Impact of ¹⁸F-FDG PET/CT for detecting recurrence of medullary thyroid carcinoma.

Objective: The aim of this study was to evaluate the value of fluorine-18 (¹⁸F) fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in the detection of recurrent medullary thyroid carcinoma (MTC) in patients with elevated calcitonin levels.

Methods: Thirty-three patients (nine men, 24 women; mean age: 50.3 ± 12 years) who were referred to undergo ¹⁸F-FDG PET/CT for restaging of MTC in patients with high calcitonin levels were included in this study. Five patients also had suspected lymph nodes detected by neck ultrasonography. The results of ¹⁸F-FDG PET/CT and clinical follow-up data were reviewed retrospectively. Histological analysis has been accepted as the gold standard in the confirmation of ¹⁸F-FDG PET/CT results. Patients were followed up for 45.6 ± 4.2 months.

Results: There were 14 negative and 19 positive scans for possible recurrence of MTC. In the positive scans, the possible recurrence sites were neck lymph nodes, thyroid bed, mediastinal lymph nodes, and the lung in 14, two, two, and one patient, respectively. Disease recurrence in 13 patients was confirmed histologically by surgical excision or fine-needle aspiration biopsy. In the remaining six patients, recurrence was excluded as it was reactive as a result of pathological examination. However, one patient had a negative scan, underwent neck lymph node excision after ¹⁸F-FDG PET/CT examination, and lymph node recurrence was detected histologically. According to these results, the sensitivity and specificity of PET/CT were calculated as 93 and 68%, respectively. According to the recommended calcitonin level by the American Thyroid Association (calcitonin levels higher than 150 pg/ml), sensitivity was calculated as 90%. Although the mean maximum standardized uptake values of the true-positive and false-positive groups were calculated as 4.72 ± 2.17 and 4.22 ± 1.02, respectively, the difference between the two groups was not statistically significant (P>0.05).

Conclusion: PET/CT is a sensitive imaging tool in the detection of MTC recurrence, especially in patients with high calcitonin levels, and it gives additional information in one third of all patients on an average by detecting an occult disease or confirming findings of other imaging tools.