PMC8635048_01
Neuroimages including contrast-enhanced MRI, FLAIR-weighted MRI, and FET PET of a patient with an anaplastic oligodendroglioma (IDH mutant, 1p/19q co-deleted). Compared to the baseline MRI (A), the follow-up MRI perfomed 9 weeks later (17 weeks after end of radiotherapy) (B) showed a substantial contrast enhancement and an extensive FLAIR signal hyperintensity in the area of the resection defect. In contrast, both the baseline and first follow-up FET PET (A, B) showed no pathologically increased tracer uptake (mean tumor-to-brain ratios in both scans, 1.8). The rationale for this is based on the fact that several FET PET studies suggested that especially in pretreated glioma patients a mean tumor-to-brain ratio of 2.0 separates best between tumor relapse and treatment-related changes. The second follow-up MRI (C) showed a further slight increase of the FLAIR signal alteration. Again, the corresponding follow-up FET PET showed no substantial change of metabolic activity compared to the previous examinations (A, B). Not earlier than 26 weeks, the third follow-up MRI (D) revealed an almost complete decrease of the contrast enhancement.
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According to current response assessment criteria, the findings of the first and second follow-up MRI were consistent with "Progressive Disease", whereas the corresponding FET PET scans remained unchanged compared to baseline (see  Figure 1 ).