Correlations between FDG-PET/CT findings and pathological risk in thymic epithelial lesions

Objectives The aim of this study was to evaluate the correlation between pathological risk and fluorodeoxyglucose (FDG) uptake, CT findings and clinical stage of FDG-positive thymic epithelial lesions.

Methods Twenty-nine (15 males, 14 females, 27-84 years old) who underwent FDG-PET/CT examinations before any treatment were enrolled. All lesions were FDG-positive and pathologically confirmed as thymic epithelial tumors. The pathological types were confirmed according to the World Health Organization categories, and classified into three subgroups of low risk (LR), high risk (HR) and thymic carcinoma (Ca). We analyzed the maximum standardized uptake value (SUVmax) using FDG-PET, and the size and shape of the lesions, and evaluated the calcification, degenerative changes/necrosis and contrast enhancement pattern on CT. Masaoka stages were also determined.

Results Nine patients were classified as LR, 10 as HR and 10 as Ca. The mean SUVmax values were: LR, 3.6; HR, 3.9; Ca, 7.7. The SUVmax of Ca was significantly higher than those of LR and HR. The mean sizes (mm) were: LR, 38; HR, 60; Ca, 77. The tumor size of Ca was significantly larger than that of LR. The number of patients with the different stages were: Stage I: LR 6, HR 1, Ca 0, Stage II: LR 2, HR 2, Ca 1, Stage III: LR 1, HR 3, Ca 3, Stage IVa: LR 0, HR 4, Ca 0, Stage IVb: LR 0, HR 0, Ca 6. No calcification was seen in LR. All 10 Ca patients had degenerative/necrotic change.

Conclusions Thymic carcinomas have very high FDG uptake, large size, heterogeneous strong contrast enhancement on CT and advanced clinical stage. Low risk thymomas have slight to moderate FDG uptake, round/oval shaped, small size, smooth margin, and can be differentiated from carcinomas by these findings. High risk thymomas have various metabolic and morphologic patterns, and sometimes mimic low risk thymomas or carcinomas. In advanced stage, FDG-PET/CT may be useful for clinical staging, because it can clearly demonstrate pleural dissemination, lymph nodes metastases and distant metastasis such as those to the liver and bone