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Background: Differentiating benign from malignant orbital lesions by imaging and clinical presentation can be challenging., Purpose: To differentiate benign from malignant orbital masses using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) based on tumor flow residence time τ calculated with the aid of a pharmacokinetic tumor model., Material and Methods: Sixty patients with orbital masses were investigated by 3-T MRI including dynamic sequences. The signal intensity-time curve after i.v. contrast medium administration within lesions was approximated by Gd-concentration profiles on the basis of model calculations where the tumor is embedded in a whole-body kinetic model. One output of the model was tumor flow residence time τ, defined as the ratio of the tumor volume and the tumor blood flow rate. Receiver operating characteristic (ROC) curves were used to analyze the diagnostic performance of τ. The results were compared with those of K trans , k ep , v e , iAUC, and ADC., Results: Thirty-one benign and 29 malignant orbital masses were identified (reference standard: histopathology, clinical characteristics). Mean τ was significantly longer for benign masses (94 ± 48 s) than for malignant masses (21 ± 19 s, P < 0.001). ROC analysis revealed the highest area under the curve (AUC = 0.94) for τ in orbital masses compared to standard methods., Conclusion: Tumor flow residence times τ of benign and malignant orbital masses are valuable in the diagnostic work-up of orbital tumors. Measures of diagnostic accuracy were superior for τ compared to ADC, K trans , v e , and iAUC.