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摘要
Purpose: To determine the validity of the assumption that the gamma dose distribution comparison tool defaults to the distance to agreement test under conditions of clinically relevant steep dose gradients and gamma test criteria. %26lt;br%26gt;Methods: The assumption was tested by computing the angle theta between the dose axis and gamma vector for clinical treatment plans. theta was a function of the evaluated dose distribution dose gradient and the ratio (alpha) of the dose difference to distance to agreement (DTA) criteria. Dose distributions from prostate, head and neck, and lung clinical treatment plans were examined: 50 treatment plans were selected for each of the prostate and head neck sites and 27 treatment plans were selected for lung. Dose-gradient histograms were prepared for each of the treatment plans using alpha = 1%/mm (e.g., 3%, 3 mm dose difference and DTA criteria, respectively). To determine how frequently different values of alpha were used in publications, papers that referenced the original gamma paper were examined to identify the dose difference and DTA criteria used in those publications. In order to compare theta calculated using alpha = 1%/mm to theta for other values of alpha, the relationship between theta and alpha was determined. %26lt;br%26gt;Results: For most of the targets and critical structures, the maximum value of theta approached 90 degrees, so the assumption that the gamma tool defaulted to the DTA test in steep dose gradients was correct. Most of the published papers using the gamma tool employed the 3%, 3 mm dose difference and DTA criteria, respectively. Most of the other evaluations used criteria such that alpha %26gt;= 1%/mm, so the conclusions relating to the examined dose distributions applied. There were a few papers employing very small values of alpha (including one where alpha = 0.17%/mm), breaking the assumption that the gamma dose comparison tool defaulted to the DTA tool in steep dose gradients. %26lt;br%26gt;Conclusions: Most published cases utilized values of alpha %26gt;= 1%/mm, and for those the implicit assumption that the gamma dose comparison tool defaults to the DTA test in steep dose gradient regions was true. There were a few cases for which alpha was small enough to potentially invalidate this assumption. Care should be taken by investigators when selecting the gamma test criteria to assure that the gamma test will appropriately default to the DTA test in the steepest dose gradients being evaluated.
- 出版日期2013-7
