AbstractID: 9305 Title: The Design and Testing of Novel Clinical Parameters for Dose
Comparison
New parameters for the clinical comparison of measured and computed doses have been developed. Normalized agreement test (NAT) values
scale differences between measured and computed dose distributions by desired accuracy goals and their clinical impact. The lesser of the
absolute value of the percent dose difference divided by the dose criterion and the distance to agreement divided by the distance criterion is
chosen at each pixel. If this value is less than one, the NAT value is zero. Otherwise, the excess over the criterion is computed by subtracting one.
NAT values are scaled by the greater of the computed or measured dose at the pixel and multiplied by 100 divided by the computed dose
maximum to convert to a percentage. NAT values that have less measured than computed dose are set to zero for areas outside the PTV region, as
these differences are considered to have no negative clinical impact. The NAT index is a single value that represents overall clinical agreement
and increases as discrepancies increase. It is calculated by dividing the average NAT value by the average dose and multiplying by 100. After
analyzing 50 clinical cases using standard dose comparison tools and NAT parameters, we found that NAT values quickly pinpointed areas of
disagreement that had clinical impact. NAT indexes for verification films with gross delivery errors were identified as outliers from normal
verification results. The use of NAT indexes for acceptance criteria could be more clinically relevant than the commonly used dose and distance
criteria.