Normal biodistribution pattern and physiologic variants of 18F-DOPA PET imaging

Abstract

Dihydroxyphenylalanine (DOPA) is a neutral amino acid that resembles natural L-dopa (dopamine precursor). It enters the catecholamine metabolic pathway of endogenous L-DOPA in the brain and peripheral tissues. It is amenable to labeling with fluorine-18(F) for PET imaging and was originally used in patients with Parkinson's disease to assess the integrity of the striatal dopaminergic system. The recent introduction and use of hybrid PET/CT scanners has contributed significantly to the management of a series of other pathologies including neuroendocrine tumors, brain tumors, and pancreatic cell hyperplasia. These pathologic entities present an increased activity of L-DOPA decarboxylase and therefore demonstrate high uptake of F-DOPA. Despite these potentially promising applications in several clinical fields, the role of F-DOPA has not been elucidated completely yet because of associated difficulties in synthesis and availability. Unfortunately, the available literature does not provide recommendations for procedures or administered activity, acquisition timing, and premedication with carbidopa. The aim of this paper is to outline the physiological biodistribution and normal variants, including possible pitfalls that may lead to misinterpretations of the scans in various clinical settings.

Figures

Fig. 1

Normal variants of l-6-[18F]fluoro-3,4-dihydroxyphenylalanine (18F-DOPA) uptake in the gallbladder. (a–d) On the left: low-dose computed tomography (CT) transaxial images; on the right: PET images. Images without carbidopa premedication. (a) Intense uptake in the gallbladder. The 18F-DOPA uptake is high in the kidneys as well. Note the focal uptake on the head of the pancreas. (b) Intense uptake in the gallbladder. Note the diffuse 18F-DOPA uptake of the body–tail of the pancreas and the mild uptake of the adrenals, especially the right one. (c) Absent uptake in the gallbladder. (d) Focal intense uptake in the gallbladder that, because of the position, could be misinterpreted as a hepatic lesion. Uptake is high in the kidneys as well. Note the uptake in the head of the pancreas.

Fig. 2

Normal variants of the l-6-[18F]fluoro-3,4-dihydroxyphenylalanine (18F-DOPA) excretion through the biliary tract. Upper series: MIP images of three patients (a–c) with medullary thyroid carcinoma (MTC) (a, c) and paraganglioma (b). Lower series: axial images of the low-dose computed tomography (CT) (on the left) and PET (on the right) of the pathologic uptake of 18F-DOPA. (a) a 78-year-old woman with MTC previously operated upon and with suspicion of relapse in the left paratracheal region well visualized at 18F-DOPA PET/CT; (b) a 68-year-old man with a suspicion of paraganglioma on morphological imaging in the aortic carefour region of the abdomen confirmed on 18F-DOPA PET/CT, which depicted an area of very high radiotracer uptake with an SUVmax of 29.9; (c) a 66-year-old woman with MTC previously operated upon, with increased calcitonin levels and hepatic and nodal metastases. In all of the above images, the variability of 18F-DOPA uptake in the physiological sites and pathologic areas, which are clearly recognized, is noteworthy. Note the very intense uptake in the gallbladder in (a) and (b), and the uptake in the biliary tracts especially in (a) and (c). In (c) note the lesions in the liver as well as the physiologic focal uptake in the gallbladder that could be misinterpreted as another hepatic lesion.

Fig. 3

Right adrenal lesion. From the upper left, clockwise: computed tomography (CT), PET, MIP, and fused PET/CT images of l-6-[18F]fluoro-3,4-dihydroxyphenylalanine (18F-DOPA) PET/CT of a 51-year-old woman presenting with very high urinary normetanephrin levels. 123I-MIBG scintigraphy revealed only a mild to moderate uptake of the tracer in the right adrenal. Instead, 18F-DOPA PET/CT shows a very intense uptake in the right adrenal, consistent with pheochromocytoma. Note the concomitant uptake in the body–tail of the pancreas, and the intense uptake in the gallbladder. Images with no carbidopa premedication.