Positron emission tomography (PET) using O-(2-[ ¹⁸ F]fluoroethyl)-L-tyrosine ( ¹⁸ F-FET) PET has been shown to be a useful tool for differentiating radiation therapy outcomes, such as brain metastasis recurrence or radiation necrosis. We present the case of a female patient with brain metastases from pulmonary mucinous adenocarcinoma with suspicion of tumor recurrence on follow-up magnetic resonance imaging (MRI) after radiosurgery. ¹⁸ F-FET PET/computed tomography (CT) was indicative of radiation necrosis. Due to the patient's medical history and the discrepancy between the brain MRI and PET/CT results, surgical biopsies were decided, which were positive for brain metastasis recurrence. The diagnosis of metastasis recurrence may also be challenging on ¹⁸ F-FET PET/CT. In case of discrepancies between MRI and PET/CT results, false-negative ¹⁸ F-FET PET/CT remains a possibility and requires careful follow-up or biopsy.