Direct comparison of super(68)Ga-DOTA-TOC and super(18)F-FDG PET/CT in the follow-up of patients with neuroendocrine tumour treated with the first full peptide receptor radionuclide therapy cycle

To determine the value of super(68)Ga-DOTA-TOC and super(18)F-FDG PET/CT for initial and follow-up evaluation of patients with neuroendocrine tumour (NET) treated with peptide receptor radionuclide therapy (PRRT). We evaluated 66 patients who had histologically proven NET and underwent both PRRT and three combined super(68)Ga-DOTA-TOC and super(18)F-FDG PET/CT studies. super(68)Ga-DOTA-TOC PET/CT was performed before PRRT, 3 months after completion of PRRT and after a further 6 - 9 months. super(18)F-FDG PET/CT was done within 2 months of super(68)Ga-DOTA-TOC PET/CT. Follow-up ranged from 11.8 to 80.0 months (mean 34.5 months). All patients were super(68)Ga-DOTA-TOC PET-positive initially and at follow-up after the first full PRRT cycle. Overall, 62 of the 198 super(18)F-FDG PET studies (31 %) were true-positive in 38 of the 66 patients (58 %). Of the 66 patients, 28 (5 grade 1, 23 grade 2) were super(18)F-FDG-negative initially and during follow-up (group 1), 24 (5 grade 1, 13 grade 2, 6 grade 3) were super(18)F-FDG-positive initially and during follow-up (group 2), 9 patients (2 grade 1, 6 grade 2, 1 grade 3) were super(18)F-FDG-negative initially but super(18)F-FDG-positive during follow-up (group 3), and 5 patients (all grade 2) were super(18)F-FDG-positive initially but super(18)F-FDG-negative during follow-up (group 4). super(18)F-FDG PET showed more and/or larger metastases than super(68)Ga-DOTA-TOC PET in five patients of group 2 and four patients of group 3, all with progressive disease. In three patients with progressive disease who died during follow-up tumour SUVmax increased by 41 - 82 % from the first to the last follow-up investigation. In NET patients, the presence of super(18)F-FDG-positive tumours correlates strongly with a higher risk of progression. Initially, patients with super(18)F-FDG-negative NET may show super(18)F-FDG-positive tumours during follow-up. Also patients with grade 1 and grade 2 NET may have super(18)F-FDG-positive tumours. Therefore, super(18)F-FDG PET/CT is a complementary tool to super(68)Ga-DOTA-TOC PET/CT with clinical relevance for molecular investigation.