A 64-year-old female patient with status post pulmonary adenocarcinoma and concomitant brain metastases previously treated with radiotherapy presented with a new contrast-enhancing lesion and perifocal edema on MRI suggestive of brain metastasis recurrence. An additional dynamic [18F]FET PET was performed and showed only faint [18F]FET uptake and constantly increasing time–activity curves in the dynamic evaluation, findings typically associated with radionecrosis rather than vital tumor tissue. A consecutive stereotactic biopsy of the lesion confirmed the PET-suspected radionecrosis without evidence of tumor recurrence. In sum, the differentiation of recurrent brain metastasis from treatment-related changes after radiotherapy can be challenging on MRI; PET with radiolabeled amino acids can significantly contribute to this clinical issue with a high diagnostic accuracy.