Tumor Immunity is Related to 18F-FDG Uptake in Thymic Epithelial Tumor

Background2-Deoxy-2-[fluorine-18]-fluoro-d-glucose (18F-FDG) positron emission tomography (18F-FDG-PET) is a convenient modality to assess metabolic activity within tumor cells. However, there is no consensus regarding the relationship between 18F-FDG uptake and the immune environment in thymic epithelial tumors (TETs). We conducted a clinicopathological study to elucidate the relationship between 18F-FDG uptake and programmed death ligands 1 and 2 (PD-L1/PD-L2) expression in patients with TETs. MethodsA total of 108 patients with histologically confirmed TETs classified as thymomas or thymic carcinomas who underwent surgical resection or biopsy or needle biopsy and 18F-FDG PET before any treatment between August 2007 and March 2020 were enrolled in this study. Tumor specimens underwent immunohistochemical staining for PD-L1, PD-L2, GLUT1, HIF-1α, VEGFR2, VEGF-C and β2 adrenergic receptor. ResultsHigh uptakes of SUVmax, SUVmean, MTV and TLG were identified in 28 (25.9%), 61 (56.5%), 55 (50.9%) and 55 (50.9%) of 108 patients, respectively. High uptake of SUVmax significantly correlated with PS of 1-2, thymic carcinoma and advanced stage, and SUVmax on 18F-FDG uptake displayed a close association with PD-L1 and PD-L2 expression, but not MTV and TLG. Multivariate analysis revealed that SUVmax was identified as an independent predictor for positive PD-L1 /PD-L2 expression. GLUT1 was clarified as a significant marker for the expression of PD-L1/PD-L2 from the biological viewpoint. Conclusion18F-FDG accumulation was closely associated with the expression of PD-L1/PD-L2, which, in turn, was correlated with glucose metabolism and hypoxia. PD-L1/PD-L2 could affect glucose metabolism and hypoxia in thymic tumor cells.