The Effects of Carbidopa on Uptake of 6-¹⁸F-Fluoro-l-DOPA in PET of Pheochromocytoma and Extraadrenal Abdominal Paraganglioma

Abstract

6-¹⁸F-fluoro-l-3,4-dihydroxyphenylalanine (¹⁸F-DOPA) PET is a useful tool for the detection of certain neuroendocrine tumors, especially with the preadministration of carbidopa, an inhibitor of DOPA decarboxylase. Whether carbidopa also improves ¹⁸F-DOPA PET of adrenal pheochromocytomas and extraadrenal paragangliomas is unknown. The aim of this study was to investigate the sensitivity of ¹⁸F-DOPA PET in the detection of paraganglioma and its metastatic lesions and to evaluate whether tracer uptake by the tumors is enhanced by carbidopa. Methods: Two patients with nonmetastatic adrenal pheochromocytoma, and 9 patients with extraadrenal abdominal paraganglioma (1 nonmetastatic, 8 metastatic), underwent whole-body CT, MRI, baseline ¹⁸F-DOPA PET, and ¹⁸F-DOPA PET with oral preadministration of 200 mg of carbidopa. The dynamics of tracer uptake by these lesions and the physiologic distribution of ¹⁸F-DOPA in normal tissues were recorded. Results: Seventy-eight lesions were detected by CT or MRI, 54 by baseline ¹⁸F-DOPA PET (P = 0.0022 vs. CT/MRI), and 57 by ¹⁸F-DOPA PET plus carbidopa (P = 0.0075 vs. CT/MRI, not statistically significant vs. baseline). In reference to findings on CT and MRI, the sensitivities of baseline ¹⁸F-DOPA PET were 47.4% for lesions and 55.6% for positive body regions, versus 50.0% (lesions) and 66.7% (regions) for ¹⁸F-DOPA PET plus carbidopa (neither is statistically significant vs. baseline). Compared with baseline, carbidopa detected additional lesions in 3 (27%) of 11 patients. Carbidopa increased the mean (±SD) peak standardized uptake value in index tumor lesions from 6.4 ± 3.9 to 9.1 ± 5.6 (P = 0.037). Pancreatic physiologic ¹⁸F-DOPA uptake, which may mask adrenal pheochromocytoma, is blocked by carbidopa. Conclusion: Carbidopa enhances the sensitivity of ¹⁸F-DOPA PET for adrenal pheochromocytomas and extraadrenal abdominal paragangliomas by increasing the tumor-to-background ratio of tracer uptake. The sensitivity of ¹⁸F-DOPA PET for metastases of paraganglioma appears to be limited.