Role of O -(2- 18 F-Fluoroethyl)-l-Tyrosine PET for Differentiation of Local Recurrent Brain Metastasis from Radiation Necrosis

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. Methods: Thirty-one patients (mean age ± SD, 53 ± 11 y) with single or multiple contrastenhancing 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 (TBRmax and TBRmean, 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 plateau, and (III) 18F-FET uptake peaking early (TTP # 20 min) followed by a constant descent. The diagnostic accuracy of the TBRmax and TBRmean of 18F-FET uptake and the curve patterns for the correct identification of recurrent brain metastasis were evaluated by receiver-operating-characteristic analyses or Fisher exact test for 2 x 2 contingency tables using subsequent histologic analysis (11 lesions in 11 patients) or clinical course and MRI findings (29 lesions in 20 patients) as reference. Results: Both TBRmax and TBRmean were significantly higher in patients with recurrent metastasis (n = 19) than in patients with radiation necrosis (n = 21) (TBRmax, 3.2 ± 0.9 vs. 2.3 ± 0.5, < 0.001; TBRmean, 2.1 ± 0.4 vs. 1.8 ± 0.2, < 0.001). The diagnostic accuracy of 18F-FET PET for the correct identification of recurrent brain metastases reached 78% using TBRmax (area under the ROC curve [AUC], 0.822 ± 0.07; sensitivity, 79%; specificity, 76%; cutoff, 2.55; P = 0.001), 83% using TBRmean (AUC, 0.851 ± 0.07; sensitivity, 74%; specificity, 90%; cutoff, 1.95; < 0.001), and 92% for curve patterns II and III versus curve pattern I (sensitivity, 84%; specificity, 100%; < 0.0001). The highest accuracy (93%) to diagnose local recurrent metastasis was obtained when both a TBRmean greater than 1.9 and curve pattern II or III were present (AUC, 0.959 ± 0.03; sensitivity, 95%; specificity, 91%; < 0.001). Conclusion: Our findings suggest that the combined evaluation of the TBRmean of 18F-FET uptake and the pattern of the time-activity curve can differentiate local brain m