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

Abstract

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.

Keywords: 18F-FDG; 18F-FLT; PET; Sarcoidosis.

Fig. 1

PET/CT images of a 57-year-old female with cardiac and extra-cardiac thoracic sarcoidosis (patient 1 in Table 1). Transverse ¹⁸F-FDG (a) and ¹⁸F-FLT (b) PET/CT fusion images show increased uptake in the mediastinal and hilar lymph nodes, and focal uptake at the anteroseptal, inferoseptal, and inferolateral myocardium

Fig. 2

The three different patterns of cardiac uptake on transverse ¹⁸F-FDG and ¹⁸F-FLT PET/CT fusion images at cardiac level include focal ¹⁸F-FDG uptake in the basal anteroseptal myocardium, with no corresponding uptake of ¹⁸F-FLT (patient 8 in Table 1) (a), diffuse ¹⁸F-FDG uptake and focal ¹⁸F-FLT uptake in the upper basal anteroseptal myocardium (patient 11 in Table 1) (b), and no ¹⁸F-FDG and ¹⁸F-FLT uptake in the myocardium (patient 14 in Table 1) (c)