SU‐FF‐T‐289: Gamma Is a Necessary, But Not Sufficient Criteria for Comparing Dose Distributions

Purpose: To demonstrate via simple test cases that although percent pass gamma (γ<1) may be a necessary condition for acceptance in dosimetric analysis, it is not a sufficient condition to ensure dose integrity and therefore should not be used as a sole of criterion to judge dosimetric acceptability. Method and Materials: Simple modifications to the delivery of treatment fields are created such that, although > 98% of points have γ<1 with respect to the reference field when 2D planar fluence/dose verification images are acquired, the resultant dose distributions are clinically unacceptable. Gamma criterion of 3%, 3 mm and 5%, 5mm are used. Reference test fields include a 10×10 cm 2 open field with 5mm gap sweeping and two modified 10×10 cm 2 fields. Deviations include shrinking the dose distribution by 3 mm, and introducing a non‐moving 5 mm wide MLC leaf in the interior of a field. Results: The two‐dimensional field that is narrower than the reference field can have 100% of the points with γ<1. A non‐moving 5 mm MLC leaf in a 10 ×10 cm 2 sliding window can have 94.5% of the points passed 3%, 3mm gamma test and 97.8% of the points passed 5%, 5mm gamma test. Similarly, fields with dose peaks or valleys in them with dimensions up to two‐times the DTA criteria can have 100% of points with γ<1. Exchanging to roles of the reference and test images in the gamma evaluation can be distinguish some, but not all, of the clinically relevant errors. Conclusion: Evaluation of the fraction of points with γ<1 may be a necessary condition, but it is not sufficient to state a plan is dosimetically acceptable. Evaluation of additional dose metrics is required to show clinical acceptability. Conflict of Interest: Research supported in part by Varian Medical Systems and NIH P01 CA116602.