18F-FDG-PET/CT in Patients Affected by Differentiated Thyroid Carcinoma with Positive Thyroglobulin Level and Negative 131I Whole Body Scan. It's Value Confirmed by a Bicentric Experience

Background: 18F-FDG-PET/CT applications in the field of differentiated thyroid cancer (DTC) are continuously growing. Objective: To retrospectively analyze in a wide population enrolled in two nuclear medicine centers the diagnostic value of 18F-FDG-PET/CT in patients treated with 131I and negative 131I-WBS but in presence of Tg levels higher than 1ng/ml after TSH stimulation. Method: From September 2005 to December 2014, 154 patients affected by DTC, treated with 131I with negative 131I-WBS and Tg≥1ng/ml underwent 18F-FDG-PET/CT. Results: 66 patients (43%) had a negative 18F-FDG-PET/CT (mean Tg=7.7ng/ml; SD=9.6) and 88 (57%) a positive scan (mean Tg=169.4; SD=294.6) documenting bone, pulmonary, lymph-node metastases and local recurrences. There was no statistically significant correlation between 18F-FDGPET/ CT results and sex, previous lymphadenectomy, the “on-” or “off-therapy” state; on the contrary, there was a statistically significant correlation between PET results and primary tumor size (T1–T2 vs. T3–T4). ROC analysis showed that the highest accuracy in terms of best compromise between sensitivity and specificity (sensitivity= 71,6%; specificity=92,4%; AUC area under the curve= 0.807) is achieved when the 12ng/mL is considered the Tg cut-off value to interpret the results. Conclusion: Our study confirm in a wide population that 18F-FDG-PET/CT is useful in evaluating patients affected by DTC, negative 131I-WBS and Tg>1ng/ml, identifying 12ng/ml as the Tg level cut-off over which is better to perform the study.