Background <p>Functional imaging using positron-emission tomography (PET) is gaining increasing importance in individualized treatment planning for oncological diseases. Compared to conventional imaging, specific tracers enable a&#xa0;more precise assessment of tumor biology and spread.</p> Objective <p>This study investigates the use of prostate-specific membrane antigen (PSMA)-, fluorodeoxyglucose (FDG)-, and fluoroethyltyrosine (FET)-PET/CT in prostate cancer (PC), non-small cell lung cancer (NSCLC), and glioblastoma (GB) and evaluates their relevance for treatment planning.</p> Materials and methods <p>We conducted a&#xa0;review and critical analysis of current studies on the integration of functional imaging into treatment planning, with a&#xa0;particular focus on radiation therapy.</p> Results <p>In recurrent prostate cancer, PSMA-PET/CT led to changes in the therapeutic strategy in up to 68% of cases due to more accurate staging. In primary high-risk PC, it is now the preferred staging modality, while it facilitates metastasis-directed stereotactic radiotherapy in oligometastatic settings. In NSCLC, FDG-PET allows for reduction of elective radiation volumes without compromising efficacy and identifies high-risk regions for dose escalation. In GB, FET-PET improves delineation of the extent of the local tumor, especially in non-contrast-enhancing areas or in cases of radionecrosis, and thereby supports radiotherapeutic and neurosurgical planning.</p> Conclusion <p>By enabling more precise staging, PET/CT allows for more individualized treatment approaches in PC, NSCLC, and GB. Further prospective studies on oncological benefits are pending.</p>