68 Ga-DOTANOC and 18 F-FDG PET/CT in metastatic medullary thyroid carcinoma: novel correlations with tumoral biomarkers.

OBJECTIVE: Metastatic disease is common in medullary thyroid carcinoma (MTC) and it is usually detected by raising calcitonin and carcinoembryonic antigen (CEA) levels. Nuclear medicine imaging has an important role in lesion identification/characterisation. We aim to compare 68 Ga-DOTANOC PET/CT and 18 F-FDG PET/CT performance and to explore the correlations between tumoral markers and functional imaging.

METHODS: This a retrospective cross-sectional study including 13 patients with MTC and high calcitonin/CEA levels that underwent both 68 Ga-DOTANOC PET/CT and 18 F-FDG PET/CT.

RESULTS: 68 Ga-DOTANOC PET/CT identified MTC metastases in 2twopatients that were 18 F-FDG-negative (sensitivity of 69.2% vs. 53.9%, respectively). 68 Ga-DOTANOC PET/CT also detected a higher number of lesions than 18 F-FDG PET/CT in seven patients, with only one patient showing the opposite pattern. Both differences lacked statistical significance (p = 0.50 and p = 0.86, respectively) but 68 Ga-DOTANOC PET/CT better performance allowed changes in patients' management. 68 Ga-positive/18 F-FDG-negative patients were the ones with the lowest calcitonin doubling time and presented a CEA doubling time >24 months, while the patient with more 18 F-FDG-positive lesions was the one with the highest CEA/calcitonin ratio. The number of lesions found in 68 Ga-DOTANOC PET/CT were correlated with calcitonin levels (r = 0.73; p < 0.01) but not with CEA ones (r = 0.42; p = 0.15). The number of 18 F-FDG hypermetabolic focus were correlated with CEA levels (r = 0.60; p < 0.05) but not with calcitonin (r = 0.48; p = 0.09).

CONCLUSIONS: This is the first study to describe a positive correlation between 68 Ga-positive lesions and calcitonin levels and between 18 F-FDG-positivity and CEA levels. Tumoral markers pattern in metastatic MTC could help clinicians to decide which exam to perform first.