Department of Molecular and Clinical Cancer Medicine, Institute of Translational Medicine, University of Liverpool, The Sherrington Building, Ashton Street, Liverpool L69 3BX, United Kingdom. Department of Physics, Clatterbridge Cancer Centre, Clatterbridge Road, Wirral CH63 4JY, United Kingdom. Department of Physics, University of Liverpool, Oliver Lodge Laboratory, Oxford Street, Liverpool L69 7ZE, United Kingdom.
Phys Med Biol. 2020 Aug 10;65(15):155011. doi: 10.1088/1361-6560/ab91d9.
In small megavoltage photon fields, the accuracies of an unmodified PTW 60017-type diode dosimeter and six diodes modified by adding airgaps of thickness 0.6-1.6 mm and diameter 3.6 mm have been comprehensively characterized experimentally and computationally. The optimally thick airgap for density compensation was determined, and detectors were micro-CT imaged to investigate differences between experimentally measured radiation responses and those predicted computationally.
Detectors were tested on- and off-axis, at 5 and 15 cm depths in 6 and 15 MV fields ≥ 0.5 × 0.5 cm. Computational studies were carried out using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Experimentally, radiation was delivered using a Varian TrueBeam linac and doses absorbed by water were measured using Gafchromic EBT3 film and ionization chambers, and compared with diode readings. Detector response was characterized via the [Formula: see text] formalism, choosing a 4 × 4 cm reference field.
For the unmodified 60017 diode, the maximum error in small field doses obtained from diode readings uncorrected by [Formula: see text] factors was determined as 11.9% computationally at +0.25 mm off-axis and 5 cm depth in a 15 MV 0.5 × 0.5 cm field, and 11.7% experimentally at -0.30 mm off-axis and 5 cm depth in the same field. A detector modified to include a 1.6 mm thick airgap performed best, with maximum computationally and experimentally determined errors of 2.2% and 4.1%. The 1.6 mm airgap deepened the modified dosimeter's effective point of measurement by 0.5 mm. For some detectors significant differences existed between responses in small fields determined computationally and experimentally, micro-CT imaging indicating that these differences were due to within-tolerance variations in the thickness of an epoxy resin layer.
The dosimetric performance of a 60017 diode detector was comprehensively improved throughout 6 and 15 MV small photon fields via density compensation. For this approach to work well with good detector-to-detector reproducibility, tolerances on dense component dimensions should be reduced to limit associated variations of response in small fields, or these components should be modified to have more water-like densities.
在小兆伏光子场中,对未经修改的 PTW 60017 型二极管剂量计和六个通过添加厚度为 0.6-1.6 毫米和直径为 3.6 毫米的气隙进行了全面的实验和计算特性描述。确定了用于密度补偿的最佳气隙厚度,并对探测器进行了微计算机断层扫描成像,以研究实验测量的辐射响应与计算预测的辐射响应之间的差异。
在 6 和 15 MV 场≥0.5×0.5 cm 中,在 5 和 15 cm 深度处,对轴上和轴外的探测器进行了测试。使用 EGSnrc/BEAMnrc 蒙特卡罗辐射传输代码进行了计算研究。在实验中,使用瓦里安 TrueBeam 直线加速器输送辐射,用水吸收的剂量使用 Gafchromic EBT3 胶片和电离室进行测量,并与二极管读数进行比较。通过[公式:见正文]形式化方法来描述探测器的响应,选择 4×4 cm 的参考场。
对于未经修正的 60017 二极管,在 15 MV 0.5×0.5 cm 场中,在+0.25 mm 轴外和 5 cm 深度处,通过[公式:见正文]因子未校正的二极管读数获得的小场剂量的最大误差,在计算上确定为 11.9%,在实验上确定为-0.30 mm 轴外和 5 cm 深度处相同的字段。包含 1.6 毫米厚气隙的探测器性能最佳,计算和实验确定的最大误差分别为 2.2%和 4.1%。1.6 毫米气隙将修改后的剂量计的有效测量点加深了 0.5 毫米。对于一些探测器,在小场中通过计算和实验确定的响应之间存在显著差异,微计算机断层扫描成像表明,这些差异是由于树脂层厚度的公差内变化所致。
通过密度补偿,60017 型二极管探测器在整个 6 和 15 MV 小光子场中的剂量性能得到了全面提高。为了使这种方法具有良好的探测器间可重复性,应减小致密组件尺寸的公差,以限制小场中响应的相关变化,或者应修改这些组件以使其具有更类似于水的密度。