A Novel, Dimensionless Efficiency Index for Evaluating Stereotactic Radiotherapy Plans: Nabaa Efficiency Index (NEI)
Abstract
Purpose
To introduce and validate the Nabaa Efficiency Index (NEI), a unified, dimensionless metric that integrates geometric conformity and dose-energy efficiency for evaluating plan quality across stereotactic radiosurgery (SRS/SBRT) and fractionated radiotherapy (IMRT/VMAT).
Methods
NEI was formulated as the product of the Conformity Index (CI) and a Dose-Efficiency Ratio (DER), defined at both 50% and 90% prescription isodose volumes. NEI₅₀ quantified gradient/halo efficiency using the 50% isodose envelope, while NEI₉₀ captured prescription-dose conformity using the 90% envelope. Analytical behavior was evaluated under ideal and non-ideal boundary conditions to confirm monotonicity, boundedness, and dimensional neutrality. Clinical validation included 100 stereotactic plans (50 Gamma Knife, 50 VMAT-SRS). Comparative performance was assessed against established indices including CI, HI, and the Paddick Efficiency Index (PEI) using ANOVA, Pearson correlations, ROC analysis, and decision-curve analysis.
Results
NEI₅₀ and NEI₉₀ showed stable, monotonic behavior and maintained strict dimensionless scaling across prescriptions and modalities. NEI₅₀ demonstrated strong discrimination of intermediate-dose halo efficiency between Gamma Knife and linac-based SRS plans (p < 0.001), reflecting gradient sensitivity beyond GI and CI. NEI₉₀ achieved the highest predictive accuracy for prescription-dose conformity (AUC = 0.94), outperforming the PEI in discrimination, interpretability, and response to variations in dose-volume structure. Both NEI formulations showed strong correlations with target dose concentration while preserving geometric-dosimetric coupling absent in traditional indices
Conclusion
NEI provides a unified, modality-agnostic metric capable of evaluating both stereotactic and fractionated radiotherapy plans. Its dimensionless formulation, physical interpretability, and automation-ready structure support seamless integration into clinical QA workflows and treatment-planning systems. NEI addresses longstanding limitations of existing conformity and homogeneity indices by simultaneously quantifying geometric accuracy and energetic efficiency.