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

Abstract

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 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 plateau, and (III) ¹⁸F-FET uptake peaking early (TTP ≤ 20 min) followed by a constant descent. The diagnostic accuracy of the TBR_max and TBR_mean 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 × 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 TBR_max and TBR_mean were significantly higher in patients with recurrent metastasis (n = 19) than in patients with radiation necrosis (n = 21) (TBR_max, 3.2 ± 0.9 vs. 2.3 ± 0.5, P < 0.001; TBR_mean, 2.1 ± 0.4 vs. 1.8 ± 0.2, P < 0.001). The diagnostic accuracy of ¹⁸F-FET PET for the correct identification of recurrent brain metastases reached 78% using TBR_max (area under the ROC curve [AUC], 0.822 ± 0.07; sensitivity, 79%; specificity, 76%; cutoff, 2.55; P = 0.001), 83% using TBR_mean (AUC, 0.851 ± 0.07; sensitivity, 74%; specificity, 90%; cutoff, 1.95; P < 0.001), and 92% for curve patterns II and III versus curve pattern I (sensitivity, 84%; specificity, 100%; P < 0.0001). The highest accuracy (93%) to diagnose local recurrent metastasis was obtained when both a TBR_mean greater than 1.9 and curve pattern II or III were present (AUC, 0.959 ± 0.03; sensitivity, 95%; specificity, 91%; P < 0.001). Conclusion: Our findings suggest that the combined evaluation of the TBR_mean of ¹⁸F-FET uptake and the pattern of the time–activity curve can differentiate local brain metastasis recurrence from radionecrosis with high accuracy. ¹⁸F-FET PET may thus contribute significantly to the management of patients with brain metastases.