Spectrum of thymic uptake at 18F-FDG PET.

Fluorine-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) has emerged as a strong diagnostic tool for the diagnosis and staging of neoplasms. Uptake in the thymus at (18)F-FDG PET complicates the assessment of mediastinal involvement by tumor in children and young adults. Increased thymic (18)F-FDG uptake may represent normal physiologic uptake but may also indicate the presence of thymic hyperplasia, lymphomatous infiltration, primary thymic neoplasm, or metastatic disease. Familiarity with the patterns of (18)F-FDG uptake that characterize these pathologic conditions is crucial to the interpretation of PET findings in the thymus. In addition, awareness of the subsets of patients in whom physiologic uptake may be seen and of the normal morphologic features and (18)F-FDG PET appearance of the thymus, along with a general sense of the upper limits of metabolic activity for physiologic thymic uptake, will aid in differentiating between physiologic thymic uptake and mediastinal disease. In equivocal cases, correlation with morphologic data from computed tomography or magnetic resonance imaging will likely continue to play a key role in diagnosis and will aid in differentiating benign thymic uptake from malignancy. (c) RSNA, 2004.