Alpsten Freja, van Veelen Bob, Valdes-Cortez Christian, Berumen Francisco, Ahnesjö Anders, Carlsson Tedgren Åsa
Department of Oncology-Pathology, Karolinska Institutet, Stockholm, Sweden.
Department of Nuclear Medicin and Medical Physics, Karolinska University Hospital, Stockholm, Sweden.
Med Phys. 2025 Jan;52(1):585-595. doi: 10.1002/mp.17434. Epub 2024 Oct 29.
Model-based dose calculation algorithms (MBDCA), such as the Advanced Collapsed cone Engine (ACE) in Oncentra Brachy® can be used to overcome the limitations of the TG-43 formalism. ACE is a point kernel superposition algorithm that calculates the total dose separated into primary, first-scatter, and multiple-scatter dose. Albeit ACE yields accurate results under most circumstances, several studies have reported underestimations of the dose to cortical bone. These underestimations are likely caused by approximations in the handling of multiple-scatter dose for non-water media. Such would result in noticeable deviations where the multiple-scatter is a considerable fraction of the total dose, that is, at greater distances from the source.
To improve and test the accuracy of the multiple-scatter dose component in the ACE algorithm to remedy its inaccuracy for non-water geometries.
A careful analysis of the transport and absorption of the multiple-scatter energy fluence revealed an inconsistency in the scaling of energy absorption ratios for non-water media of the multiple-scatter kernel. We implemented an updated algorithm version, ACE, and tested it for three different geometries. All had a single Ir-source at the center of a cubic water phantom with a box-shaped heterogeneity of either cortical bone or air, positioned at different distances from the source. Dose distributions for the three cases were calculated with ACE and ACE and compared to Monte Carlo simulations, using the percentage dose difference ratio as figure-of-merit. All dose calculation methods scored separately the dose deposited by primary, first-scattered, and multiple-scattered photons.
The accuracy of the updated algorithm ACE was superior to ACE. In the cortical bone heterogeneity, the mean percentage dose difference ratio for the total dose improved from to (in the worst case) by our update. Less impact was seen in the air heterogeneity, where both ACE and ACE deviated less than 2% from the Monte Carlo results. The algorithm update mainly concerns the multiple-scattered dose component, but an accompanying data processing update also had a small effect ( 0.5% difference) on the primary and first-scattered dose. The calculation times were not affected.
The updates to ACE improved the accuracy of multiple-scatter dose calculation for non-water media, without increasing calculation times.
基于模型的剂量计算算法（MBDCA），如Oncentra Brachy®中的高级坍缩圆锥引擎（ACE），可用于克服TG-43形式体系的局限性。ACE是一种点核叠加算法，可计算分为原发射线剂量、首次散射剂量和多次散射剂量的总剂量。尽管ACE在大多数情况下能产生准确结果，但几项研究报告称其低估了皮质骨的剂量。这些低估可能是由于非水介质中多次散射剂量处理的近似性导致的。在多次散射占总剂量相当大比例的情况下，即在离源较远的距离处，这会导致明显的偏差。
改进并测试ACE算法中多次散射剂量分量的准确性，以纠正其在非水几何形状中的不准确性。
对多次散射能量注量的传输和吸收进行仔细分析后发现，多次散射核的非水介质能量吸收比缩放存在不一致性。我们实现了一个更新的算法版本ACE，并针对三种不同的几何形状进行了测试。所有几何形状均在立方水体模中心有一个单一铱源，其中有一个皮质骨或空气的盒状不均匀性区域，位于距源不同距离处。使用百分比剂量差异率作为品质因数，用ACE和ACE计算三种情况下的剂量分布，并与蒙特卡罗模拟结果进行比较。所有剂量计算方法分别对原发射线光子、首次散射光子和多次散射光子沉积的剂量进行评分。
更新后的算法ACE的准确性优于ACE。在皮质骨不均匀性区域，通过我们的更新，总剂量的平均百分比剂量差异率从（在最坏情况下）提高到了。在空气不均匀性区域，影响较小，ACE和ACE与蒙特卡罗结果的偏差均小于2%。算法更新主要涉及多次散射剂量分量，但伴随的数据处理更新对原发射线剂量和首次散射剂量也有较小影响（差异<0.5%）。计算时间未受影响。
ACE的更新提高了非水介质中多次散射剂量计算的准确性，且未增加计算时间。
