The University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA.
Med Phys. 2011 May;38(5):2619-28. doi: 10.1118/1.3575419.
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.
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.
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.
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.
放射治疗束吸收剂量的测量是提供安全且可重复治疗的重要组成部分。对于像热释光剂量计（TLD）这样的能量依赖性剂量计，通常假定在整个治疗场中能谱是恒定的，不受场大小、深度、场调制或不均匀性的干扰。然而，这并不反映实际情况，并会给临床剂量测量带来误差。本研究的目的是评估瓦里安 6 MV 射束的能谱变化，并评估光子能谱变化对常见能量依赖性剂量计 TLD 响应的影响。
使用蒙特卡罗方法，我们计算了治疗参数变化（包括场大小、测量位置、存在不均匀性和场调制）对 6 MV 射束光子能谱的变化。TLD 响应的这种光谱变化的影响主要基于剂量计中光电效应的增加，并且使用 Burlin 腔理论计算了这种影响。还进行了能量响应的测量，以确定由于所有固有和次级效应引起的额外能量响应。
对于大多数场内测量，无论治疗参数如何，剂量计响应都不会受到光谱变化的显著影响（<1%的影响）。对于治疗场之外的测量点，由于 TLD 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，光谱较软，TLD 的响应会过度增加 12%。
在治疗场内的位置，通常可以忽略光子光谱变化对测量剂量的影响。然而，在治疗场之外，光谱要软得多，通常需要校正因子。这项工作的结果确定了该因子的值，具体取决于特定的照射条件，范围从 0.88 到 0.99。
