参数可能适用于比较IMRT中的不同优化算法。
