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。我们只调查固定的领域。这些字段既用于生成剂量计算软件的物理参数，也用于验证此类软件的剂量计算。对于不同的探测器类型，我们研究和量化的响应inregions二次带电粒子平衡不存在。除了半径小于横向电子射程的场外，我们还考虑了场的边缘和表面区域。在此基础上，研究了具有中心阻断的场，以确定对纯散射辐射的响应效应。因此，在知道初级辐射与散射辐射的比率的情况下，可以计算检测器响应。在下一步中，我们希望在类似于解剖结构的体模中测量更真实的剂量分布（如应用于患者）：通常，仅针对简单的固定射束测试治疗计划系统的剂量计算精度。然而，在临床实践中，递送部分非常小且不规则形成的场的复杂序列。通过在均质塑料体模中测量来验证这种复杂的治疗计划，例如脊椎骨计划，通常显示出测量的剂量分布和计划的剂量分布之间的大偏差。这可能有各种原因：仅对治疗单元的准直系统进行了近似建模，动态准直控制不完全符合计划中的规定，或者剂量计算存在缺陷。然而，测量也受到误差的影响。太大的探测器在其体积上的平均值太大，而非水等效的探测器导致场的扰动。后者主要取决于测量点处次级粒子的光谱。因此，提高复杂场形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，在体模中的每个测量点处确定校正因子。合理地，相关指南建议随机测试计算的剂量分布，也在具有不均匀性的体模中。尽管如此，缺乏简单的临床可用程序。制定这种程序是该项目的另一个重点。关键区域是气腔、肺部病变、骨骼、牙齿和植入物。我们计划用3D打印机设计和生产幻影，以便现实地测试治疗计划。这里的挑战将再次是以足够的精度确定每个测量点的响应，以便识别交付错误。应采用不同的治疗计划系统制定治疗计划。这将能够量化计算算法的各个参数的影响。

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.