Diagnostic Value of 18F-Fluorocholine PET/CT for the Detection of Medullary Thyroid Cancer

Medullary thyroid cancer (MTC) is a relatively rare type of cancer that represents up to 10% of all primary thyroid cancers. It is a neuroendocrine tumour derived from parafollicular C-cells of the thyroid. It occurs either sporadically or in a hereditary form as a component of the type 2 multiple endocrine neoplasia (MEN) syndromes, MEN2A and MEN2B, and the related syndrome, familial MTC (FMTC).

Currently, the diagnosis of MTC is suspected based on the results of fine-needle aspiration (FNA) cytology, immunohistochemical analysis and elevated laboratory values of tumour markers calcitonin (Ctn) and carcinoembryonic antigen (CEA). According to the 2015 ATA guidelines, the preoperative imaging workup in all patients should include ultrasound examination of the neck; in selected patients, contrast-enhanced CT of the neck and chest, three-phase contrast-enhanced multi-detector liver CT, or contrast-enhanced MRI of the liver, and axial MRI and bone scintigraphy is also recommended. The curative therapy of choice is surgical removal of the tumour and/or metastases.

Nodal metastases are detected in 35-50% and distal metastases in about 15% of patients with primary MTC. Even with currently recommended diagnostic imaging techniques, about 50% of patients have persistent/recurrent disease after surgical treatment. This implies that currently available diagnostic imaging studies are suboptimal for accurate disease staging. New hybrid molecular imaging techniques based on SPECT/CT and especially PET/CT could improve disease detection by visualising pathophysiological processes in vivo. The most studied PET radiopharmaceutical for MTC imaging to date has been 18F-FDOPA, with recent studies focusing also on somatostatin receptor imaging using 68Ga-DOTATATE/TOC/NOC radiotracers.

18F-fluorocholine is a structural analogue of choline. It accumulates in cells with active membrane synthesis and overexpressed intracellular signal transduction, processes that are overactive in benign and malignant neoplasms. 18F-fluorocholine is currently primarily used for prostate cancer imaging. In contrast to radiotracers such as18F-fluorodeoxyglucose (18F-FDG), it is also taken up by well-differentiated neoplasms in which 18F-FDG uptake is unreliable. Similarly to 18F-FDG, 18F-fluorocholine is also known to accumulate in inflamed and infected tissue. However, this limitation could be overcome by performing multi-time-point imaging and using basic kinetic analysis. The working hypothesis is that 18F-fluorocholine might be efficiently taken up by primary MTC tumour as well as loco-regional and distant metastases.

The aim of the trial is to investigate the diagnostic accuracy of 18F-fluorocholine PET/CT in comparison to existing imaging modalities (US, CT and MRI).