Purpose <p>Amino acid PET with [¹⁸F]-fluoroethylthyrosine ([¹⁸F]FET-PET) is frequently utilized in gliomas. Most studies on prognostication based on amino acid PET comprise mixed cohorts of brain tumors with low- and high-grade features. The objective of this study was to assess the potential prognostic value of [¹⁸F]FET-PET-based markers in the group of grade 2 adult-type diffuse gliomas, as defined by the WHO CNS 2021 classification.</p> Methods <p>Retrospectively, all therapy-naive patients having undergone [¹⁸F]FET-PET before maximal safe resection of grade II / 2 gliomas over a time of 2012—2022 were included. Diagnoses were updated according to the WHO CNS 2021 classification. [¹⁸F]FET-PET were quantitatively evaluated, including dynamic PET acquisition if available. The primary outcome measure was progression-free survival (PFS), progression was defined by RANO 2.0 criteria.</p> Results <p>In the cohort of WHO grade 2 gliomas, 57 (69%) patients were diagnosed with astrocytoma, IDH-mutant. Twenty-six (31%) patients were diagnosed with oligodendroglioma, IDH-mutant and 1p/19q-codeleted. Quantitative PET uptake parameters (TBR_max, TBR_mean, BTV) were significantly higher in oligodendroglioma compared to astrocytoma (p &lt; 0.001). In all patients, Cox regression analysis of clinical and imaging parameters did not identify any factor that significantly impacted PFS. In the subgroup of astrocytoma without adjuvant treatment, for patients with TBR_max above 1.9 PFS was significantly shorter (p &lt; 0.001).</p> Conclusion <p>Preoperative [¹⁸F]FET-PET can provide prognostic information in distinct subgroups of diffuse low-grade gliomas not having undergone adjuvant therapies. Following external validation, preoperative [¹⁸F]FET-PET may possibly be employed as a decision-support tool to inform the choice of adjuvant therapies in astrocytoma.</p>