Working Group Medical Radiation Physics, Carl von Ossietzky University Oldenburg, Germany.
Z Med Phys. 2012 Sep;22(3):181-96. doi: 10.1016/j.zemedi.2012.05.001. Epub 2012 May 31.
According to accepted dosimetry protocols, the "radiation quality correction factor"k(Q) accounts for the energy-dependent changes of detector responses under the conditions of clinical dosimetry for high-energy photon radiations. More precisely, a factor k(QR) is valid under reference conditions, i.e. at a point on the beam axis at depth 10 cm in a large water phantom, for 10×10 cm(2) field size, SSD 100 cm and the given radiation quality with quality index Q. Therefore, a further correction factor k(NR) has been introduced to correct for the influences of spectral quality changes when detectors are used under non-reference conditions such as other depths, field sizes and off-axis distances, while under reference conditions k(NR) is normalized to unity. In this paper, values of k(NR) are calculated for 6 and 15 MV photon beams, using published data of the energy-dependent responses of various radiation detectors to monoenergetic photon radiations, and weighting these responses with validated photon spectra of clinical high-energy photon beams from own Monte-Carlo-calculations for a wide variation of the non-reference conditions within a large water phantom. Our results confirm the observation by Scarboro et al. [26] that k(NR) can be represented by a unique function of the mean energy Em, weighted by the spectral photon fluence. Accordingly, the numerical variations of Em with depth, field size and off-axis distance have been provided. Throughout all considered conditions, the deviations of the k(NR) values from unity are at most 2% for a Farmer type ion chamber, and they remain below 15% for the thermoluminescent detectors LiF:Mg,Ti and LiF:Mg,Cu,P. For the shielded diode EDP-10, k(NR) varies from unity up to 20%, while the unshielded diode EDD-5 shows deviations up to 60% in the peripheral region. Thereby, the restricted application field of unshielded diodes has been clarified. For small field dosimetry purposes k(NR) can be converted into k(NCSF), the non-calibration condition correction factor normalized to unity for a 4×4 cm(2) calibration field. For the unshielded Si diodes needed in small-field dosimetry, the values of k(NCSF) are closer to unity than the associated k(NR) values.
根据公认的剂量学协议，“辐射质量校正因子”k(Q) 用于在高能光子辐射的临床剂量学条件下，对探测器响应的能量依赖性变化进行校正。更准确地说，在参考条件下有效因子 k(QR)，即在大水箱中的射束轴上 10cm 深度处、10×10cm(2)射野大小、SSD 100cm 和给定射线质 Q 的情况下。因此，引入了另一个校正因子 k(NR)，以校正当探测器在非参考条件下（例如其他深度、射野大小和离轴距离）使用时光谱质量变化的影响，而在参考条件下 k(NR)归一化为 1。本文使用各种辐射探测器对单能光子辐射的能量依赖性响应的已发表数据，计算了 6MV 和 15MV 光子束的 k(NR)值，并使用我们自己的 Monte-Carlo 计算的临床高能光子束的验证光子谱对这些响应进行加权，以大水箱内各种非参考条件下的广泛变化。我们的结果证实了 Scarboro 等人的观察结果。[26]，k(NR)可以由加权光谱光子通量的平均能量 Em 的唯一函数表示。因此，提供了 Em 随深度、射野大小和离轴距离的数值变化。在所有考虑的条件下，对于 Farmer 型电离室，k(NR)值与 1 的偏差最大为 2%，对于 LiF:Mg、Ti 和 LiF:Mg、Cu、P 型热释光探测器，偏差小于 15%。对于屏蔽二极管 EDP-10，k(NR)从 1 变化到 20%，而未屏蔽二极管 EDD-5 在周边区域的偏差高达 60%。从而阐明了未屏蔽二极管的应用领域受限。对于小射野剂量学目的，k(NR)可以转换为 k(NCSF)，即归一化为 1 的非校准条件校正因子对于 4×4cm(2)校准射野。对于小射野剂量学所需的未屏蔽 Si 二极管，k(NCSF)的值比相关的 k(NR)值更接近 1。
