Institut für Medizinische Physik und Strahlenschutz, IMPS University of Applied Sciences, Giessen, Germany.
Phys Med Biol. 2013 Apr 21;58(8):2431-44. doi: 10.1088/0031-9155/58/8/2431. Epub 2013 Mar 21.
The application of small photon fields in modern radiotherapy requires the determination of total scatter factors Scp or field factors Ω(f(clin), f(msr))(Q(clin), Q(msr)) with high precision. Both quantities require the knowledge of the field-size-dependent and detector-dependent correction factor k(f(clin), f(msr))(Q(clin), Q(msr)). The aim of this study is the determination of the correction factor k(f(clin), f(msr))(Q(clin), Q(msr)) for different types of detectors in a clinical 6 MV photon beam of a Siemens KD linear accelerator. The EGSnrc Monte Carlo code was used to calculate the dose to water and the dose to different detectors to determine the field factor as well as the mentioned correction factor for different small square field sizes. Besides this, the mean water to air stopping power ratio as well as the ratio of the mean energy absorption coefficients for the relevant materials was calculated for different small field sizes. As the beam source, a Monte Carlo based model of a Siemens KD linear accelerator was used. The results show that in the case of ionization chambers the detector volume has the largest impact on the correction factor k(f(clin), f(msr))(Q(clin), Q(msr)); this perturbation may contribute up to 50% to the correction factor. Field-dependent changes in stopping-power ratios are negligible. The magnitude of k(f(clin), f(msr))(Q(clin), Q(msr)) is of the order of 1.2 at a field size of 1 × 1 cm(2) for the large volume ion chamber PTW31010 and is still in the range of 1.05-1.07 for the PinPoint chambers PTW31014 and PTW31016. For the diode detectors included in this study (PTW60016, PTW 60017), the correction factor deviates no more than 2% from unity in field sizes between 10 × 10 and 1 × 1 cm(2), but below this field size there is a steep decrease of k(f(clin), f(msr))(Q(clin), Q(msr)) below unity, i.e. a strong overestimation of dose. Besides the field size and detector dependence, the results reveal a clear dependence of the correction factor on the accelerator geometry for field sizes below 1 × 1 cm(2), i.e. on the beam spot size of the primary electrons hitting the target. This effect is especially pronounced for the ionization chambers. In conclusion, comparing all detectors, the unshielded diode PTW60017 is highly recommended for small field dosimetry, since its correction factor k(f(clin), f(msr))(Q(clin), Q(msr)) is closest to unity in small fields and mainly independent of the electron beam spot size.
在现代放射治疗中，小光子场的应用需要高精度地确定总散射因子 Scp 或场因子 Ω(f(clin), f(msr))(Q(clin), Q(msr))。这两个量都需要知道与射野大小相关和与探测器相关的校正因子 k(f(clin), f(msr))(Q(clin), Q(msr))。本研究的目的是确定不同类型探测器在西门子 KD 线性加速器临床 6 MV 光子束中的校正因子 k(f(clin), f(msr))(Q(clin), Q(msr))。使用 EGSnrc 蒙特卡罗代码计算水剂量和不同探测器的剂量，以确定场因子以及不同小方形射野大小的所述校正因子。此外，还计算了不同小射野大小下的水与空气平均阻止本领比以及相关材料平均能量吸收系数比。作为束源，使用了基于蒙特卡罗的西门子 KD 线性加速器模型。结果表明，在电离室的情况下，探测器体积对校正因子 k(f(clin), f(msr))(Q(clin), Q(msr))的影响最大；这种干扰可能会对校正因子贡献高达 50%。与射野相关的阻止本领比的变化可以忽略不计。对于大体积电离室 PTW31010，在 1×1cm²的射野大小下，k(f(clin), f(msr))(Q(clin), Q(msr))的值约为 1.2，而对于 PinPoint 电离室 PTW31014 和 PTW31016，其值仍在 1.05-1.07 范围内。对于本研究中包括的二极管探测器（PTW60016、PTW60017），在 10×10 至 1×1cm²的射野大小范围内，校正因子偏离 1 的偏差不超过 2%，但在该射野大小以下，k(f(clin), f(msr))(Q(clin), Q(msr))急剧低于 1，即剂量存在严重高估。除了射野大小和探测器依赖性之外，结果还揭示了校正因子对低于 1×1cm²的射野大小下的加速器几何形状的明显依赖性，即对击中靶的初级电子的束斑大小的依赖性。这种效应在电离室中尤为明显。总之，与所有探测器相比，未屏蔽的二极管 PTW60017 非常适合小射野剂量测量，因为其在小射野中校正因子 k(f(clin), f(msr))(Q(clin), Q(msr))最接近 1，并且主要与电子束斑大小无关。
