18F-Fluorodihydroxyphenylalanine PET/CT in Patients with Neuroendocrine Tumors of Unknown Origin: Relation to Tumor Origin and Differentiation

This work was performed to evaluate the performance of F-18-fluorodihydroxyphenylalanine (F-18-FDOPA) PET/CT in detecting primary neuroendocrine tumors (NETs) occult on morphologic and functional imaging, in relation to tumor origin and differentiation. Methods: A retrospective study of NET patients who were investigated with F-18-FDOPA PET/CT imaging in 2 academic endocrine tumor centers was conducted. Only patients with negative conventional and somatostatin receptor scintigraphy (SRS) results were studied. Results: Twenty-seven patients were evaluated with F-18-FDOPA PET/CT, 23 at their initial staging and 4 during their follow-up. The primary occult NET was localized by F-18-FDOPA PET/CT in 12 patients (overall sensitivity, 44%; 52% in patients evaluated at initial diagnosis), leading to tumor resection in all cases. The primary tumors were distributed and graded as follows: 1 duodenum G2 lesion, 7 ileum G2 lesions, 2 terminal ileum 01 lesions, 1 pancreas 02 lesion, and 1 gallbladder G3 lesion. Patients with positive F-18-FDOPA PET/CT results had higher values of serum chromogranin A (100% vs. 20%, P = 0.0003), serotonin, or urinary 5-hydroxyindolacetic acid (83% vs. 20%, P = 0.003). Two false-negative results were related to poorly differentiated duodenal and prostatic NETs (G3). F-18-FDOPA PET/CT showed more metastatic anatomic regions than SRS in 17 patients. Conclusion: F-18-FDOPA PET appears to be a sensitive functional imaging tool for the detection of primary NETs occult on SRS, especially tumors with a well-differentiated pattern and serotonin secretion.