18F‐fluorodeoxyglucose and 11C‐acetate positron emission tomography are useful modalities for diagnosing the histologic type of thymoma

BACKGROUND: The objective of this study was to clarify the usefulness of positron emission tomography (PET) using(18)F-fluorodeoxyglucose (FDG) and carbon 11-labeled acetate (AC) for predicting the histologic types and tumor invasiveness of thymoma in a multicenter study. METHODS: Forty thymomas were examined using both FDG-PET and AC-PET before surgery. The histologic types were type A in 1 thymoma, type AB in 12 thymomas, type B1 in 11 thymomas, type B2 in 7 thymomas, type B3 in 6 thymomas, and type C in 3 thymomas. Tumor invasiveness was assessed by pathologic tumor stage and was identified as stage I in 17 tumors, stage II in 17 tumors, stage III in 4 tumors, and stage IV in 2 tumors. FDG and AC uptake was measured as the maximum standard uptake value (SUV). RESULTS: The FDG-SUV in type C thymomas was significantly higher than that in the other types (A-B3; P = .001 - P = .048). The AC-SUV in type A/AB thymomas was significantly higher than that in the other tumor types (B1-C; P < .001 - P = .002). All 3 type C tumors had an FDG-SUV >or=6.3, and all 13 type A/AB tumors had an FDG-SUV <6.3 and an AC-SUV >or=5.7. All 17 thymomas that had an FDG-SUV <6.3 and an AC-SUV <5.7 were type B1, B2, or B3. Neither the FDG-SUV nor the AC-SUV differed significantly between the stages I/II tumors and stage III/IV tumors. CONCLUSIONS: Although neither the FDG-SUV nor the AC-SUV can predict the invasiveness of thymomas assessed by tumor stage, they are useful for predicting histologic types of thymoma. Thymomas with an FDG-SUV <6.3 and an AC-SUV >or=5.7 almost certainly are types A/AB, which is of considerable prognostic and management significance.