Determination of complex dose distributions in inhomogenious phantoms, irratiated by high-energy photons

在我们之前的项目中，我们研究了高能光子束在水中剂量分布测量的准确性。我们只调查了固定的领域。这些字段既用于剂量计算软件的物理参数的生成，也用于用该软件进行剂量计算的验证。对于不同类型的探测器，我们研究并量化了不存在二次带电粒子平衡区域的响应。除了半径小于横向电子范围的场外，我们还考虑了场的边缘和表面区域。在此基础上，研究了中心遮挡场对纯散射辐射的响应效应。这样，在知道主辐射与散射辐射之比的情况下，就可以计算出探测器的响应。在下一步，我们想要测量更真实的剂量分布，就像应用于病人一样，在类似于解剖结构的幻影中：一般来说，治疗计划系统的剂量计算的准确性仅针对简单的固定光束进行测试。然而，在临床实践中，部分非常小且不规则形成的场的复杂序列被传递。通过在均匀塑料模型中测量来验证这种复杂的治疗计划，例如脊椎骨计划，通常显示测量剂量分布与计划剂量分布之间存在很大偏差。这可能有各种原因：处理单元的准直系统只是近似建模，动态准直控制不完全按照计划规定或剂量计算有缺陷。然而，测量也受到误差的影响。过大的探测器在其体积上的平均强度过大，而非水当量探测器则会导致场的扰动。后者主要取决于测点处二次粒子的光谱。因此，提高复杂地层结构测量的精度是一个重要的目标。为此，有必要根据特定的场配置，在模体中的每个测量点确定校正因子。明智的是，相关指南建议随机测试计算出的剂量分布，也适用于具有不均匀性的幻影。尽管如此，缺乏简单的临床可用的方法。制定这样的程序是该项目的进一步重点。关键部位是空气腔、肺病变、骨骼、牙齿和植入物。我们计划设计幻影并用3D打印机制造出来，以便真实地测试治疗方案。这里的挑战将再次是在每个测量点以足够的精度确定响应，以便识别交付错误。治疗方案应采用不同的治疗计划制度。这将使我们能够量化计算算法中各个参数的影响。

In our preceding project, we investigated the accuracy of measurements of dose distributions in water for high-energy Photon beams. We investigated fixed fields only. Such fields are used for both, the generation of the physical parameters for dose calculation software and for verification of dose calculation with such software. For different detector types, we studied and quantified the response inregions where secondary charged particle equilibrium does not exist. Besides fields with radii smaller than the lateral electron range, weconsidered field edge and surface regions. Over that, fields with central blocking were investigated in order to identify the Response effects to pure scattered radiation. Thus and with knowledge of the ratio of primary to scatter radiation detector response may be calculated. In a next step, we want to measure more realistic dose distributions, as applied to patients, in phantoms similar to the anatomy: In general, the accuracy of the dose calculation of a treatment planning system is tested for simple fixed beams only. However, in clinical practice complex sequences of partly very small and irregularly formed fields are delivered. Verification of such complex treatment plans, e.g. spinal vertebrae plans, by measurement in homogeneous plastic phantoms, often shows large deviations between measured and planned dose distribution. This may have various reasons: the collimation system of the Treatment unit is modeled approximately only, the dynamic collimation control is not exactly as specified in the plan or the dose calculation is defective. However, also the measurement is afflicted with errors. Too large detectors average too heavily across their volumes, while nonwater- equivalent probes lead to perturbations of the field. The latter mainly depend on the spectrum of the secondary particles at the measurement point. Therefore, it is a major goal to improve the accuracy for the measurement of complex field configurations. For that, it is necessary to determine correction factors, depending on the particular field configuration, at every measurement point in a phantom. Sensibly, pertinent guidelines recommend testing calculated dose distributions randomly, also in phantoms with inhomogeneities. Despite of that, simple clinically usable procedures are lacking. To develop such procedures is a further focus of the project. Critical regions are air cavities, lung lesions, bones, teeth and implants. Weplan to design phantoms and produce them with a 3D printer in order to test treatment plans realistically. Here the challenge will be again to determine the response at every measurement point with sufficient accuracy in order to recognize delivery errors. The treatment plans shall be made with different treatment planning systems. This will enable to quantify the influence of individual parameters of the calculation algorithms.