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An accurate assessment of the radiotherapy plans requires an accurate dose to the target and efficient confinement of the doses. However, present indices do not completely cover these aspects in a single framework.

To develop a unified, dimensionless metric that integrates geometric conformity and dosimetric efficiency for evaluating stereotactic and fractionated radiotherapy plans aims to reduce QA workload by providing a rapid, quantitative estimate of plan acceptability, allowing early prediction of treatment plan performance.

In this study, the Nabaa Efficiency Index η N a b a a is introduced and defined as the product of the Conformity Index (CI) and a Dose-Efficiency Ratio (DER), evaluated at 50% and 90% isodose levels ( η 50 % N a b a a , η 90 % N a b a a ). We analyzed theoretical behavior against normalized dose-volume ratios and compared η N a b a a with established indices of conformity, homogeneity, and the Paddick efficiency index ( η 50 % P a d d i c k ) . Clinical validation used 100 plans (50 Gamma Knife, 50 VMAT-SRS). Statistics included ANOVA, correlations, ROC/AUC, and decision-curve analysis.

η 50 % N a b a a and η 90 % N a b a a were monotonic, bounded, and dimensionless. η 50 % N a b a a distinguished gradient/halo efficiency between stereotactic and linac plans (p < 0.001). η 90 % N a b a a most strongly reflected prescription-dose conformity (pooled AUC = 0.94) and outperformed the Paddick Efficiency Index ( η 50 % P a d d i c k ) in discrimination and interpretability.

η N a b a a unifies geometric and dosimetric assessment across modalities. Its simplicity, physical interpretability, and software-ready formulation support integration into automated QA and treatment-planning pipelines.