为了更好地理解伽马指数：使用基于表面的距离方法研究参数。
Toward a better understanding of the gamma index: Investigation of parameters with a surface-based distance method.
机构信息
Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA.
出版信息
Med Phys. 2011 Dec;38(12):6730-41. doi: 10.1118/1.3659707.
PURPOSE
The purpose of this work was to clarify the interactions between the parameters used in the γ index with the surface-based distance method, which itself can be viewed as a generalized version of the γ index. The examined parameters included the distance to agreement (DTA)/dose difference (DD) criteria, the percentage used as a passing criterion, and the passing percentage for given DTA/DD criteria. The specific aims of our work were (1) to understand the relationships between the parameters used in the γ index, (2) to determine the detection limit, or the minimum detectable error, of the γ index with a given set of parameters, and (3) to establish a procedure to determine parameters that are consistent with the capacity of an IMRT QA system.
METHODS
The surface-based distance technique with dose gradient factor was derived, and then the relationship between surface-based distance and γ index was established. The dose gradient factor for plans and measurements of 10 IMRT patients, 10 spine stereotactic radiosurgery (SRS) patients, and 3 Radiological Physics Center (RPC) head and neck phantom were calculated and evaluated. The detection limits of the surface-based distance and γ index methods were examined by introducing known shifts to the 10 IMRT plans.
RESULTS
The means of the dose gradient factors were 0.434 mm/% and 0.956 mm/% for the SRS and IMRT plans, respectively. Key quantities (including the mean and 90th and 99th percentiles of the distance distribution) of the surface-based distance distribution between two dose distributions were linearly proportional to the actual shifts. However, the passing percentage of the γ index for a given set of DTA/DD criteria was not associated with the actual shift. For IMRT, using the standard quality assurance criteria of 3 mm/3% DTA/DD and a 90% passing rate, we found that the detection limit of the γ index in terms of global shift was 4.07 mm/4.07 % without noise.
CONCLUSIONS
Surface-based distance is a direct measure of the difference between two dose distributions and can be used to evaluate or determine parameters for use in calculating the γ index. The dose gradient factor represents the weighting between spatial and dose shift and should be determined before DTA/DD criteria are set. The authors also present a procedure to determine γ index parameters from measurements.
目的
本研究旨在阐明γ指数与基于表面的距离方法中使用的参数之间的相互作用，该方法本身可以被视为γ指数的广义版本。所研究的参数包括距离一致性（DTA）/剂量差（DD）标准、用作通过标准的百分比以及给定 DTA/DD 标准的通过百分比。我们研究的具体目标是：（1）理解γ指数中使用的参数之间的关系；（2）确定具有给定参数集的γ指数的检测极限或最小可检测误差；（3）建立一种确定与 IMRT QA 系统能力一致的参数的程序。
方法
推导了基于表面的距离技术和剂量梯度因子，然后建立了基于表面的距离与γ指数之间的关系。计算并评估了 10 例 IMRT 患者、10 例脊柱立体定向放射外科（SRS）患者和 3 例放射物理中心（RPC）头颈部体模的计划和测量的剂量梯度因子。通过向 10 例 IMRT 计划引入已知的移位来检查基于表面的距离和γ指数方法的检测极限。
结果
SRS 和 IMRT 计划的剂量梯度因子的平均值分别为 0.434 mm/%和 0.956 mm/%。两个剂量分布之间基于表面的距离分布的关键量（包括距离分布的均值和 90%和 99%分位数）与实际移位线性相关。然而，给定 DTA/DD 标准的γ指数的通过百分比与实际移位无关。对于 IMRT，使用 3 mm/3% DTA/DD 的标准质量保证标准和 90%的通过率，我们发现，在没有噪声的情况下，γ指数的全局移位检测极限为 4.07 mm/4.07%。
结论
基于表面的距离是两个剂量分布之间差异的直接度量，可以用于评估或确定用于计算γ指数的参数。剂量梯度因子表示空间和剂量移位之间的权重，应在设置 DTA/DD 标准之前确定。作者还提出了一种从测量值确定γ指数参数的程序。
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