Value of 18F-FDG PET negativity and Tg suppressibility as markers of prognosis in patients with elevated Tg and 131I-negative differentiated thyroid carcinoma (TENIS syndrome).

Abstract

The aim of the study was to investigate the prognostic value of fluorine-18 fluorodeoxyglucose (18F-FDG) PET negativity and thyroglobulin (Tg) suppressibility in differentiated thyroid carcinoma patients with elevated Tg and a negative radioiodine scan. The study population was selected from thyroid cancer patients registered at a large tertiary cancer care center for management and consisted of patients with metastatic thyroid cancer with elevated Tg on follow-up, negative 131I whole-body scan and negative 18F-FDG PET/computed tomography (CT) study. Patients with thyroid carcinoma were subjected to a thyroid-stimulating hormone-stimulated assessment on the basis of a 131I whole-body scan, serum Tg level and whole-body 18F-FDG PET/CT scan for evaluation of metastatic disease burden. The same patients were subjected to a follow-up evaluation of serum Tg and whole-body 18F-FDG PET/CT scan under thyroid-stimulating hormone suppression while on thyroxine sodium. Comparison was also made between the findings of 18F-FDG PET/CT in patients demonstrating suppressible Tg. A total of 40 (25 male and 15 female) patients were included in the study. All patients had a negative whole-body 18F-FDG PET/CT study but had stimulated Tg more than 5 ng/dl (range: 5.1-> 250 ng/ml), indicating the presence of disease. The patients demonstrated variable Tg suppressibility and were classified on the basis of the extent of Tg suppressibility (%Tg suppressibility > 90% in 21 patients; %Tg suppressibility 65-90% in 12 patients; and %Tg suppressibility < 65% in five patients; and no suppressibility in two patients). 18F-FDG PET was normal in all of these patients both on stimulation and on suppression. All patients were asymptomatic during this period. No definite correlation could be established between the status of metastasis or the histopathology and suppressibility of Tg. The average follow-up data available were for more than 3 years in 26 patients (two patients had no Tg suppressibility in this group), for 1-3 years in 10 patients and for less than 1 year in four patients. At the time of analysis in this study the patients were asymptomatic during the aforementioned follow-up periods (based upon follow-up data available). In this study, we observed that 'elevated Tg but normal 18F-FDG PET' exists as a definitive entity in differentiated thyroid carcinoma. On the basis of the studied follow-up, a negative 18F-FDG PET in the setting of elevated Tg level could be regarded as a favorable prognostic indicator to predict symptom-free status during the follow-up period in this group of patients. Suppressibility of Tg (> 65%) is observed in a significant fraction of these patients, which appears to be independent of the status of metastasis or the histopathology. Also patients who show no Tg suppressibility but had a negative 18F-FDG PET/CT scan still had a better prognosis indicated by the disease-free interval in these patients as indicated in our study. Whether there exists any relation between the extent of suppressibility and their long-term outcome requires to be further examined in future prospective studies.