Differential diagnosis of thymic tumors using a combination of C-11-methionine PET and FDG PET

We assessed the usefulness of PET studies in making a differential diagnosi s of thymic tumors by using C-11-methionine (MET) and F-18-fluorodeoxygluco se (FDG). Methods: We examined 31 patients with thymic tumors, including 14 patients with thymic cancer, 9 with invasive thymoma, 5 with noninvasive t hymoma and 3 with thymic cysts. The histological diagnosis was confirmed by either surgery or biopsy. MET PET and FDG PET were performed in 28 and 29 patients, respectively. Both the MET and FDG uptakes were evaluated by the standardized uptake value (SUV). Results: MET uptake was not substantially different among thymic cancer (4.8 +/- 1.4), invasive thymoma (4.3 +/- 1.1) and noninvasive thymoma (4.5 +/- 1.2), but MET uptake in thymic cysts (0.9 +/- 0.1) was lower than that in the other three tumors (P < 0.01). The FDG uptake in thymic cancer (7.2 +/- 2.9) was higher than that in invasive thy moma (3.8 +/- 1.3), noninvasive thymoma (3.0 +/- 1.0) and thymic cysts (0.9 ) (P < 0.01). MET uptake in thymic tumors correlated with the FDG uptake (r = 0.65), whereas MET uptake in thymic cancer was tower than FDG uptake (FD G/MET ratio = 1.52 +/- 0.52) but was higher than FDG uptake in both invasiv e and noninvasive thymoma (FDG/MET ratio = 0.86 +/- 0.33), To differentiate thymic cancer from thymoma, a receiver operating characteristic (ROC) anal ysis was performed. The area under the curve of FDG PET was 0.90, whereas t he FDG/MET ratio was 0.87. Conclusion: The MET PET, FDG PET and the FDG/MET ratios were unable to differentiate benign thymic tumors from malignant on es, although FDG PET was considered to be useful in the differential diagno sis between thymic cancer and thymoma. Although the difference in the uptak e ratio between FDG and MET suggests a different origin of the tumors, the FDG/MET ratio is not considered to be useful as a complementary method for the differential diagnosis of thymic tumors.