sup 18^F-FDG and ^sup 18^F-FLT PET/CT imaging in the characterization of mediastinal lymph nodes

Purpose There is currently no single modality for accurate characterization of enlarged mediastinal lymph nodes into benign or malignant. Recently ^sup 18^F-fluorothymidine (FLT) has been used as a proliferation marker. In this prospective study, we examined the role of ^sup 18^F-fluorodeoxyglucose (^sup 18^F-FDG) positron emission tomography/computed tomography (PET/CT) and ^sup 18^F-FLT PET/CT in categorizing mediastinal lymph nodes as benign or malignant. Materials and methods A total of 70 consecutive patients with mediastinal lymphadenopathy detected on computed tomography (CT) or chest radiograph underwent whole body ^sup 18^F-FLT PET/CT and ^sup 18^F-FDG PET/CT (within 1 week of each other). Lymph nodal tracer uptake was determined by calculation of standardized uptake value (SUV) with both the tracers. Results of PET/CT were compared with histopathology of the lymph nodes. Results Histopathology results showed thirty-seven patients with sarcoidosis, seven patients with tuberculosis, nine patients with non-small cell lung cancer, five patients with Hodgkin's lymphoma and twelve patients with non-Hodgkin's lymphoma. The mean FDG SUV^sub max^ of sarcoidosis, tuberculosis, Hodgkin's and non-Hodgkin's lymphoma was 12.7, 13.4, 8.2, and 8.8, respectively, and the mean FLT SUV^sub max^ was 6.0, 5.4, 4.4, and 3.8, respectively. It was not possible to characterize mediastinal lymphadenopathy as benign or malignant solely based on FDG SUV^sub max^ values (p > 0.05) or FLT SUV^sub max^ values (p > 0.05). There was no significant difference in FDG uptake (p > 0.9) or FLT uptake (p > 0.9) between sarcoidosis and tuberculosis. In lung cancer patients, the FDG SUV^sub max^ and FLT SUV^sub max^ of those lymph nodes with tumor infiltration on biopsy was 6.7 and 3.9, respectively, and those without nodal infiltration was 6.4 and 3.7, respectively, and both the tracers were not able to characterize the nodal status as malignant or benign (p > 0.05). Conclusion Though ^sup 18^F-FLT PET/CT and ^sup 18^F-FDG PET/CT reflect different aspects of biology, i.e., proliferation and metabolism, respectively, neither tracer could provide satisfactory categorization of benign and malignant lymph nodes. The results of this study clearly suggest that differentiation of mediastinal nodes into benign and malignant solely based on SUV^sub max^ values cannot be relied upon, especially in settings where tuberculosis and sarcoidosis are common.