Variations in photon energy spectra of a 6 MV beam and their impact on TLD response
Variations in photon energy spectra of a 6 MV beam and their impact on TLD response
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DOI：
10.1118/1.3575419
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发表时间：
2011-05-01
期刊：
影响因子：
3.8
通讯作者：
Kry, Stephen F.
中科院分区：
文献类型：
--
作者：
Scarboro, Sarah B.;Followill, David S.;Kry, Stephen F.
目的：测量放射治疗射束的吸收剂量是提供安全和可重复治疗的重要组成部分。对于热释光剂量计（TLD）等能量依赖型剂量计，通常假设能谱在整个治疗野中是恒定的，并且不受野大小、深度、场调制或不均匀性的干扰。然而，这并不反映现实，并将误差引入临床剂量测量。本研究的目的是评估Varian 6 MV射束的能谱的可变性，并评估光子能谱的这些变化对普通能量依赖剂量计的响应的影响。方法：利用蒙特卡罗方法，我们计算了6 MV光束的光子能谱随治疗参数变化的变化，包括射野大小、测量位置、不均匀性和场调制的存在。这些光谱变化对剂量计响应的影响主要是基于剂量计中光电效应的增加，并且使用伯林腔理论计算了这种影响。能量响应的测量也作出了确定的额外的能量响应，由于所有的内在和二次effects.Results：对于大多数现场测量，无论治疗参数，剂量计响应的光谱变化（< 1%的效果）没有显着影响。对于治疗野外的测量点，光谱较软，由于在光子晶体材料中光电效应的概率增加以及在低光子能量下起作用的固有电离密度效应，光子晶体的过度响应高达12%。通常可以忽略光子光谱变化对治疗野内位置的测量剂量的影响。然而，在治疗野之外，光谱要柔和得多，并且校正因子通常是适当的。这项工作的结果已经确定了该因子的值，其范围从0.88到0.99取决于特定的照射条件。(C)2011年美国医学物理学家协会。[DOI：10.1118/1.3575419]
Purpose: Measurement of the absorbed dose from radiotherapy beams is an essential component of providing safe and reproducible treatment. For an energy-dependent dosimeter such as thermoluminescent dosimeters (TLDs), it is generally assumed that the energy spectrum is constant throughout the treatment field and is unperturbed by field size, depth, field modulation, or heterogeneities. However, this does not reflect reality and introduces error into clinical dose measurements. The purpose of this study was to evaluate the variability in the energy spectrum of a Varian 6 MV beam and to evaluate the impact of these variations in photon energy spectra on the response of a common energy-dependent dosimeter, TLD.Methods: Using Monte Carlo methods, we calculated variations in the photon energy spectra of a 6 MV beam as a result of variations of treatment parameters, including field size, measurement location, the presence of heterogeneities, and field modulation. The impact of these spectral variations on the response of the TLD is largely based on increased photoelectric effect in the dosimeter, and this impact was calculated using Burlin cavity theory. Measurements of the energy response were also made to determine the additional energy response due to all intrinsic and secondary effects.Results: For most in-field measurements, regardless of treatment parameter, the dosimeter response was not significantly affected by the spectral variations (< 1% effect). For measurement points outside of the treatment field, where the spectrum is softer, the TLD over-responded by up to 12% due to an increased probability of photoelectric effect in the TLD material as well as inherent ionization density effects that play a role at low photon energies.Conclusions: It is generally acceptable to ignore the impact of variations in the photon spectrum on the measured dose for locations within the treatment field. However, outside the treatment field, the spectra are much softer, and a correction factor is generally appropriate. The results of this work have determined values for this factor, which range from 0.88 to 0.99 depending on the specific irradiation conditions. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3575419]