Blanpain Baptiste, Mercier David
CEA, LIST Multisensor Intelligence and Machine Learning Laboratory, F-91191 Gif-sur-Yvette, France.
Med Phys. 2009 Mar;36(3):797-808. doi: 10.1118/1.3070546.
The gamma index is a tool that compares a dose distribution with a reference distribution by combining dose-difference and distance-to-agreement criteria. It has been widely used for ten years despite its high computational cost. This cost is due to both a search process for each reference point and the necessity to remove overestimations caused by the discrete nature of dose grids. The method proposed in this paper is much faster since it avoids both these problems. It consists in computing the delta envelope formed by the gamma ellipsoids around the points of the reference distribution. This delta envelope provides dose-difference tolerances that are then used to create new indices, called the delta indices, that provide useful information to interpret the deviations. Applied to both 1D and 2D test cases and compared to the gamma index, the S indices proved to be very accurate and intuitive. Their computational efficiency was evaluated on a 3D case: the delta envelope can be computed in 8 s on a 250 x 250 x 50 grid. Moreover it can be precomputed if the reference dose is known in advance. Then the delta indices are obtained in less than 2 s.
γ指数是一种通过结合剂量差异和距离一致性标准，将剂量分布与参考分布进行比较的工具。尽管其计算成本高昂，但已被广泛使用了十年。这种成本既源于对每个参考点的搜索过程，也源于消除剂量网格离散性质所导致的高估的必要性。本文提出的方法要快得多，因为它避免了这两个问题。它包括计算围绕参考分布各点的γ椭球体形成的增量包络。这个增量包络提供了剂量差异容限，然后用于创建新的指数，称为增量指数，这些指数为解释偏差提供了有用信息。应用于一维和二维测试案例并与γ指数进行比较后，S指数被证明非常准确且直观。在一个三维案例中评估了它们的计算效率：在250×250×50的网格上，增量包络可以在8秒内计算出来。此外，如果预先知道参考剂量，它可以预先计算。然后在不到2秒的时间内获得增量指数。
