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The aim of this study was to investigate the potential of O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) PET for differentiating local recurrent brain metastasis from radiation necrosis after radiation therapy because the use of contrast-enhanced MRI for this issue is often difficult. Thirty-one patients (mean age ± SD, 53 ± 11 y) with single or multiple contrast-enhancing brain lesions (n = 40) on MRI after radiation therapy of brain metastases were investigated with dynamic ¹⁸F-FET PET. Maximum and mean tumor-to-brain ratios (TBR_max and TBR_mean, respectively; 20–40 min after injection) of ¹⁸F-FET uptake were determined. Time–activity curves were generated, and the time to peak (TTP) was calculated. Furthermore, time–activity curves of each lesion were assigned to one of the following curve patterns: (I) constantly increasing ¹⁸F-FET uptake, (II) ¹⁸F-FET uptake peaking early (TTP ≤ 20 min) followed by a …