The effects of molar activity on [ 18 F]FDOPA uptake in patients with neuroendocrine tumors.

Background: 6-[ ¹⁸ F]fluoro-L-3,4-dihydroxyphenyl alanine ([ ¹⁸ F]FDOPA) is a commonly used PET tracer for the detection and staging of neuroendocrine tumors. In neuroendocrine tumors, [ ¹⁸ F]FDOPA is decarboxylated to [ ¹⁸ F]dopamine via the enzyme amino acid decarboxylase (AADC), leading to increased uptake when there is increased AADC activity. Recently, in our hospital, a new GMP compliant multi-dose production of [ ¹⁸ F]FDOPA has been developed, [ ¹⁸ F]FDOPA-H, resulting in a higher activity yield, improved molar activity and a lower administered mass than the conventional method ([ ¹⁸ F]FDOPA-L).
Aims: This study aimed to investigate whether the difference in molar activity affects the [ ¹⁸ F]FDOPA uptake at physiological sites and in tumor lesions, in patients with NET. It was anticipated that the specific uptake of [ ¹⁸ F]FDOPA-H would be equal to or higher than [ ¹⁸ F]FDOPA-L.
Methods: We retrospectively analyzed 49 patients with pathologically confirmed NETs and stable disease who underwent PET scanning using both [ ¹⁸ F]FDOPA-H and [ ¹⁸ F]FDOPA-L within a time span of 5 years. A total of 98 [ ¹⁸ F]FDOPA scans (49 [ ¹⁸ F]FDOPA-L and 49 [ ¹⁸ F]FDOPA-H with average molar activities of 8 and 107 GBq/mmol) were analyzed. The SUVmean was calculated for physiological organ uptake and SUVmax for tumor lesions in both groups for comparison, and separately in subjects with low tumor load (1-2 lesions) and higher tumor load (3-10 lesions).
Results: Comparable or slightly higher uptake was demonstrated in various physiological uptake sites in subjects scanned with [ ¹⁸ F]FDOPA-H compared to [ ¹⁸ F]FDOPA-L, with large overlap being present in the interquartile ranges. Tumor uptake was slightly higher in the [ ¹⁸ F]FDOPA-H group with 3-10 lesion (SUVmax 6.83 vs. 5.19, p < 0.001). In the other groups, no significant differences were seen between H and L.
Conclusion: [18F]FDOPA-H provides a higher activity yield, offering the possibility to scan more patients with one single production. Minor differences were observed in SUV's, with slight increases in uptake of [ ¹⁸ F]FDOPA-H in comparison to [ ¹⁸ F]FDOPA-L. This finding is not a concern for clinical practice, but could be of importance when quantifying follow-up scans while introducing new production methods with a higher molar activity of [ ¹⁸ F]FDOPA.
(© 2021. The Author(s).)