Purpose <p>This study aimed to assess the diagnostic and prognostic utility of baseline and post-therapy [18F] FET (O-(2-[18F]fluoroethyl)-l-tyrosine) PET imaging parameters for tumor detection, grading, recurrence assessment, residual tumor evaluation, and survival prediction in pediatric and adolescent patients with brain tumors.</p> Methods <p>This systematic review and meta-analysis followed PRISMA and Cochrane guidelines. Electronic databases were searched up to September 9, 2024, without language restrictions. The diagnostic and prognostic value of baseline and post-therapy [18F] FET PET parameters, such as tumor-to-brain ratio (TBRmax and TBRmean), time-to-peak (TTP), kinetic pattern (KP), maximum standardized uptake value (SUVmax), and metabolic tumor volume (MTV), was examined. Bayesian diagnostic test accuracy meta-analysis and paired meta-analysis were performed.</p> Results <p>A total of 12 studies (352 patients) published between 2005 and 2023 were included. Baseline TBRmax demonstrated weak diagnostic performance for tumor grade evaluation, with an area under the summary receiver operating characteristic curve (AUSROC) of 0.639, sensitivity of 73.8% (52.8%–88.4%), and specificity of 53.9% (27.5%–75.7%). Paired meta-analysis showed significantly higher baseline TBRmax (mean difference: 1.93, <i>p</i> &lt; 0.001) and TBRmean (mean difference: 1.25, <i>p</i> = 0.02) in patients with high-grade tumors. Post-therapy TBRmax demonstrated robust diagnostic accuracy for tumor recurrence (AUSROC: 0.850, sensitivity: 81.5% [67.8%–90.9%], specificity: 88.5% [66.4%–97.8%]). Other parameters and outcomes lacked sufficient data for meta-analysis.</p> Conclusion <p>This systematic review and meta-analysis demonstrate the potential of [18F] FET PET imaging in diagnosing and prognosticating pediatric and adolescent brain tumors. Standardized methodologies and multicenter studies are essential to improve reliability and clinical application.</p>