School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, Qld 4001, Australia.
Phys Med Biol. 2013 Jul 7;58(13):4501-12. doi: 10.1088/0031-9155/58/13/4501. Epub 2013 Jun 13.
Due to their small collecting volume, diodes are commonly used in small field dosimetry. However, the relative sensitivity of a diode increases with decreasing small field size. Conversely, small air gaps have been shown to cause a significant decrease in the sensitivity of a detector as the field size is decreased. Therefore, this study uses Monte Carlo simulations to look at introducing air upstream to diodes such that they measure with a constant sensitivity across all field sizes in small field dosimetry. Varying thicknesses of air were introduced onto the upstream end of two commercial diodes (PTW 60016 photon diode and PTW 60017 electron diode), as well as a theoretical unenclosed silicon chip using field sizes as small as 5 mm × 5 mm. The metric D(w,Q)/D(Det,Q) used in this study represents the ratio of the dose to a point of water to the dose to the diode active volume, for a particular field size and location. The optimal thickness of air required to provide a constant sensitivity across all small field sizes was found by plotting D(w,Q)/D(Det,Q) as a function of introduced air gap size for various field sizes, and finding the intersection point of these plots. That is, the point at which D(w,Q)/D(Det,Q) was constant for all field sizes was found. The optimal thickness of air was calculated to be 3.3, 1.15 and 0.10 mm for the photon diode, electron diode and unenclosed silicon chip, respectively. The variation in these results was due to the different design of each detector. When calculated with the new diode design incorporating the upstream air gap, k(f(clin),f(msr))(Q(clin),Q(msr)) was equal to unity to within statistical uncertainty (0.5%) for all three diodes. Cross-axis profile measurements were also improved with the new detector design. The upstream air gap could be implanted on the commercial diodes via a cap consisting of the air cavity surrounded by water equivalent material. The results for the unclosed silicon chip show that an ideal small field dosimetry diode could be created by using a silicon chip with a small amount of air above it.
由于收集体积小，二极管通常用于小野剂量测定。然而，二极管的相对灵敏度随小野尺寸的减小而增加。相反，小气隙已被证明会导致探测器的灵敏度显著降低，随着野尺寸的减小。因此，本研究使用蒙特卡罗模拟来研究在二极管上游引入空气，以使它们在小野剂量测定中所有野尺寸下具有恒定的灵敏度。在两个商业二极管（PTW 60016 光子二极管和 PTW 60017 电子二极管）以及一个理论上未封闭的硅芯片的上游端引入了不同厚度的空气，使用的野尺寸小至 5mm×5mm。本研究中使用的度量 D(w,Q)/D(Det,Q) 表示特定野尺寸和位置下水点处的剂量与二极管有效体积处的剂量之比。通过绘制 D(w,Q)/D(Det,Q) 作为引入空气间隙大小的函数对于各种野尺寸，并找到这些图的交点，可以找到在所有小野尺寸下提供恒定灵敏度所需的最佳空气厚度。也就是说，找到了 D(w,Q)/D(Det,Q) 对于所有野尺寸都恒定的点。计算出光子二极管、电子二极管和未封闭硅芯片的最佳空气厚度分别为 3.3、1.15 和 0.10mm。这些结果的变化是由于每个探测器的不同设计。当用新的二极管设计结合上游气隙进行计算时，k(f(clin),f(msr))(Q(clin),Q(msr)) 对于三个二极管在统计不确定度（0.5%）内等于 1。新的探测器设计还改善了交叉轴轮廓测量。可以通过由水等效材料包围的空气腔的帽将上游气隙植入商业二极管中。未封闭硅芯片的结果表明，可以通过在其上方使用少量空气的硅芯片来创建理想的小野剂量测定二极管。
