Purpose <p>Recent advancements in imaging, particularly 18F-fluorodeoxyglucose positron-emission tomography–computed tomography (FDG-PET/CT), have improved the detection of involved lymph nodes, thus influencing staging accuracy and potentially treatment outcomes. This study is a&#xa0;post hoc analysis of the GAZAI trial data to evaluate the impact of FDG-PET/CT versus computed tomography (CT) alone on radiation target volumes for involved-site radiotherapy (IS-RT) in early-stage follicular lymphoma (FL).</p> Methods <p>All patients in the GAZAI trial underwent pretherapeutic FDG-PET/CT examinations, which were subject to central quality control. Lymph nodes with pathological metabolism were assessed for CT morphology. Differential regional involvement and the impact on radiation target volume for IS-RT were compared between PET/CT-based to solely CT-based staging.</p> Results <p>In 54&#xa0;patients with PET-positive lymph nodes after initial surgery, 170 involved lymph nodes were identified in total. FDG-PET/CT identified additionally involved lymph nodes not detected by CT in 61% of the patients, leading to a&#xa0;significant change in radiation treatment fields for 30% of the cohort. Only 58% of all involved lymph nodes exhibited pathological CT morphology. The findings were robust across different Deauville score thresholds and CT morphological metrics.</p> Conclusion <p>The findings confirm the essential role of FDG-PET/CT in accurately defining the radiation volume for treatment of early-stage follicular lymphomas with radiotherapy. These results support the integration of FDG-PET/CT into the standard diagnostic pathway and its inclusion in the service catalogue of statutory health insurance, emphasizing its importance for optimal treatment planning and the potential impact on patient outcomes.</p>