Diagnostic impact of PET with ¹⁸F-FDG, ¹⁸F-DOPA and 3-O-methyl-6-[¹⁸F]fluoro-DOPA in recurrent or metastatic medullary thyroid carcinoma

Purpose
In patients with medullary thyroid carcinoma (MTC), rising levels of the tumour markers calcitonin and CEA after primary surgery indicate tumour recurrence or metastases. The only chance of cure is the resection of localised tumour tissue. For positron emission tomography (PET) with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) and ¹⁸F-dihydroxyphenylalanine (¹⁸F-DOPA), sensitivities of 78% and 63% have been reported, but in a considerable percentage of MTC patients the source of tumour marker elevation is not detected. The aim of this retrospective data evaluation was to compare the value of PET with ¹⁸F-FDG, ¹⁸F-DOPA and the amino acid tracer 3-O-methyl-6-[¹⁸F] fluoro-DOPA (¹⁸F-OMFD) in the detection of MTC recurrence.

Methods
Fifteen patients with elevated calcitonin were investigated with PET as part of their individual clinical work-up. All patients underwent ¹⁸F-FDG PET and ¹⁸FDOPA PET, and ten patients underwent ¹⁸F-OMFD PET.

Results
With ¹⁸F-FDG, seven patients showed foci in the neck, mediastinum, upper abdomen or bone. In seven patients, ¹⁸F-DOPA revealed suspicious foci; five of these seven patients showed partially corresponding uptake of ¹⁸F-FDG in the neck and mediastinum. Two of these patients underwent surgery and metastases were verified. With ¹⁸F-OMFD, a small focus in the liver was suspected in one patient without a correlate on ¹⁸F-FDG PET, ¹⁸FDOPA PET or conventional imaging.

Conclusion
¹⁸F-FDG and ¹⁸F-DOPA showed foci that were highly suspicious for local recurrence or metastasis of MTC, although histological verification in these patients with numerous previous surgical interventions was performed in only two patients. The amino acid tracer ¹⁸FOMFD had no diagnostic impact in these patients.