Introduction: The Paddick gradient index was devised in 2006 as a complement to several previously described conformity index measures. Beautiful in its simplicity, the gradient index’s purpose was to quantify the steepness of dose falloff by computing the ratio of the prescription isodose volume to the volume associated with half that dose (e.g PIV_50%/PIV_25% for a typical Gamma Knife plan). One caveat of the gradient index is its impropriety in comparing plans of incongruous conformity. Although Paddick warned against this in his original treatise, it is nevertheless routinely employed. Here we report a metric suitable for direct comparison of dose falloff between separate radiosurgical plans, particularly those with disparate conformity – the area under the DVH curve between the prescription isodose and 50% prescription isodose (AUC_100%-50%).
Methods: The following metrics (Fig. 1) were evaluated for 28 cases among two separate plan types (multi-arc VMAT and gamma knife) with statistically different conformity and gradient index, but similar coverage and 50% isodose spill. Plans had been previously judged clinically equivalent through DVH inspection and isodose spill review.
Results: Mean conformity was lower for the multi-arc VMAT than gamma knife (CI_VMAT=1.89, CI_GK=2.30; p=0.0052). Mean 50% prescription isodose volumes were not significantly different (PV50%_VMAT=24.98, PV50%_GK=23.42; p=0.79). As expected with higher conformity, and similar 50% isodose spill, gradient index was lower for gamma knife (GI_VMAT=6.05, GI_GK=3.00; p<0.0001). AUC_100%-50% was not significantly different between plans (AUC_VMAT=115.65,AUC_GK=120.90; p=0.86)
Conclusions: Here we validate that plans with differing Paddick gradient indices do not necessarily have significantly differing dose falloff, and present a more robust surrogate measure for comparatively evaluating plans in such cases. The metric can be used to evaluate the extent of any other dose spill. Further analysis is needed to determine whether this metric can be used to improve upon existing models for normal tissue complication probability.
Patient Care: The improved radiosurgery plan quality metric we present here will allow providers an accurate and robust way to compare the dose fall-off between multiple types of plans.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Appreciate the impropriety of using the Paddick GI for comparison of plans with disparate conformity, 2) Understand the rationale for utilizing the area under the DVH curve as a surrogate dose falloff metric.