Section of Oncology, Department of Oncology, Radiology and Clinical Immunology, Uppsala University, Uppsala, Sweden.
Phys Med Biol. 2010 Dec 21;55(24):7411-23. doi: 10.1088/0031-9155/55/24/002. Epub 2010 Nov 19.
The dosimetry of small fields is important for the use of high resolution photon radiotherapy. Silicon diodes yield a high signal from a small detecting volume which makes them suitable for use in small fields and high dose gradients. Unshielded diodes used in large fields are known to give a varying dose response depending on the proportion of low energy scattered photons in the field. Response variations in small fields can be caused by both spectral variations, and disturbances of the local level of lateral electron equilibrium. We present a model that includes the effects from lack of charged particle equilibrium. The local spectra are calculated by use of fluence pencil kernels and divided into a low and a high energy component. The low energy part is treated with large cavity theory and the high energy part with the Spencer-Attix small cavity theory. Monte Carlo-derived correction factors are used to account for both the local level of electron equilibrium in the field, and deviations from this level in the silicon disk cavity. Results for field sizes ranging from 0.5 × 0.5 to 20 × 20 cm² are compared to data from full Monte Carlo simulations and measurements. The achieved dose response accuracy is for the smallest fields 1-2%, and for larger fields 0.5%. Spectral variations were of little importance for the small field response, implying that volume averaging, and to some extent interface transient effects, are of importance for use of unshielded diodes in non-equilibrium conditions. The results indicate that diodes should preferably be designed to have the thin layer of active volume padded in between inactive layers of the silicon base material.
小野区剂量学对于高分辨率光子放射治疗的应用非常重要。硅二极管从较小的探测体积中产生高信号，这使得它们适合用于小野区和高剂量梯度。在大野区中使用未屏蔽的二极管会根据野区内低能散射光子的比例而产生不同的剂量响应。小野区中的响应变化可能是由光谱变化和局部水平侧向电子平衡的干扰引起的。我们提出了一个包含缺乏带电粒子平衡影响的模型。局部光谱通过使用注量铅笔核来计算，并分为低能和高能分量。低能部分用大空腔理论处理，高能部分用 Spencer-Attix 小空腔理论处理。使用蒙特卡罗衍生的修正因子来考虑场中的局部电子平衡水平以及硅盘腔中的这种水平的偏差。对 0.5×0.5 至 20×20cm² 的野区尺寸的结果与全蒙特卡罗模拟和测量数据进行了比较。对于最小的野区，实现的剂量响应精度为 1-2%，对于较大的野区，精度为 0.5%。光谱变化对小野区响应的重要性不大，这意味着体积平均和在某种程度上界面瞬态效应对于在非平衡条件下使用未屏蔽的二极管很重要。结果表明，二极管最好设计成在硅基底材料的不活跃层之间填充有薄的活性体积层。
