Comparative evaluation of 18F-FLT and 18F-FDG for detecting cardiac and extra-cardiac thoracic involvement in patients with newly diagnosed sarcoidosis

BACKGROUND 18F-FDG PET has been used in sarcoidosis for diagnosis and determination of the extent of the disease. However, assessing inflammatory lesions in cardiac sarcoidosis using 18F-FDG can be challenging because it accumulates physiologically in normal myocardium. Another radiotracer, 3'-deoxy-3'-18F-fluorothymidine (18F-FLT), has been investigated as a promising PET tracer for evaluating tumor proliferative activity. In contrast to 18F-FDG, 18F-FLT uptake in the normal myocardium is low. The purpose of this retrospective study was to compare the uptake of 18F-FLT and 18F-FDG in the evaluation of cardiac and extra-cardiac thoracic involvement in patients with newly diagnosed sarcoidosis. Data for 20 patients with newly diagnosed sarcoidosis were examined. 18F-FLT and 18F-FDG PET/CT studies had been performed at 1 h after each radiotracer injection. The patients had fasted for at least 18 h before 18F-FDG PET/CT but were given no special dietary instructions regarding the period before 18F-FLT PET/CT. Uptake of 18F-FLT and 18F-FDG was examined visually and semiquantitatively using maximal standardized uptake value (SUVmax). RESULTS Two patients had cardiac sarcoidosis, 7 had extra-cardiac thoracic sarcoidosis, and 11 had both cardiac and extra-cardiac thoracic sarcoidosis. On visual analysis for diagnosis of cardiac sarcoidosis, 4/20 18F-FDG scans were rated as inconclusive because the 18F-FDG pattern was diffuse, whereas no FLT scans were rated as inconclusive. The sensitivity of 18F-FDG PET/CT for detection of cardiac sarcoidosis was 85%; specificity, 100%; and accuracy, 90%. The corresponding values for 18F-FLT PET/CT were 92, 100, and 95%, respectively. Using semiquantitative analysis of cardiac sarcoidosis, the mean 18F-FDG SUVmax was significantly higher than the mean 18F-FLT SUVmax (P < 0.005). Both 18F-FDG and 18F-FLT PET/CT studies detected all 24 extra-cardiac lesions. Using semiquantitative analysis of extra-cardiac sarcoidosis, the mean 18F-FDG SUVmax was significantly higher than the mean 18F-FLT SUVmax (P < 0.001). CONCLUSIONS The results of this preliminary study suggest that 18F-FLT PET/CT can detect cardiac and extra-cardiac thoracic involvement in patients with newly diagnosed sarcoidosis as well as 18F-FDG PET/CT, although uptake of 18F-FLT in lesions was significantly lower than that of 18F-FDG. However, 18F-FLT PET/CT may be easier to perform since it requires neither prolonged fasting nor a special diet prior to imaging.