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.