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Clinical Investigations |
1 Service Hospitalier Frédéric Joliot, Département de Recherche Médicale, Direction des Sciences du Vivant, Commissariat à lEnergie Atomique, Orsay, France
2 Département de Métabolisme et Pédiatrie, Hôpital Necker-Enfants Malades, Paris, France
3 Service de Radiologie Pédiatrique, Hôpital Necker-Enfants Malades, Paris, France
4 Laboratoire de Anatomopathologie, Hôpital Necker-Enfants Malades, Paris, France
5 Département de Chirurgie Infantile, Hôpital Necker-Enfants Malades, Paris, France
| ABSTRACT |
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Key Words: hyperinsulinism of infancy PET 18F-fluoro-L-DOPA
| INTRODUCTION |
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Control of HI is attempted through medical treatment with diazoxide, nifedipine, or octreotide (1315), but pancreatectomy is the only issue for patients who are resistant to these treatments (10,16). Therefore, the differential diagnosis between the 2 forms becomes of major importance since their surgical treatment and the outcome differ considerably. Focal HI is totally cured by selective resection of the adenoma, whereas diffuse forms of HI require a subtotal pancreatectomy, with severe iatrogenic diabetes as a consequence (17,18).
The localization of insulin hypersecretion before surgery is possible only through pancreatic venous catheterization (PVS), allowing a pancreatic map of insulin concentrations, with an eventual additional pancreatic arterial calcium stimulation (1921). PVS is an invasive method, which is technically difficult to perform and requires general anesthesia. The concentrations of plasmatic glucose must be maintained between 2 and 3 mmol/L before and during PVS. Moreover, all medical treatments must be stopped 5 d before the study. Therefore, it is of major interest to find a less invasive way to differentiate between focal and diffuse HI. This method should precisely localize the pathologic area of focal HI to guide the surgeon.
L-Dihydroxyphenylalanine (L-DOPA) is a precursor of catecholamines that is converted to dopamine by the aromatic amino acid decarboxylase (AADC). In addition to its role as a precursor of noradrenaline and adrenaline, dopamine is a transmitter substance in the central and peripheral nervous system. The capacity to take up and decarboxylate amine precursors such as L-DOPA and 5-hydroxytryptophan and to store their biogenic amine (dopamine and serotonin) is characteristic of neuroendocrine cells.
Pancreatic cells contain markers usually associated with neuroendocrine cells, such as tyrosine hydroxylase, dopamine, neuronal dopamine transporter, vesicular dopamine transporter, and monoamine oxidases A and B (2224). Pancreatic islets have been shown to take up L-DOPA and convert it to dopamine through the AADC (2527).
PET performed with 18F-fluoro-L-dihydroxyphenylalanine (18F-fluoro-L-DOPA) has been extensively used to study the central dopaminergic system. Nevertheless, several recent studies have demonstrated the usefulness of this radiotracer to detect neuroendocrine tumors such as pheochromocytomas, thyroid medullar carcinomas, or gastrointestinal carcinoid tumors that usually contain secretory granules and have the ability to produce biogenic amines (28,29).
The aim of this work was to evaluate the use of whole-body PET with 18F-fluoro-L-DOPA to detect the hyperfunctional pancreatic islet tissue and to test its ability to differentiate between focal and diffuse HI.
| MATERIALS AND METHODS |
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-hydrazino-
-methyl-ß-(3,4-dihydroxyphenyl)propionic acid or carbidopa). Another patient had a PET study 72 h after drug withdrawal and another PET study after administration of octreotide and diazoxide. During all PET studies, normoglycemia was maintained by glucose infusion, which was carefully adjusted according to frequent blood glucose monitoring. Maximal glucose infusion rates between 6.4 and 13.2 mg/kg/min were needed. PET acquisition was performed under light sedation (pentobarbital associated with or without chloral). The 5 patients for whom 18F-fluoro-L-DOPA uptake results strongly suggested focal HI and the patients with diffuse HI resistant to medical treatment (n = 4/10) underwent surgery. Pancreatic tissue obtained from surgical resections was fixed in formalin and embedded in paraffin, and serial sections were studied by immunohistochemistry after a water bath antigen retrieval step. The primary antibodies used were antiproinsulin (1/400 mouse monoclonal antibody, 1G4; Novocastra), antichromogranin A (1/200 mouse monoclonal antibody DAK-A3; DAKO), antisynaptophysin (1/50 rabbit polyclonal antibody A0010; DAKO), and anti-DOPA decarboxylase or anti-AADC (1/100 rabbit polyclonal antibody; Chemicon International).
Data Acquisition
MRI.
Six patients underwent MRI of the abdomen before or after the PET study, using a 1.5-T imager (Signa; General Electric). T1-weighed SPGR (spoiled gradient acquisition at the steady state) acquisition with inversion recovery was performed to allow 3-dimensional (3D) reconstruction of MR images. MRI was used to reveal potential signal abnormalities in the pancreas and to allow the coregistration between PET and MRI.
PET.
The PET studies were performed using an ECAT EXACT HR+ scanner (Siemens/CTI) that collects 63 simultaneous 2.4-mm-thick slices with an intrinsic in-plane resolution of 4.3 mm. The patients were placed in supine position in the tomograph using a 3D laser alignment. To ensure the optimal position in the scanner and to avoid movement artifacts, the children were comfortably immobilized during the study acquisition by placing them in a vacuum mattress. The synthesis of 18F-fluoro-L-DOPA followed a previously described electrophilic procedure (30). Intravenous bolus injection of 18F-fluoro-L-DOPA (a mean of 4.0 MBq/kg weight) was done 3050 min before transmission acquisition.
Tissue attenuation was measured using three 68Ge rod sources (approximately 450 MBq). Transmission scans (2-dimensional acquisition mode) lasted 2.5 min per bed position (field of view [FOV] of 15 cm), with 2 or 3 steps, according to the height of the patient, from the neck to the hip. After segmentation, the transmission scans were used for subsequent correction of attenuation of emission scans. Thoraxabdomen emission scans (3D acquisition mode) starting 4565 min after the radiotracer injection (2.5-min step acquisition, 2 or 3 steps for 1 scan) were acquired over 30 min.
Data Analysis
The emission sets were corrected for scatter using a model-based correction, allowing the simulation of the map of single scatter events. The images were reconstructed using an attenuation-weighted ordered-subset expectation maximization iterative algorithm with 4 iterations and 6 subsets. The final spatial resolution in reconstructed images was approximately 6.0 mm.
The reconstructed images were evaluated in a 3D display using axial, coronal, and sagittal views to visualize the pancreas, which always presented a high uptake of 18F-fluoro-L-DOPA, and to distinguish it from the surrounding organs in the abdomen.
For each patient, all thoraxabdomen emission scans were assembled with bed position overlap, and this integrated image was used to define regions of interest (ROIs) over the pancreas, liver, kidneys, and lungs. The mean activity concentration value in each ROI was calculated and used to generate regional timeactivity curves. These curves were used to evaluate the contrast between the pancreas and other tissues and the biologic clearance of 18F-fluoro-L-DOPA.
The mean activity concentration measured in each ROI, 60 min after injection, was used to calculate standardized uptake values (SUVs). The mean radioactivity concentration in each ROI was divided by the injected dose of 18F-fluoro-L-DOPA (corrected to the beginning of the emission acquisition) and the body weight. Based on visual analysis, the patients HI was classified in 2 groups, focal or diffuse, and quantitatively compared using the SUVs. Comparisons were done using an ANOVA test for repeated measures.
The assembled image was also used to achieve the coregistration with MRI slices. With regard to MRI acquisitions, a specific FOV was determined to include the pancreas, liver, and kidneys, which were the only organs visible in PET images in the abdominal area. A corresponding FOV was then extracted from PET images. Due to the low contrast observed between kidneys and surrounding organs on MR images, an enhancement of the gray level intensity of these structures based on a manual segmentation of the kidneys was performed. Volume-based coregistration of PET and enhanced MR images were done using mutual information as the matching criterion (3133). Global rigid transformation was considered for the spatial alignment (34). The coregistration task was evaluated visually using a fusion mode taking into account the superimposition of the liver and the kidneys in both modalities (35). Finally, the improvement of pancreatic uptake targeting on the pancreas was achieved by fusing anatomic MRI and functional coaligned PET information on the pancreas.
| RESULTS |
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The PET images showed that most of the radioactivity injected was found in the kidneys and urinary bladder (Figs. 1A, 2A, and 2B). Variable uptakes were also seen in the liver, gallbladder, biliary duct, and duodenum, all of which could be discerned from the pancreas. In 5 of 15 patients, a focal uptake (hot spot) of 18F-fluoro-L-DOPA was observed in the pancreatic area (Fig. 1A, patient 3). The focal localization was the head of the pancreas (n = 4) and in 1 patient in the body. The 5 patients with a focal increase of the radiotracer uptake underwent a limited pancreatic resection that was followed by a complete clinical remission. In all 5 patients, the PET data were in accordance with immunohistochemical analysis. The abnormal ß-cells, identified by their overexpression of proinsulin, synaptophysin, and chromogranin A, also overexpressed AADC (Fig. 1B, patient 3). The distribution of abnormal ß-cells was restricted to the adenoma.
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No significant differences in 18F-fluoro-L-DOPA uptake were observed between the 2 PET studies performed with and without octreotide and diazoxide (Figs. 3A and 3B).
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| DISCUSSION |
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For all 6 patients who underwent MRI, the images obtained did not distinguish between focal and diffuse disease. However, our study emphasized that coregistration of MRI with PET confirmed the radiopharmaceutical accumulation in the pancreas. Slight misregistrations due to the acquisition on separated imaging devices (different patient bed support) and to the softness of the tissues in the abdominal area have been observed (Fig. 4, coregistration images). However, the coregistration between MRI and PET images improves the localization of anatomic regions in PET as well the identification of the lesions.
The PET images showed that most of the injected radioactivity accumulated in the kidneys and urinary bladder, the main route of elimination of the radiotracer. Consequently, the high radioactivity concentrated in the kidneysparticularly in the left kidneymight make the identification of focal forms localized in the tail of the pancreas difficult.
The timeactivity curves showed that the pancreatic radioactivity remained rather constant during the whole dynamic acquisition, independent of the type of HI. This result emphasizes that a 30-min dynamic acquisition is not useful; a 5-min scan should be informative enough. Furthermore, the emission acquisition could start between 45 and 90 min after injection. A similar observation was recently published in which 18F-fluoro-L-DOPA was also used to study neuroendocrine tumors in adults (36).
We have used SUVs to distinguish the 2 forms of HI and to complete the visual inspection. However, although the pancreas SUVs seemed to be higher in focal HI than in diffuse HI, the difference was not statistically significant. This result may be due to the small number of patients with focal HI studied.
In patient 12, the diffuse 18F-fluoro-L-DOPA uptake observed in the pancreas before treatment with carbidopa was no longer detectable after the administration of carbidopa. This result demonstrates, in vivo, that pancreas ß-cells are able to take up L-DOPA, an amino precursor, and contain the enzyme AADC, which is responsible for the conversion of 18F-fluoro-L-DOPA into 18F-fluoro-dopamine. 18F-Fluoro-L-DOPA is probably transported across the cell membrane by the amino acid transporter. Then it is decarboxylated into 18F-fluoro-dopamine, which is stored in vesicles. When decarboxylation is prevented by an AADC inhibitor, such carbidopa, it is possible that 18F-fluoro-L-DOPA is released from the tissue. Thus, the diffuse fixation shown by PET before treatment disappeared completely after the administration of carbidopa.
The effect of octreotide and diazoxide, medications generally used for HI, was tested in 1 patient (patient 8). The uptake of 18F-fluoro-L-DOPA was unchanged. Thus, in contrast to PVS, PET studies could be performed without discontinuing the medication.
Surprisingly, when 11C-L-DOPA was used instead of 18F-fluoro-L-DOPA to detect pancreatic neuroendocrine tumors (37,38), only 1 of 3 insulinomas was diagnosed. One explanation might be that most adult insulinomas contain poorly differentiated ß-cells with a low level of insulin synthesis and secretion. In contrast, in infantile hyperinsulinemic disease, the pancreatic ß-cells are highly differentiated and hyperfunctioning.
| CONCLUSION |
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The localization provided by PET seems to be as precise as that obtained by PVS and, in most cases, should be enough to guide surgical resection. However, for focal HI localized at the tail of the pancreas, coregistration between PET and MR images appears to be necessary for optimal surgery planning.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Maria-João Ribeiro, MD, PhD, Service Hospitalier Frédéric Joliot, DRM/DSV, CEA, 4, place du Général Leclerc, F-91406 Orsay, France.
E-mail: ribeiro{at}shfj.cea.fr
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