(upper row) A 59-year-old male patient diagnosed with an IDH-wild-type glioblastoma (WHO CNS grade 4). Following resection and chemoradiation with temozolomide, the contrast-enhanced MRI (CE-T1w MRI) suggested tumor relapse in the right parietal region 7 months after completing radiotherapy. Accordingly, the dynamic FET PET scan revealed pathologically increased FET uptake right parietal (TBRmax, 4.2) and decreased time–activity curve; (lower row). A 37-year-old female patient diagnosed with an IDH-wild-type glioblastoma (WHO CNS grade 4). Following resection and chemoradiation with temozolomide, the contrast-enhanced MRI suggested tumor relapse in the left frontal region 7 months after completing radiotherapy. In contrast to the patient in the upper row, the FET uptake in the left frontal region was not pathologically increased (TBRmax, 1.6) with a steadily increasing time–activity curve, indicating reactive treatment-related changes. SUV = standardized uptake value.