AbstractOBJECTIVESWe evaluated the potential of preoperative fluorine-18-fluorodeoxyglucose positron emission tomography to predict invasive thymic epithelial tumours in patients with computed tomography-defined clinical stage I thymic epithelial tumours ≤5 cm in size who are generally considered to be candidates for minimally invasive approaches.METHODSFrom January 2012 to July 2022, we retrospectively analysed patients who exhibited tumour-node-metastasis (TNM) clinical stage I thymic epithelial tumours with lesion sizes ≤5 cm as determined by computed tomography. All patients underwent fluorine-18-fluorodeoxyglucose positron emission tomography preoperatively. We analysed the association of maximum standardized uptake values with both the World Health Organization histological classification and the TNM staging classification.RESULTSA total of 107 patients with thymic epithelial tumours (thymomas, 91; thymic carcinomas, 14; carcinoids, 2) were evaluated. Nine patients (8.4%) were pathologically upstaged: TNM pathological stage II in 3 (2.8%), III in 4 (3.7%) and IV in 2 (1.9%). Among these 9 upstaged patients, 5 had thymic carcinoma with stage III/IV, 3 had type B2/B3 thymoma with stage II/III and 1 had type B1 thymoma with stage II. Maximum standardized uptake values were a predictive factor that distinguished pathological stage >I thymic epithelial tumours from pathological stage I [best cut-off value, 4.2; area under the curve = 0.820] and thymic carcinomas from other thymic tumours (best cut-off value, 4.5; area under the curve = 0.882).CONCLUSIONSThoracic surgeons should carefully determine the surgical approach for high fluorodeoxyglucose-uptake thymic epithelial tumours and keep in mind the issues associated with thymic carcinoma and potential combined resections of neighbouring structures.