Department of Radiation Oncology, University Hospital Wurzburg, Wuerzburg, Germany.
Med Phys. 2023 Dec;50(12):8044-8056. doi: 10.1002/mp.16675. Epub 2023 Aug 30.
Beam data commissioning is a core task of radiotherapy physicists. Despite multiple detectors available, a feasible measurement program compromises between detector properties and time constraints. Therefore, it is important to understand how nonideal measurement data propagates into patient dose calculation.
We simulated the effects of realistic errors, due to beam commissioning with presumably nonoptimal detectors, on the resulting patient dose distributions. Additionally, the detectability of such beam commissioning errors during patient plan quality assurance (QA) was evaluated.
A clinically used beam model was re-commissioned introducing changes to depth dose curves, output factors, profiles or combinations of those. Seventeen altered beam models with incremental changes of the modelling parameters were created to analyze dose changes on simplified anatomical phantoms. Additionally, fourteen altered models incorporate changes in the order of signal differences reported for typically used detectors. Eighteen treatment plans of different types were recalculated on patient CT data sets using the altered beam models.
For the majority of clinical plans, dose distributions in the target volume recalculated on the patient computed tomography data were similar between the original and the modified beam models, yielding global 2%/2 mm gamma pass rates above 98.9%. Larger changes were observed for certain combinations of beam modelling errors and anatomical sites, most extreme for output factor changes in a small target volume plan with a pass rate of 80.6%. Modelling an enlarged penumbra as if measured with a 0.125 cm ion chamber had the largest effect on the dose distribution (average pass rate of 96.5%, lowest 85.4%). On different QA phantom geometries, dose distributions between calculations with modified and unmodified models typically changed too little to be detected in actual measurements.
While the simulated errors during beam modelling had little effect on most plans, in some cases changes were considerable. High-quality penumbra and small field output factor should be a main focus of commissioning measurements. Detecting modelling issues using standard patient QA phantoms is unlikely. Verification of a beam model should be performed especially for plans with high modulation and in different depths or geometries representing the variety of situations expected clinically.
束流数据调试是放射治疗物理学家的核心任务。尽管有多种探测器可供选择，但可行的测量方案需要在探测器性能和时间限制之间进行权衡。因此，了解非理想测量数据如何传播到患者剂量计算中是很重要的。
我们模拟了由于使用可能不理想的探测器进行束流调试而导致的实际误差对患者剂量分布的影响。此外，还评估了在患者计划质量保证（QA）期间检测这种束流调试误差的能力。
重新调试了一种临床使用的束流模型，引入了对深度剂量曲线、输出因子、曲线或这些曲线的组合的改变。创建了 17 个具有增量建模参数变化的改变的束流模型，以分析简化解剖体模上的剂量变化。此外，14 个改变的模型纳入了通常使用的探测器报告的信号差异顺序的改变。使用改变的束流模型重新计算了 18 种不同类型的治疗计划的患者 CT 数据集。
对于大多数临床计划，在原始和修改后的束流模型上重新计算的靶区剂量分布在目标体积内相似，全局 2%/2 毫米伽马通过率高于 98.9%。对于某些束流建模误差和解剖部位的组合，观察到更大的变化，对于具有通过率为 80.6%的小靶区计划的输出因子变化则更为极端。将扩大的半影模拟为如果使用 0.125cm 离子室测量的效果最大（平均通过率为 96.5%，最低为 85.4%）。在不同的 QA 体模几何形状上，计算用修改和未修改模型之间的剂量分布变化通常太小，无法在实际测量中检测到。
虽然束流建模过程中的模拟误差对大多数计划的影响很小，但在某些情况下，变化是相当大的。高质量的半影和小的射野输出因子应该是调试测量的主要关注点。使用标准的患者 QA 体模检测建模问题不太可能。特别是对于具有高调制和不同深度或几何形状的计划，应该执行束流模型的验证，以代表临床预期的各种情况。
