Does the γ dose distribution comparison technique default to the distance to agreement test in clinical dose distributions

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