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PET using O -(2- 18 F-fluoroethyl)-l-tyrosine ( 18 F-FET) is useful to detect residual tumor tissue after glioma resection. Recent animal experiments detected reactive changes in 18 F-FET uptake at the rim of the resection cavity within the first 2 wk after resection of gliomas. In the present study, we evaluated pre- and postoperative 18 F-FET PET scans of glioma patients with particular emphasis on the identification of reactive changes after surgery. Methods: Forty-three patients with cerebral gliomas (9 low-grade, 34 high-grade; 9 primary tumors, 34 recurrent tumors) who had preoperative (time before surgery: median, 23 d; range, 6-44 d) and postoperative 18 F-FET PET (time after surgery: median, 14 d; range, 5-28 d) were included. PET scans (20-40 min after injection) were evaluated visually for complete or incomplete resection and compared with MRI. Changes in 18 F-FET uptake were evaluated by tumor-to-brain ratios in residual tumor and by maximum lesion-to-brain ratios near the resection cavity. Results: Visual analysis of 18 F-FET PET scans revealed complete resection in 16 of 43 patients and incomplete resection in the remaining patients. PET results were concordant with MRI in 69% of the patients. The maximum lesion-to-brain ratio for 18 F-FET uptake near the resection cavity was significantly higher than preoperative values (1.59 ± 0.36 vs. 1.14 ± 0.17; n = 43; P < 0.001). In 11 patients (26%), a flare phenomenon was observed, with a considerable increase in 18 F-FET uptake compared with preoperative values in either the residual tumor ( n = 5) or areas remote from the tumor on the preoperative PET scan ( n = 6) (2.92 ± 1.24 vs. 1.62 ± 0.75; P < 0.001). Further follow-up in 5 patients showed decreasing 18 F-FET uptake in the flare areas in 4 patients and progress in 1 patient. Conclusion: Our study confirmed that 18 F-FET PET provides valuable information for assessing the success of glioma resection. Postoperative reactive changes at the rim of the resection cavity appear to be mild. However, in 23% of the patients, a postoperative flare phenomenon was observed that warrants further investigation., (© 2020 by the Society of Nuclear Medicine and Molecular Imaging.)