P1.17-017 Usefulness of FDG-PET for Differentiating Thymic Epithelial Tumors from Malignant Lymphomas

Abstract

It is difficult to diagnose the tumor in the anterior mediastinum by computed tomography. Distinguishing between thymic epithelial tumors and malignant lymphoma is important, because therapeutic strategy is difficult in each disease. The objective of this study was to clarify the usefulness of positron emission tomography (PET) using 18F-fluorodeoxyglucose (FDG) for distinguishing thymic epithelial tumors and malignant lymphoma. We retrospectively reviewed FDG PET-CT scans of 62 patients pathologically diagnosed by surgery or biopsy as thymic epithelial tumors or malignant lymphoma. FDG uptake was measured as the maximum standard uptake value (SUVmax). Student t tests were used to assess association between SUVmax and pathological diagnosis. Among the 62 patients, 36 patients had a pathological diagnosis of thymoma: WHO classification type A in 3 patients (11%), type AB in 9 patients (19%), type B1 in 6 patients (19%), type B2 in 15 patients (42%), and type B3 in 3 patients (7%). Eleven patients had the thymic carcinoma. Fifteen patients had the malignant lymphoma. The SUVmax in malignant lymphoma (14.9 ± 6.4) was significantly higher than that in the thymic epithelial tumors (5.1 ± 2.5) (p＜0.001). The SUVmax in thymic carcinoma (7.8 ± 3.2) was higher than that in the thymoma (4.0 ± 1.5) (p=0.002). The ROC curve of SUVmax for predicting malignant lymphoma indicated that the optimal cutoff value was 7.3. This value had a sensitivity of 0.89 and a specificity of 0.87. FDG PET-CT is helpful for distinguishing malignant lymphoma from thymic epithelial tumors with cut off value of 7.3.