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

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