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首页> 外文期刊>Radiotherapy and oncology: Journal of the European Society for Therapeutic Radiology and Oncology >Head scatter factors for small MV photon fields. Part II: The effects of source size and detector.
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Head scatter factors for small MV photon fields. Part II: The effects of source size and detector.

机译：小MV光子场的磁头散射因子。第二部分：光源尺寸和探测器的影响。

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BACKGROUND AND PURPOSE: Small field dosimetry continues to be problematic for the planning and delivery of both circular stereotactic radiotherapy (SRT) fields and intensity modulated radiotherapy (IMRT) fields. Although the separation of head and phantom scatter is primarily of interest in IMRT fields, measurements can often been difficult. However, the set-up in fields formed by stereotactic collimators is more precise and total scatter and head scatter factors can be more easily measured. Phantom scatter factors calculated from these measurements can be extrapolated from the stereotactic situation to the IMRT one. However, the problem of measuring small field head scatter factors must be overcome first. In this work, four solid-state detectors (photon, electron, stereotactic diodes and a diamond) were examined in the measurement of small (0.5-4cm width) field S(c), to determine whether differences in S(c) values were due to real differences in scatter and source size, or due to apparent differences caused by the detector and experimental set-up. METHODS AND MATERIALS: Measurements were carried out on the 6MV beams of three different linacs (Varian 600CD, BBC CH6 and CH20), in open fields produced by the movable linac collimators and in fields produced by stereotactic collimators and the MLC. A small plastic top, equal in thickness to build-up and 5cm deep and equal in diameter to that of each detector, was used in all experiments. In-air profiles were measured for each measurement situation and these were used to remove the effects of volume averaging in each detector. RESULTS: Real changes in S(c) were found to be due to changes in the amount of scatter produced in the linac head and in the air, with particular reference to the amount of the opening at the top of the primary collimator 'seen' from the point of measurement. Apparent changes were found to be caused by the experimental set-up, in particular the positional accuracy, the detector size (sensitive volume and size of outer casing) and the width of phantom used. CONCLUSIONS: Each of the four solid-state detectors investigated was found to be suitable for measurements in fields 1cm width, when used with the build-up top and when volume averaging is accounted for. However, phantom diameter is a problem in fields <1cm width. It is not experimentally accurate to measure S(c) in fields <1cm width with any of the above detectors, without further modification of experimental technique.
机译：背景与目的：小剂量剂量学对于圆形立体定向放射治疗（SRT）场和强度调制放射治疗（IMRT）场的规划和交付仍然存在问题。尽管头部和幻影散布的分离是IMRT领域中的主要关注点，但测量通常很困难。但是，由立体定向准直器形成的场的设置更为精确，总散射和头部散射因子也可以更容易地测量。从这些测量结果计算出的幻影散射因子可以从立体定向情况推断到IMRT。但是，必须首先克服测量小场头散射因子的问题。在这项工作中，在小（0.5-4cm宽）场S（c）的测量中检查了四个固态检测器（光子，电子，立体定向二极管和钻石），以确定S（c）值是否存在差异归因于散射和源尺寸的实际差异，或者归因于检测器和实验装置的明显差异。方法和材料：在三种直线加速器（Varian 600CD，BBC CH6和CH20）的6MV光束，可移动直线加速器准直仪产生的空旷场以及立体定向准直器和MLC产生的场中进行了测量。在所有实验中都使用一个小的塑料顶部，其厚度等于堆积物，深度为5cm，直径等于每个探测器的直径。针对每种测量情况测量了空气分布，并使用这些分布来消除每个检测器中体积平均的影响。结果：发现S（c）的实际变化是由于直线加速器头部和空气中产生的散射量的变化所致，特别是参考了“准直”主准直仪顶部的开口量从测量的角度来看。发现表面变化是由实验装置引起的，特别是位置精度，检测器尺寸（灵敏体积和外壳尺寸）以及所用幻像的宽度。结论：研究的四个固态检测器中的每一个都适合在1cm宽的区域中进行测量，当与组合顶部一起使用时，或考虑到体积平均时。但是，幻影直径是宽度小于1厘米的区域中的问题。在没有进一步修改实验技术的情况下，使用任何上述检测器在小于1cm宽度的场中测量S（c）的实验精度都不高。

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