Carbidopa pretreatment improves image interpretation and visualisation of carcinoid tumours with 11C-5-hydroxytryptophan positron emission tomography

PC3T: a signature-driven predictor of chemical compounds for cellular transition

Cellular transitions hold great promise in translational medicine research. However, therapeutic applications are limited by the low efficiency and safety concerns of using transcription factors. Small molecules provide a temporal and highly tunable approach to overcome these issues. Here, we present PC3T, a computational framework to enrich molecules that induce desired cellular transitions, and PC3T was able to consistently enrich small molecules that had been experimentally validated in both bulk and single-cell datasets. We then predicted small molecule reprogramming of fibroblasts into hepatic progenitor-like cells (HPLCs). The converted cells exhibited epithelial cell-like morphology and HPLC-like gene expression pattern. Hepatic functions were also observed, such as glycogen storage and lipid accumulation. Finally, we collected and manually curated a cell state transition resource containing 224 time-course gene expression datasets and 153 cell types. Our framework, together with the data resource, is freely available at http://pc3t.idrug.net.cn/ . We believe that PC3T is a powerful tool to promote chemical-induced cell state transitions.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

68 Ga-DOTATOC-PET/MRI and 11 C-5-HTP-PET/MRI are superior to 68 Ga-DOTATOC-PET/CT for neuroendocrine tumour imaging

The present study aimed to assess gadoxetate disodium contrast-enhanced (CE) positron emission tomography (PET)/magnetic resonance imaging (MRI) with 68 Ga-DOTATOC and 11 C-5-Hydroxy-tryptophan (11 C-5-HTP) in comparison with iodine CE 68 Ga-DOTATOC-PET/computed tomography (CT) for neuroendocrine tumour imaging. Detection rate and reader's confidence were evaluated for each separate image volume: CE-CT, CE-MRI including diffusion-weighted imaging, 68 Ga-DOTATOC-PET performed at PET/CT, 68 Ga-DOTATOC-PET performed at PET/MRI and 11 C-5-HTP-PET, and for the three combined hybrid examinations 68 Ga-DOTATOC-PET/MRI, 11 C-5-HTP-PET/MRI and 68 Ga-DOTATOC-PET/CT. In 11 patients, 255 lesions were depicted. 68 Ga-DOTATOC-PET performed at PET/MRI depicted 72.5%, 68 Ga-DOTATOC-PET performed at PET/CT depicted 62.7%, 11 C-5-HTP-PET depicted 68.2% and CE-CT depicted 53% of lesions. 68 Ga-DOTATOC-PET performed at PET/MRI (P < 0.001) and PET/CT (P = 0.02), 11 C-5-HTP-PET (P < 0.001) and MRI (P < 0.001) were superior to CT. 68 Ga-DOTATOC-PET/MRI and 11 C-5-HTP-PET/MRI detected 92.5% and 92% of lesions, respectively, and both outperformed 68 Ga-DOTATOC-PET/CT (65%) (P < 0.001). For liver metastasis imaging, MRI alone was unsurpassed (P < 0.01) and 68 Ga-DOTATOC-PET/MRI and 11 C-5-HTP-PET/MRI outperformed 68 Ga-DOTATOC-PET/CT (P < 0.001). For lymph node metastasis diagnosis, 68 Ga-DOTATOC-PET performed at PET/MRI and PET/CT and 11 C-5-HTP-PET detected 94%, 94% and 94% of lesions, respectively, and outperformed MRI and CE-CT alone (P < 0.001). For bone metastasis imaging, 68 Ga-DOTATOC-PET performed at PET/MRI and PET/CT and 11 C-5-HTP-PET performed equally well (P = 0.05) and better than MRI. Reader's confidence was better for 68 Ga-DOTATOC-PET/MRI and 11 C-5-HTP-PET/MRI than for 68 Ga-DOTATOC-PET/CT. The tumour maximum standardised uptake value and tumour-to-liver ratio were both approximately twice as high as for 68 Ga-DOTATOC than for 11 C-5-HTP. 68 Ga-DOTATOC-PET/MRI and 11 C-5-HTP-PET/MRI provided the highest detection rates and reader's confidence and were both superior to 68 Ga-DOTATOC-PET/CT, mainly because of the MRI component. The imaging contrast with 68 Ga-DOTATOC was superior to that of 11 C-5-HTP.

Comparison of 18F-DOPA Versus 68Ga-DOTATOC as Preferred PET Imaging Tracer in Well-Differentiated Neuroendocrine Neoplasms

PURPOSE

The aim of this study was to retrospectively compare 18F-FDOPA versus 68Ga-DOTATOC PET in lesion detection rates and laboratory tumor markers in patients with neuroendocrine neoplasms (NENs).

PATIENTS AND METHODS

All patients with histologically proven NEN between May 2015 and February 2019 were included who underwent both 18F-DOPA and 68Ga-DOTATOC PET scans within 6 months from each other (mean, 75; median, 38; range, 2-168 days). All patients, except those with pancreatic NEN, received carbidopa before 18F-DOPA PET. Based on the number of lesions on both modalities, patients were divided into 3 categories: more lesions on 18F-DOPA (DOPA > DOTA), more lesions on 68Ga-DOTATOC (DOTA > DOPA), and equal number of lesions (DOPA = DOTA). Tumor markers chromogranin A, serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) within a maximum of 3 months around either scan were retrieved from the patients' charts.

RESULTS

18F-DOPA revealed significantly more lesions compared with 68Ga-DOTATOC (611 vs 385, P < 0.05). Twenty-four patients were included in the DOPA > DOTA group with 16 small intestinal (SI) NENs, 3 large intestinal, 4 pancreatic, and 1 tumor of unknown origin (TUO). For the 9 patients in the DOTA > DOPA group, 4 were SI, 2 pancreatic, 1 lung, and 2 TUOs. Twelve patients in the DOPA = DOTA group had 6 pancreatic tumors, 3 SI, 1 ovarian, and 2 TUOs. Only serotonin and 5-HIAA showed significant higher values for DOPA > DOTA compared with DOTA > DOPA (mean 24 vs 4, P < 0.05, and 320 vs 81, P < 0.05, respectively). Cutoff values of 20 nmol/109 for serotonin, 185 μg/L for chromogranin A, and 200 nmol/L for 5-HIAA were found to include almost exclusively DOPA > DOTA patients.

CONCLUSIONS

There is an advantage of carbidopa pretreated 18F-DOPA over 68Ga-DOTATOC PET, especially for large intestinal NENs with high levels of biomarkers. There seems to be a relationship between increased biomarker value and improved lesion detection rates with the 18F-DOPA PET scan, which requires further prospective analysis.

Molecular imaging of β-cells: diabetes and beyond

Since diabetes is becoming a global epidemic, there is a great need to develop early β-cell specific diagnostic techniques for this disorder. There are two types of diabetes (i.e., type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM)). In T1DM, the destruction of pancreatic β-cells leads to reduced insulin production or even absolute insulin deficiency, which consequently results in hyperglycemia. Actually, a central issue in the pathophysiology of all types of diabetes is the relative reduction of β-cell mass (BCM) and/or impairment of the function of individual β-cells. In the past two decades, scientists have been trying to develop imaging techniques for noninvasive measurement of the viability and mass of pancreatic β-cells. Despite intense scientific efforts, only two tracers for positron emission tomography (PET) and one contrast agent for magnetic resonance (MR) imaging are currently under clinical evaluation. β-cell specific imaging probes may also allow us to precisely and specifically visualize transplanted β-cells and to improve transplantation outcomes, as transplantation of pancreatic islets has shown promise in treating T1DM. In addition, some of these probes can be applied to the preoperative detection of hidden insulinomas as well. In the present review, we primarily summarize potential tracers under development for imaging β-cells with a focus on tracers for PET, SPECT, MRI, and optical imaging. We will discuss the advantages and limitations of the various imaging probes and extend an outlook on future developments in the field.

Fluorine-18-fluorodihydroxyphenylalanine Positron-emission Tomography Scans of Neuroendocrine Tumors (Carcinoids and Pheochromocytomas)

Objectives:

Conventional methods of imaging neuroendocrine tumors with computed tomography, magnetic resonance imaging, indium-111-octreotide, or radiolabeled metaiodobenzilguanidine scintigraphy have limitations. This pilot study tried to improve the localization of these tumors with fluorine-18-fluorodihydroxyphenylalanine (F-DOPA) positron-emission tomography (PET) scanning.

Materials and Methods:

We studied 22 patients, the majority of whom were referred with clinical diagnosis or suspicion of carcinoid (n = 11), neuroendocrine tumors (n = 7) or pheochromocytoma/paraganglioma (PGL) (n = 4). The comparison was made with the prior conventional imaging.

Results:

The F-DOPA findings were compared with the results of subsequent surgery (2), endoscopy (1), or a long-term follow-up (mean duration, 49 months) for 17 patients. Two patients were lost to follow-up. Foci of F-DOPA deposition were detected in eight patients (final diagnosis of carcinoid in six, of neuroendocrine tumors in one, and of PGL in another). Comparison with the final diagnoses revealed concordance in 16 of the 22 patients. F-DOPA results appeared superior to those obtained with conventional imaging. Despite the small number and diagnostic heterogeneity, in a substantial fraction of patients F-DOPA PET added information relevant to clinical management.

Conclusion:

F-DOPA scanning added prognostic value, particularly when multiple abnormal foci versus a negative examination were considered.

[11C]5-hydroxy-tryptophan PET for Assessment of Islet Mass During Progression of Type 2 Diabetes

[¹¹C]5-hydroxy-tryptophan ([¹¹C]5-HTP) positron emission tomography of the pancreas has been shown to be a surrogate imaging biomarker of pancreatic islet mass. The change in islet mass in different stages of type 2 diabetes (T2D) as measured by noninvasive imaging is currently unknown. Here, we describe a cross-sectional study where subjects at different stages of T2D development with expected stratification of pancreatic islet mass were examined in relation to individuals without diabetes. The primary outcome was the [¹¹C]5-HTP uptake and retention in pancreas, as a surrogate marker for the endogenous islet mass. We found that metabolic testing indicated a progressive loss of β-cell function, but this was not mirrored by a decrease in [¹¹C]5-HTP tracer accumulation in the pancreas. This provides evidence of retained islet mass despite decreased β-cell function. The results herein indicate that β-cell dedifferentiation, and not necessarily endocrine cell loss, constitutes a major cause of β-cell failure in T2D.

Whole-body MRI including diffusion-weighted MRI compared with 5-HTP PET/CT in the detection of neuroendocrine tumors

AIM

We wanted to explore if whole-body magnetic resonance imaging (MRI) including diffusion-weighted (DW) and liver-specific contrast agent-enhanced imaging could be valuable in lesion detection of neuroendocrine tumors (NET). [11C]-5-Hydroxytryptophan positron emission tomography/computed tomography (5-HTP PET/CT) was used for comparison.

MATERIALS AND METHODS

Twenty-one patients with NET were investigated with whole-body MRI, including DW imaging (DWI) and contrast-enhanced imaging of the liver, and whole-body 5-HTP PET/CT. Seven additional patients underwent upper abdomen MRI including DWI, liver-specific contrast agent-enhanced imaging, and 5-HTP PET/CT.

RESULTS

There was a patient-based concordance of 61% and a lesion-based concordance of 53% between the modalities. MRI showed good concordance with PET in detecting bone metastases but was less sensitive in detecting metastases in mediastinal lymph nodes. MRI detected more liver metastases than 5-HTP PET/CT.

CONCLUSION

Whole-body MRI with DWI did not detect all NET lesions found with whole-body 5-HTP PET/CT. Our findings indicate that MRI of the liver including liver-specific contrast agent-enhanced imaging and DWI could be a useful complement to whole-body 5-HTP PET/CT.

Effect of Carbidopa on 18F-FDOPA Uptake in Insulinoma: From Cell Culture to Small-Animal PET Imaging

Patient premedication with carbidopa seems to improve the accuracy of 6-¹⁸F-fluoro-3,4-dihydroxy-l-phenylalanine (¹⁸F-FDOPA) PET for insulinoma diagnosis. However, the risk of PET false-negative results in the presence of carbidopa is a concern. Consequently, we aimed to evaluate the effect of carbidopa on ¹⁸F-FDOPA uptake in insulinoma β-cells and an insulinoma xenograft model in mice.

METHODS

¹⁸F-FDOPA in vitro accumulation was assessed in the murine β-cell line RIN-m5F. In vivo small-animal PET experiments were performed on tumor-bearing nude mice after subcutaneous injection of RIN-m5F cells. Experiments were conducted with and without carbidopa pretreatment.

RESULTS

Incubation of RIN-m5F cells with 80 μM carbidopa did not significantly affect the cellular accumulation of ¹⁸F-FDOPA. Tumor xenografts were clearly detectable by small-animal PET in all cases. Insulinoma xenografts in carbidopa-treated mice showed significantly higher ¹⁸F-FDOPA uptake than those in nontreated mice. Regardless of carbidopa premedication, the xenografts were characterized by an early increase in ¹⁸F-FDOPA uptake and then a progressive reduction over time.

CONCLUSION

Carbidopa did not influence in vitro ¹⁸F-FDOPA accumulation in RIN-m5F cells but improved insulinoma imaging in vivo. Our findings increase current knowledge about the ¹⁸F-FDOPA uptake profile of RIN-m5F cells and a related xenograft model. To our knowledge, the present work represents the first preclinical research specifically focused on insulinomas, with potential translational implications.

Strategy to develop a MAO-A-resistant 5-hydroxy-L-[β-(11)C]tryptophan isotopologue based on deuterium kinetic isotope effects

Background

The serotonin precursor 5-hydroxy-l-[β-¹¹C]tryptophan ([¹¹C]HTP) is in clinical use for localization of neuroendocrine tumors and has been suggested as a proxy marker for pancreatic islet cells. However, degradation by monoamine oxidase-A (MAO-A) reduces retention and the contrast to non-endocrine tissue.

Methods

A synthesis method was developed for 5-hydroxy-l-[β-¹¹C²H]tryptophan ([¹¹C]DHTP), an isotopologue of [¹¹C]HTP, labeled with ¹¹C and ²H at the β-position adjacent to the carbon involved in MAO-A decarboxylation. MAO-A-mediated degradation of [¹¹C]DHTP was evaluated and compared to non-deuterated [¹¹C]HTP.

Results

[¹¹C]DHTP was synthesized with a radiochemical purity of >98%, radioactivity of 620 ± 190 MBq, and deuterium (²H or ²H₂) incorporation at the β-position of 22% ±5%. Retention and resistance to MAO-A-mediated degradation of [¹¹C]DHTP were increased in cells but not in non-human primate pancreas.

Conclusions

Partial deuteration of the β-position yields improved resistance to MAO-A-mediated degradation in vitro but not in vivo.

Strategy to develop a MAO-A-resistant 5-hydroxy-L-[β-(11)C]tryptophan isotopologue based on deuterium kinetic isotope effects

BACKGROUND

The serotonin precursor 5-hydroxy-L-[β-(11)C]tryptophan ([(11)C]HTP) is in clinical use for localization of neuroendocrine tumors and has been suggested as a proxy marker for pancreatic islet cells. However, degradation by monoamine oxidase-A (MAO-A) reduces retention and the contrast to non-endocrine tissue.

METHODS

A synthesis method was developed for 5-hydroxy-L-[β- (11)C(2)H]tryptophan ([(11)C]DHTP), an isotopologue of [(11)C]HTP, labeled with (11)C and (2)H at the β-position adjacent to the carbon involved in MAO-A decarboxylation. MAO-A-mediated degradation of [(11)C]DHTP was evaluated and compared to non-deuterated [(11)C]HTP.

RESULTS

[(11)C]DHTP was synthesized with a radiochemical purity of >98%, radioactivity of 620 ± 190 MBq, and deuterium ((2)H or (2)H2) incorporation at the β-position of 22% ±5%. Retention and resistance to MAO-A-mediated degradation of [(11)C]DHTP were increased in cells but not in non-human primate pancreas.

CONCLUSIONS

Partial deuteration of the β-position yields improved resistance to MAO-A-mediated degradation in vitro but not in vivo.

18F-FDOPA PET/CT imaging of insulinoma revisited

PURPOSE

(18)F-FDOPA PET imaging is increasingly used in the work-up of patients with neuroendocrine tumours. It has been shown to be of limited value in localizing pancreatic insulin-secreting tumours in adults with hyperinsulinaemic hypoglycaemia (HH) mainly due to (18)F-FDOPA uptake by the whole pancreatic gland. The objective of this study was to review our experience with (18)F-FDOPA PET/CT imaging with carbidopa (CD) premedication in patients with HH in comparison with PET/CT studies performed without CD premedication in an independent population.

METHODS

A retrospective study including 16 HH patients who were investigated between January 2011 and December 2013 using (18)F-FDOPA PET/CT (17 examinations) in two academic endocrine tumour centres was conducted. All PET/CT examinations were performed under CD premedication (200 mg orally, 1 - 2 h prior to tracer injection). The PET/CT acquisition protocol included an early acquisition (5 min after (18)F-FDOPA injection) centred over the upper abdomen and a delayed whole-body acquisition starting 20 - 30 min later. An independent series of eight consecutive patients with HH and investigated before 2011 were considered for comparison. All patients had a reference whole-body PET/CT scan performed about 1 h after (18)F-FDOPA injection. In all cases, PET/CT was performed without CD premedication.

RESULTS

In the study group, (18)F-FDOPA PET/CT with CD premedication was positive in 8 out of 11 patients with histologically proven insulinoma (73 %). All (18)F-FDOPA PET/CT-avid insulinomas were detected on early images and 5 of 11 (45 %) on delayed ones. The tumour/normal pancreas uptake ratio was not significantly different between early and delayed acquisitions. Considering all patients with HH, including those without imaging evidence of disease, the detection rate of the primary lesions using CD-assisted (18)F-FDOPA PET/CT was 53 %, showing 9 insulinomas in 17 studies performed. In the control group (without CD premedication, eight patients), the final diagnosis was benign insulinoma in four, nesidioblastosis in one, and no definitive diagnosis in the remainder. (18)F-FDOPA PET/CT failed to detect any tumour in these patients.

CONCLUSION

According to our experience, CD administration before (18)F-FDOPA injection leads to low residual pancreatic (18)F-FDOPA activity preserving tumoral uptake with consequent insulinoma detection in more than half of adult patients with HH and more than 70 % of patients with a final diagnosis of insulinoma. If (18)F-FDOPA PET/CT is indicated, we strongly recommend combining CD premedication with early acquisition centred over the pancreas.

Positron emission tomography ligand [11C]5-hydroxy-tryptophan can be used as a surrogate marker for the human endocrine pancreas

In humans, a well-developed serotonin system is localized to the pancreatic islets while being absent in exocrine pancreas. Assessment of pancreatic serotonin biosynthesis could therefore be used to estimate the human endocrine pancreas. Proof of concept was tested in a prospective clinical trial by comparisons of type 1 diabetic (T1D) patients, with extensive reduction of β-cells, with healthy volunteers (HVs). C-peptide-negative (i.e., insulin-deficient) T1D subjects (n = 10) and HVs (n = 9) underwent dynamic positron emission tomography with the radiolabeled serotonin precursor [(11)C]5-hydroxy-tryptophan ([(11)C]5-HTP). A significant accumulation of [(11)C]5-HTP was obtained in the pancreas of the HVs, with large interindividual variation. A substantial and highly significant reduction (66%) in the pancreatic uptake of [(11)C]5-HTP in T1D subjects was observed, and this was most evident in the corpus and caudal regions of the pancreas where β-cells normally are the major constituent of the islets. [(11)C]5-HTP retention in the pancreas was reduced in T1D compared with nondiabetic subjects. Accumulation of [(11)C]5-HTP in the pancreas of both HVs and subjects with T1D was in agreement with previously reported morphological observations on the β-cell volume, implying that [(11)C]5-HTP retention is a useful noninvasive surrogate marker for the human endocrine pancreas.

Quantitative imaging of serotonergic biosynthesis and degradation in the endocrine pancreas

Serotonergic biosynthesis in the endocrine pancreas, of which the islets of Langerhans is the major constituent, has been implicated in insulin release and β cell proliferation. In this study, we investigated the feasibility of quantitative noninvasive imaging of the serotonergic metabolism in the pancreas using the PET tracer (11)C-5-hydroxy-l-tryptophan ((11)C-5-HTP).

METHODS

Uptake of (11)C-5-HTP, and its specificity for key enzymes in the serotonergic metabolic pathway, was assessed in vitro (INS-1 and PANC1 cells and human islet and exocrine preparations) and in vivo (nonhuman primates and healthy and diabetic rats).

RESULTS

In vitro tracer uptake in endocrine cells (INS-1 and human islets), but not PANC1 and exocrine cells, was mediated specifically by intracellular conversion into serotonin. Pancreatic uptake of (11)C-5-HTP in nonhuman primates was markedly decreased by inhibition of the enzyme dopa decarboxylase, which converts (11)C-5-HTP to (11)C-serotonin and increased after inhibition of monoamine oxidase-A, the main enzyme responsible for serotonin degradation. Uptake in the rat pancreas was similarly modulated by inhibition of monoamine oxidase-A and was reduced in animals with induced diabetes.

CONCLUSION

The PET tracer (11)C-5-HTP can be used for quantitative imaging of the serotonergic system in the endocrine pancreas.

Neuroendocrine tumours: the role of imaging for diagnosis and therapy

In patients with neuroendocrine tumours (NETs), a combination of morphological imaging and nuclear medicine techniques is mandatory for primary tumour visualization, staging and evaluation of somatostatin receptor status. CT and MRI are well-suited for discerning small lesions that might escape detection by single photon emission tomography (SPECT) or PET, as well as for assessing the local invasiveness of the tumour or the response to therapy. Somatostatin receptor imaging, by (111)In-pentetreotide scintigraphy or PET with (68)Ga-labelled somatostatin analogues, frequently identifies additional lesions that are not visible on CT or MRI scans. Currently, somatostatin receptor scintigraphy with (111)In-pentetreotide is the more frequently available of the two techniques to determine somatostatin receptor expression and is needed to select patients for peptide receptor radionuclide therapy. In the future, because of its higher sensitivity, PET with (68)Ga-labelled somatostatin analogues is expected to replace somatostatin receptor scintigraphy. Whereas (18)F-FDG-PET is only used in high-grade neuroendocrine cancers, PET-CT with (18)F-dihydroxy-L-phenylalanine or (11)C-5-hydroxy-L-tryptophan is a useful problem-solving tool and could be considered for the evaluation of therapy response in the future. This article reviews the role of imaging for the diagnosis and management of intestinal and pancreatic NETs. Response evaluation and controversies in NET imaging will also be discussed.

On the use of [18F]DOPA as an imaging biomarker for transplanted islet mass

AIM

Islet transplantation is being developed as a potential cure for patients with type 1 diabetes. There is a need for non-invasive imaging techniques for the quantification of transplanted islets, as current transplantation sites are associated with a substantial loss of islet viability. The dopaminergic metabolic pathway is present in the islets; therefore, we propose Fluorine-18 labeled L-3,4-dihydroxyphenylalanine ([18F]DOPA) as a biomarker for transplanted islet mass.

METHODS

The expression of enzymes involved in the dopaminergic metabolic pathway was investigated in both native and transplanted human islets. The specific uptake of [18F]DOPA in islets and immortalized beta cells was studied in vitro by selective blocking of dopa decarboxylase (DDC). Initial in vivo PET imaging of viable subcutaneous human islets was performed using [18F]DOPA.

RESULTS

DDC and vesicular monoamine transporter 2 are co-localized with insulin in the native human pancreas, and the expression is retained after transplantation. Islet uptake of the [18F]DOPA could be modulated by inhibiting DDC, indicating that the uptake followed the normal dopaminergic metabolic pathway. In vivo imaging revealed [18F]DOPA uptake at the site of the functional islet graft.

CONCLUSION

Based on the in vitro and in vivo results presented in this study, we propose to further validate [18F]DOPA-PET as a sensitive imaging modality for imaging extrahepatically transplanted islets.

Other PET tracers for neuroendocrine tumors

In this article the applicability of (124)I-MIBG and (11)C-5-HTP PET for the detection of abdominal gastro-enteropancreatic neuroendocrine tumors is discussed. (124)I-MIBG is a positron-emitting variant of (123)I-MIBG and therefore suited for PET imaging. Due to the better intrinsic characteristics of PET, (124)I-MIBG PET has a higher spatial resolution than (123)I-MIBG and may therefore lead to better lesion detection in patients with neuroblastoma and pheochromocytoma. Research is ongoing to develop (18)F-labeled analogues and to assess its place in staging patients with gastro-enteropancreatic neuroendocrine tumors when compared with (18)F-FDOPA and (68)Ga-somatostatin PET analogues.

Role of PET/CT in the functional imaging of endocrine pancreatic tumors

Endocrine pancreatic tumors (EPTs) are a heterogeneous group of neoplasms with variable clinical and biological features and prognosis, ranging from very slow-growing tumors to highly aggressive and very malignant ones. As other neuroendocrine tumors, EPTs are characterized by the presence of neuroamine uptake mechanisms and/or peptide receptors at the cell membrane and these features constitute the basis of the clinical use of specific radiolabeled ligands, both for imaging and therapy. The more widespread use of hybrid machines, i.e., positron emission tomography/computed tomography (PET/CT), allows to perform imaging with high resolution and high diagnostic accuracy especially for small lesions, and to correlate anatomic location with function. The recent WHO recommendations for classification and prognostic factors help the selection of tracers likely to show a positive image on PET; therefore, tracers exploiting specific metabolic patterns ((18)F-DOPA and (11)C-5-HTP) or specific receptor expression ((68)Ga-DOTA-peptides) are suited to well-differentiated tumors, while the use of (18)F-FDG is preferred for poorly-differentiated neoplasms with high proliferative activity and loss of neuroendocrine features. In differentiated EPTs, (11)C-5-HTP performs better than (18)F-DOPA even though its use is hampered by its complex production and limited availability and experience; (68)Ga-peptides are indicated for all type of gastroenteropancreatic (GEP) neuroendocrine tumors, regardless of their functional activity. In addition, (68)Ga-DOTA-peptides play a distinctive role in planning peptide receptor radionuclide therapy.

Enzyme inhibition of dopamine metabolism alters 6-[18F]FDOPA uptake in orthotopic pancreatic adenocarcinoma

Background

An unknown location hampers removal of pancreatic tumours. We studied the effects of enzyme inhibitors on the uptake of 6-[¹⁸F]fluoro-l-3,4-dihydroxyphenylalanine ([¹⁸F]FDOPA) in the pancreas, aiming at improved imaging of pancreatic adenocarcinoma.

Methods

Mice bearing orthotopic BxPC3 pancreatic adenocarcinoma were injected with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) and scanned with positron emission tomography/computed tomography (PET/CT). For [¹⁸F]FDOPA studies, tumour-bearing mice and sham-operated controls were pretreated with enzyme inhibitors of aromatic amino acid decarboxylase (AADC), catechol-O-methyl transferase (COMT), monoamine oxidase A (MAO-A) or a combination of COMT and MAO-A. Mice were injected with [¹⁸F]FDOPA and scanned with PET/CT. The absolute [¹⁸F]FDOPA uptake was determined from selected tissues using a gamma counter. The intratumoural biodistribution of [¹⁸F]FDOPA was recorded by autoradiography. The main [¹⁸F]FDOPA metabolites present in the pancreata were determined with radio-high-performance liquid chromatography.

Results

[¹⁸F]FDG uptake was high in pancreatic tumours, while [¹⁸F]FDOPA uptake was highest in the healthy pancreas and significantly lower in tumours. [¹⁸F]FDOPA uptake in the pancreas was lowest with vehicle pretreatment and highest with pretreatment with the inhibitor of AADC. When mice received COMT + MAO-A inhibitors, the uptake was high in the healthy pancreas but low in the tumour-bearing pancreas.

Conclusions

Combined use of [¹⁸F]FDG and [¹⁸F]FDOPA is suitable for imaging pancreatic tumours. Unequal pancreatic uptake after the employed enzyme inhibitors is due to the blockade of metabolism and therefore increased availability of [¹⁸F]FDOPA metabolites, in which uptake differs from that of [¹⁸F]FDOPA. Pretreatment with COMT + MAO-A inhibitors improved the differentiation of pancreas from the surrounding tissue and healthy pancreas from tumour. Similar advantage was not achieved using AADC enzyme inhibitor, carbidopa.

Improving the diagnosis and management of neuroendocrine tumors: utilizing new advances in biomarker and molecular imaging science

Neuroendocrine tumors (NET) are malignant solid tumors that arise in hormone-secreting tissue of the diffuse neuroendocrine system or endocrine glands. Although traditionally understood to be a rare disease, the incidence and prevalence of NET have increased greatly in the past 3 decades. However, during this time, progress in diagnosis and outcome of NET has generally been modest. In order to achieve improved outcome in NET, a better understanding of NET biology combined with more reliable serum markers and better techniques to identify tumor localization and small lesions are needed. Although some NET biomarkers exist, sensitive and specific markers that predict tumor growth and behavior are generally lacking. In addition, the integration of new molecular imaging technologies in patient diagnosis and follow-up has the potential to enhance care. To discuss developments and issues required to improve diagnostics and management of NET patients, with specific focus on the latest advances in molecular imaging and biomarker science, 17 global leaders in the fields of NET, molecular imaging and biomarker technology gathered to participate in a 2-day meeting hosted by Prof. Kjell Öberg at the University of Uppsala in Sweden. During this time, findings were presented regarding methods with potential prognostic and treatment applications in NET or other types of cancers. This paper describes the symposium presentations and resulting discussions.

11C-hydroxytryptophan uptake and metabolism in endocrine and exocrine pancreas

Determination of the residual β-cell mass using noninvasive tools might help to follow up the efficacy of new treatments in both type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes mellitus, including islet transplantation. β-cells are neuroendocrine cells that can take up and metabolize the serotonin precursor 5-hydroxytryptophan. The serotonin pathway may therefore be an attractive target for the development of an imaging tracer for residual β-cell mass. The aim of this study was to evaluate the uptake mechanism and the retention of the PET tracer (11)C-hydroxytryptophan in endocrine and exocrine pancreas in vitro and in vivo.

METHODS

The exocrine human pancreas carcinoma cell line (PANC-1) and the endocrine human insulinoma cell line (CM) were applied for in vitro (11)C-hydroxytryptophan accumulation/efflux experiments and blocking studies using inhibitors of key enzymes and transporters involved in the serotonin pathway. Animal experiments were performed on normal Wistar rats and on rats pretreated with the monoamine oxidase (MAO) inhibitor clorgyline. After intravenous injection of (11)C-hydroxytryptophan, a 60-min dynamic PET scan was acquired followed by an ex vivo biodistribution study. Autoradiography and hematoxylin-eosin staining were performed on the dissected pancreas to localize the radioactivity within the pancreatic tissue.

RESULTS

(11)C-hydroxytryptophan accumulated rapidly in both endocrine CM cells and exocrine PANC-1 cells. In the exocrine cells, a rapid efflux of radioactivity was observed, whereas most radioactivity remained trapped in the endocrine cells. PET images showed clear accumulation of (11)C-hydroxytryptophan in the pancreas in both animal groups, but with a significant 3-fold higher retention of the radiopharmaceutical in clorgyline-treated animals. Ex vivo biodistribution studies confirmed the results obtained by PET. Autoradiographs did not discriminate between the exocrine and endocrine pancreas in control animals, whereas autoradiographs showed intense radioactive spots colocalized with the islets of Langerhans in clorgyline-treated animals.

CONCLUSION

(11)C-hydroxytryptophan is trapped in β-cells but not in exocrine pancreatic cells. β-cell selectivity can be strongly enhanced by inhibition of MAO-A. This observation offers perspectives for the development of a more selective PET tracer for β-cell mass, based on an (11)C-hydroxytryptophan derivative with increased resistance toward degradation by MAO-A.

Nuclear medicine imaging of gastro-entero-pancreatic neuroendocrine tumors. The key role of cellular differentiation and tumor grade: from theory to clinical practice

Nuclear medicine imaging is a powerful diagnostic tool for the management of patients with gastro-entero-pancreatic neuroendocrine tumors, mainly developed considering some cellular characteristics that are specific to the neuroendocrine phenotype. Hence, overexpression of specific trans membrane receptors as well as the cellular ability to take up, accumulate, and decarboxylate amine precursors have been considered for diagnostic radiotracer development. Moreover, the glycolytic metabolism, which is not a specific energetic pathway of neuroendocrine tumors, has been proposed for radionuclide imaging of neuroendocrine tumors. The results of scintigraphic examinations reflect the pathologic features and tumor metabolic properties, allowing the in vivo characterization of the disease. In this article, the influence of both cellular differentiation and tumor grade in the scintigraphic pattern is reviewed according to the literature data. The relationship between nuclear imaging results and prognosis is also discussed. Despite the existence of a relationship between the results of scintigraphic imaging and cellular differentiation, tumor grade and patient outcome, the mechanism explaining the variability of the results needs further investigation.

11C-5-hydroxytryptophan positron emission tomography after radiofrequency ablation of neuroendocrine tumor liver metastases

AIM

The aim was to assess the feasibility of (11)C-5-hydroxy-tryptophan positron emission tomography ((11)C-5-HTP-PET) in the follow-up after radiofrequency ablation (RFA) of liver metastases from neuroendocrine tumors (NETs).

BACKGROUND

Contrast-enhanced computed tomography (CECT) and contrast-enhanced ultrasound (CEUS) are commonly used to evaluate the liver after RFA of NETs. In general, (11)C-5-HTP-PET is more sensitive in the visualization of NETs, but no studies have investigated its role after RFA.

METHODS

Six consecutive patients with liver metastases from NETs were subjected to RFA treatment. All patients underwent baseline imaging before RFA and on two occasions (1-2 and 6-11 months) after RFA. The imaging consisted of (11)C-5-HTP-PET, CEUS and CECT on all three occasions.

RESULTS

Thirty RFA areas were evaluated, and residual tumors (RTs) were depicted in eight areas (22%). (11)C-5-HTP-PET depicted RTs after RFA with maximum sensitivity (100%) and specificity (100%), using radiological follow-up as the gold standard. (11)C-5-HTP-PET detected five out of eight RTs earlier than CECT or CEUS. In general, the sensitivity of (11)C-5-HTP-PET exceeded that of CECT and CEUS for early visualization of NET liver metastases.

CONCLUSION

(11)C-5-HTP-PET can be used in the follow-up after RFA for the purpose of detecting RT, and it provides additional information to CEUS and CECT by detecting new lesions.

PET-Guided Surgery - High Correlation between Positron Emission Tomography with 11C-5-Hydroxytryptophane (5-HTP) and Surgical Findings in Abdominal Neuroendocrine Tumours

Positron emission tomography (PET) with 11C-labeled 5-hydroxytryptophane (5-HTP) is a sensitive technique to visualize neuroendocrine tumours (NETs), due to high intracellular uptake of amine-precursors like L-dihydroxyphenylalanine (L-DOPA) and 5-HTP. NETs are often small and difficult to localize in spite of overt clinical symptoms due to hormonal excess. In our study, 38 consecutive NET patients underwent 11C-5-HTP-PET and morphological imaging by CT within 12 weeks prior to surgery. Surgical, histopathological and 5-HTP PET findings were correlated. 11C-5-HTP-PET corresponded to the surgical findings in 31 cases, was false negative in six, and true negative in one case resulting in 83.8% sensitivity and 100% specificity. Positive predicted value was 100%. In 11 patients 11C-5-HTP-PET was the only imaging method applied to localize the tumour. Thus, we could demonstrate that functional imaging by 11C-5-HTP-PET in many cases adds vital preoperative diagnostic information and in more than every fourth patient was the only imaging method that will guide the surgeon in finding the NET-lesion. Although the present results demonstrates that 11C-5-HTP may be used as an universal NET tracer, the sensitivity to visualize benign insulinomas and non functioning pancreatic NETs was lower.

Therapeutic monitoring of gastroenteropancreatic neuroendocrine tumors: the challenges ahead

BACKGROUND

Gastroenteropancreatic neuroendocrine tumors (NETs), a heterogeneous family of tumors arising in a variety of anatomic sites, are generally well differentiated and often metastatic at diagnosis. Morphologic and functional imaging modalities have vastly improved the understanding and diagnosis of NETs. However, use of conventional imaging techniques and response criteria to assess treatment response is often complicated by the clinical course and cytostatic nature of oncologic treatments for NETs.

MATERIALS AND METHODS

The means of therapeutic monitoring discussed in this review were based on a PubMed search of the medical literature and on the clinical expertise of the authors.

RESULTS

Morphology-based criteria for assessing tumor response in general oncology are presented, along with their limitations for assessing response in gastrointestinal and pancreatic NETs. Functional imaging and preliminary response criteria incorporating functional imaging are presented as possible solutions to monitoring treatment response in NETs.

CONCLUSIONS

Morphology-based criteria to assess tumor response have limitations for NETs, which are often slow growing and frequently demonstrate low response rates when based on conventional radiological criteria. Furthermore, many NET treatments do not induce cytotoxic effects despite demonstrated clinical benefit. Novel imaging techniques are available which have the potential to measure changes in tumor physiology and metabolism. These include (68)Ga-labelled somatostatin analogs for PET/CT-based monitoring of NET, molecular imaging with PET tracers that are not based on somatostatin receptor targeting, and functional MRI. These techniques should be explored as options for monitoring treatment in patients with NET.

Complementary roles of 18F-DOPA PET/CT and 18F-FDG PET/CT in medullary thyroid cancer

Serum calcitonin and carcinoembryonic antigen (CEA) are markers of recurrent or persistent disease in medullary thyroid cancer (MTC). However, conventional imaging often fails to localize metastatic disease. Our aim was to compare fluorine-labeled dihydroxyphenylalanine ((18)F-DOPA) and (18)F-FDG PET/CT with multidetector CT (MDCT) and MRI in recurrent or persistent MTC.

METHODS

Nineteen MTC patients with increased calcitonin or CEA on follow-up (mean ± SD, 93 ± 91 mo; range, 4-300 mo) after primary therapy were prospectively imaged with 4 techniques: (18)F-DOPA PET/CT, (18)F-FDG PET/CT, MDCT, and MRI. Images were analyzed for pathologic lesions, which were surgically removed when possible. The correlation between the detection rate for each method and the calcitonin and CEA concentrations and histopathologic findings was investigated.

RESULTS

On the basis of histology and follow-up, one or more imaging methods accurately localized metastatic disease in 12 (63%) of 19 patients. The corresponding figures for (18)F-DOPA PET/CT, (18)F-FDG PET/CT, MDCT, and MRI were 11 (58%) of 19, 10 (53%) of 19, 9 (47%) of 19, and 10 (59%) of 17, respectively. Calcitonin and CEA correlated with (18)F-DOPA PET/CT (P = 0.0007 and P = 0.0263, respectively) and (18)F-FDG PET/CT findings (both P < 0.0001). In patients with an unstable calcitonin doubling time (n = 8), (18)F-DOPA and (18)F-FDG PET/CT were equally sensitive. In contrast, for patients with an unstable CEA doubling time (n = 4), (18)F-FDG PET/CT was more accurate.

CONCLUSION

For most MTC patients with occult disease, (18)F-DOPA PET/CT accurately detects metastases. In patients with an unstable calcitonin level, (18)F-DOPA PET/CT and (18)F-FDG PET/CT are complementary. For patients with an unstable CEA doubling time, (18)F-FDG PET/CT may be more feasible. MRI is sensitive but has the highest rate of false-positive results.

Tailored imaging of islet cell tumors of the pancreas amidst increasing options

Pancreatic islet cell tumors are neuroendocrine tumors, which can produce hormones and can arise as part of multiple endocrine neoplasia type 1 or von-Hippel-Lindau-disease, two genetically well-defined hereditary cancer syndromes. Currently, technical innovation improves conventional and specific molecular imaging techniques. To organize the heterogeneous results described for the imaging of these tumors, we distinguished three indications (1) imaging of a patient with hormone hypersecretion, (2) search for a pancreatic primary in case of proven neuroendocrine cancer of unknown primary, and (3) screening of asymptomatic mutation carriers. We searched for publications on imaging of islet cell tumors between 1995 and January 2010 and defined a Level of Evidence (LOE) for the applicability of each technique. For each technique, data were analyzed in a Forest plot and arranged per imaging indication and tumor subtype. LOEs are weak for all imaging techniques. Analyses indicate a prominent role for endoscopic ultrasound for all three indications.

Total 18F-dopa PET tumour uptake reflects metabolic endocrine tumour activity in patients with a carcinoid tumour

Purpose

Positron emission tomography (PET) using 6-[¹⁸F]fluoro-L-dihydroxyphenylalanine (¹⁸F-dopa) has an excellent sensitivity to detect carcinoid tumour lesions. ¹⁸F-dopa tumour uptake and the levels of biochemical tumour markers are mediated by tumour endocrine metabolic activity. We evaluated whether total ¹⁸F-dopa tumour uptake on PET, defined as whole-body metabolic tumour burden (WBMTB), reflects tumour load per patient, as measured with tumour markers.

Methods

Seventy-seven consecutive carcinoid patients who underwent an ¹⁸F-dopa PET scan in two previously published studies were analysed. For all tumour lesions mean standardised uptake values (SUVs) at 40% of the maximal SUV and tumour volume on ¹⁸F-dopa PET were determined and multiplied to calculate a metabolic burden per lesion. WBMTB was the sum of the metabolic burden of all individual lesions per patient. The 24-h urinary serotonin, urine and plasma 5-hydroxindoleacetic acid (5-HIAA), catecholamines (nor)epinephrine, dopamine and their metabolites, measured in urine and plasma, and serum chromogranin A served as tumour markers.

Results

All but 1 were evaluable for WBMTB; 74 patients had metastatic disease. ¹⁸F-dopa PET detected 979 lesions. SUV_max on ¹⁸F-dopa PET varied up to 29-fold between individual lesions within the same patients. WBMTB correlated with urinary serotonin (r = 0.51) and urinary and plasma 5-HIAA (r = 0.78 and 0.66). WBMTB also correlated with urinary norepinephrine, epinephrine, dopamine and plasma dopamine, but not with serum chromogranin A.

Conclusion

Tumour load per patient measured with ¹⁸F-dopa PET correlates with tumour markers of the serotonin and catecholamine pathway in urine and plasma in carcinoid patients, reflecting metabolic tumour activity.

Functional imaging of neuroendocrine tumors

Neuroendocrine tumors (NET) have several distinct pathophysiological features that can be addressed by specific radiolabeled probes. An overview on the different radiopharmaceuticals that have been developed for positron emission tomography (PET) of NET are presented. The focus is on fluordeoxyglucose (F-18 FDG), biogenic amine precursors, somatostatin analogs, and hormone syntheses markers. Due to the highly specific tracers lacking any clear anatomical landmarking, the advantages of integrated functional and morphological imaging systems such as PET-CT are obvious. Based on the up to now published literature and one's own experience, it is concluded that amine precursors (e.g. fluor-dihydroxyphenylalanin and hydroxytryptophane) should be employed in most gastroenteropancreatic NET, whereas F-18 FDG should be preserved for more aggressive less-differentiated NETs. Hormone syntheses markers have up to now only been used in few centers and their broad clinical value remains uncertain. The different available somatostatin analogs are the most promising tracers, since they can improve dosimetry in cases where peptide receptor radiotherapies are planned. Of specific interest are the somatostatin analogs addressing several subtypes of the somatostatin receptor (e.g. DOTANOC) that allow detecting also subtypes not expressing the "classically" addressed subtype 2 and 5. Since NET have a high variety of different features, the individual diagnostic approach using PET or integrated PET-CT should be tailored, depending on the histological classification and the differentiation of the tumor.

Measuring serotonin synthesis: from conventional methods to PET tracers and their (pre)clinical implications

The serotonergic system of the brain is complex, with an extensive innervation pattern covering all brain regions and endowed with at least 15 different receptors (each with their particular distribution patterns), specific reuptake mechanisms and synthetic processes. Many aspects of the functioning of the serotonergic system are still unclear, partially because of the difficulty of measuring physiological processes in the living brain. In this review we give an overview of the conventional methods of measuring serotonin synthesis and methods using positron emission tomography (PET) tracers, more specifically with respect to serotonergic function in affective disorders. Conventional methods are invasive and do not directly measure synthesis rates. Although they may give insight into turnover rates, a more direct measurement may be preferred. PET is a noninvasive technique which can trace metabolic processes, like serotonin synthesis. Tracers developed for this purpose are α-[¹¹C]methyltryptophan ([¹¹C]AMT) and 5-hydroxy-L-[β-¹¹C]tryptophan ([¹¹C]5-HTP). Both tracers have advantages and disadvantages. [¹¹C]AMT can enter the kynurenine pathway under inflammatory conditions (and thus provide a false signal), but this tracer has been used in many studies leading to novel insights regarding antidepressant action. [¹¹C]5-HTP is difficult to produce, but trapping of this compound may better represent serotonin synthesis. AMT and 5-HTP kinetics are differently affected by tryptophan depletion and changes of mood. This may indicate that both tracers are associated with different enzymatic processes. In conclusion, PET with radiolabelled substrates for the serotonergic pathway is the only direct way to detect changes of serotonin synthesis in the living brain.

Clinical value of 18-fluorine-fluorodihydroxyphenylalanine positron emission tomography/computed tomography in the follow-up of medullary thyroid carcinoma

BACKGROUND

18-Fluorine-fluorodihydroxyphenylalanine positron emission tomography ((18)F-DOPA PET) is a sensitive method for detecting medullary thyroid carcinoma (MTC). The advent of PET/computed tomography (CT) has enabled more sensitive and specific lesion identification using various tracers in many other tumors. The aim of this study was therefore to test the hypothesis that combined (18)F-DOPA PET/CT more accurately detects MTC lesions than each modality does alone.

METHODS

Twenty-eight consecutive (18)F-DOPA PET, CT, and (18)F-DOPA PET/CT scans of patients followed up for sporadic MTC or multiple endocrine neoplasia 2 syndrome-associated MTC were reviewed retrospectively in randomized sequence by two blinded readers, one a nuclear medicine physician and the other a radiologist, with extensive experience interpreting such images.

RESULTS

Of 18 lesions detected concurrently by the three modalities, PET identified all as positive for MTC, but was unable to definitively localize 4 (22%) lesions. CT could definitively localize all 18 lesions, but could not definitively diagnose or exclude MTC in 6 (33%) lesions. Further, CT falsely identified as MTC-negative one lesion that was judged to be MTC-positive by both PET and PET/CT. Only PET/CT scans accurately characterized and localized all 18 lesions. On a per patient basis, the sensitivity of (18)F-DOPA PET/CT for MTC was 74% and the specificity, 100%. In the present series, no truly MTC-positive (18)F-DOPA PET/CT case was found in patients with basal human calcitonin (hCt) levels under 60 pg/mL, and conversely, no truly MTC-negative PET/CT case was found in patients with basal hCt over 120 pg/mL. Between hCt concentrations of 60 and 150 pg/mL true-negative, false-negative, and true-positive scans all were obtained. (18)F-DOPA PET/CT had 100% sensitivity and specificity when hCt at the time of scanning was over 150 pg/mL, the threshold at which the 2009 American Thyroid Association guidelines recommend performing additional imaging including (18)F-DOPA PET/CT.

CONCLUSIONS

(18)F-DOPA PET/CT allows for a more accurate diagnosis and localization of MTC lesions than do (18)F-DOPA PET or CT alone. Supporting the recent American Thyroid Association recommendations on additional imaging in MTC, (18)F-DOPA PET/CT appears to have 100% sensitivity in patients with hCt over 150 pg/mL.

PET and PET/CT in endocrine tumours

Functional information provided by PET tracers together with the superior image quality and the better data quantification by PET technology had a changing effect on the significance of nuclear medicine in medical issues. Recently introduced hybrid PET/CT systems together with the introduction of novel PET radiopharmaceuticals have contributed to the fact that nuclear medicine has become a growing diagnostic impact on endocrinology. In this review imaging strategies, different radiopharmaceuticals including the basic mechanism of their cell uptake, and the diagnostic value of PET and PET/CT in endocrine tumours except differentiated thyroid carcinomas will be discussed.

6-[F-18]Fluoro-L-dihydroxyphenylalanine positron emission tomography is superior to conventional imaging with (123)I-metaiodobenzylguanidine scintigraphy, computer tomography, and magnetic resonance imaging in localizing tumors causing catecholamine excess

CONTEXT

Catecholamine excess is rare, but symptoms may be life threatening.

OBJECTIVE

The objective of the study was to investigate the sensitivity of 6-[F-18]fluoro-l-dihydroxyphenylalanine positron emission tomography ((18)F-DOPA PET), compared with (123)I-metaiodobenzylguanidine ((123)I-MIBG) scintigraphy and computer tomography (CT)/magnetic resonance imaging (MRI) for tumor localization in patients with catecholamine excess.

DESIGN AND SETTING

All consecutive patients with catecholamine excess visiting the University Medical Center Groningen, Groningen, The Netherlands, between March 2003 and January 2008 were eligible.

PATIENTS

Forty-eight patients were included. The final diagnosis was pheochromocytoma in 40, adrenal hyperplasia in two, paraganglioma in two, ganglioneuroma in one, and unknown in three.

MAIN OUTCOME MEASURES

Sensitivities and discordancy between (18)F-DOPA PET, (123)I-MIBG, and CT or MRI were analyzed for individual patients and lesions. Metanephrines and 3-methoxytyramine in plasma and urine and uptake of (18)F-DOPA with PET were measured to determine the whole-body metabolic burden and correlated with biochemical tumor activity. The gold standard was a composite reference standard.

RESULTS

(18)F-DOPA PET showed lesions in 43 patients, (123)I-MIBG in 31, and CT/MRI in 32. Patient-based sensitivity for (18)F-DOPA PET, (123)I-MIBG, and CT/MRI was 90, 65, and 67% (P < 0.01 for (18)F-DOPA PET vs. both (123)I-MIBG and CT/MRI, P = 1.0 (123)I-MIBG vs. CT/MRI). Lesion-based sensitivities were 73, 48, and 44% (P < 0.001 for (18)F-DOPA PET vs. both (123)I-MIBG and CT/MRI, P = 0.51 (123)I-MIBG vs. CT/MRI). The combination of (18)F-DOPA PET with CT/MRI was superior to (123)I-MIBG with CT/MRI (93 vs. 76%, P < 0.001). Whole-body metabolic burden measured with (18)F-DOPA PET correlated with plasma normetanephrine (r = 0.82), urinary normetanephrine (r = 0.84), and metanephrine (r = 0.57).

CONCLUSION

To localize tumors causing catecholamine excess, (18)F-DOPA PET is superior to (123)I-MIBG scintigraphy and CT/MRI.

Molecular imaging in neuroendocrine tumors: molecular uptake mechanisms and clinical results

Neuroendocrine tumors can originate almost everywhere in the body and consist of a great variety of subtypes. This paper focuses on molecular imaging methods using nuclear medicine techniques in neuroendocrine tumors, coupling molecular uptake mechanisms of radiotracers with clinical results. A non-systematic review is presented on receptor based and metabolic imaging methods. Receptor-based imaging covers the molecular backgrounds of somatostatin, vaso-intestinal peptide (VIP), bombesin and cholecystokinin (CCK) receptors and their link with nuclear imaging. Imaging methods based on specific metabolic properties include meta-iodo-benzylguanide (MIBG) and dimercapto-sulphuric acid (DMSA-V) scintigraphy as well as more modern positron emission tomography (PET)-based methods using radio-labeled analogues of amino acids, glucose, dihydroxyphenylalanine (DOPA), dopamine and tryptophan. Diagnostic sensitivities are presented for each imaging method and for each neuroendocrine tumor subtype. Finally, a Forest plot analysis of diagnostic performance is presented for each tumor type in order to provide a comprehensive overview for clinical use.

Combining 6-fluoro-[18F]l-dihydroxyphenylalanine and [18F]fluoro-2-deoxy-d-glucose positron emission tomography for distinction of non-carcinoid malignancies in carcinoid patients

AIM

Carcinoid patients frequently develop a second primary malignancy (SPM), which can deserve full treatment. Distinguishing a SPM from carcinoid lesions is therefore important. Differentiation can be achieved using the difference in uptake between different positron emission tomography (PET) tracers.

METHODS AND RESULTS

Between January 2005 and August 2008, 105 carcinoid patients were seen at the Department of Medical Oncology for treatment and follow-up. We identified 3 patients who presented with a new SPM in whom differentiation between carcinoid lesions and the SPM was guided by functional imaging of the catecholamine pathway with 6-fluoro-[(18)F]l-dihydroxyphenylalanine ((18)F-DOPA) PET and [(18)F]fluoro-2-deoxy-d-glucose ((18)F-FDG) PET as radiotracer for the glucose metabolism. All 3 patients had metastatic carcinoid disease and localised adenocarcinoma based on the PET-scans. For the adenocarcinoma they received curative treatment.

CONCLUSION

The difference in uptake between these PET techniques can be used for decision making when a primary or metastatic SPM is suspected.

Intraindividual comparison of 68Ga-DOTA-TATE and 18F-DOPA PET in patients with well-differentiated metastatic neuroendocrine tumours

AIM

To compare the diagnostic impact of (68)Ga-DOTA-TATE and (18)F-DOPA PET in the diagnosis of well-differentiated metastatic neuroendocrine tumours (NET).

METHODS

PET/CT using both (68)Ga-DOTA-TATE and (18)F-DOPA was performed in 25 patients with histologically proven metastatic NET (nine gut, five pancreas, six lung, one paranasal sinus, four with unknown primary). Analyses of PET examinations were patient-based (pathological uptake: yes/no), and based on tumour regions (primary tumour if present and metastases of liver, lung, bones and lymph nodes). The results were compared with the results of contrast enhanced CT, and with plasma serotonin levels, which were available in 24 of the 25 patients.

RESULTS

Patient-based sensitivities were 96% for (68)Ga-DOTA-TATE PET and 56% for (18)F-DOPA PET. (68)Ga-DOTA-TATE PET delineated metastases in 54 of 55 positive metastatic tumour regions in contrast to 29 of 55 delineated by (18)F-DOPA PET. Overall, (68)Ga-DOTA-TATE was superior to (18)F-DOPA in 13 patients (two patients showed fewer positive tumour regions with (18)F-DOPA PET). The results were comparable in 12 patients. In 13 of 24 patients, plasma serotonin levels were elevated, and 11 of these 13 patients showed pathological uptake of (18)F-DOPA. Of the 11 patients with normal levels of serotonin, 3 also showed positive (18)F-DOPA uptake. In patients positive for (18)F-DOPA uptake the maximum tumour SUVs were correlated with the levels of serotonin (r=0.66, p=0.01).

CONCLUSION

In this study (68)Ga-DOTA-TATE PET proved clearly superior to (18)F-DOPA PET for detection and staging of NET. (18)F-DOPA uptake tended to be increased in those patients with elevated plasma serotonin. We conclude that (18)F-DOPA PET should be employed in patients with NET with negative (68)Ga-DOTA-TATE PET and elevated plasma serotonin.

Molecular imaging of neuroendocrine tumors

Molecular imaging represents tissue-specific imaging and quantification of physiologically functional and molecular events in tumors, utilizing new noninvasive imaging modalities. It combines anatomic, physiologic and metabolic information in a single imaging session. Neuroendocrine tumors (NETs) present unique features to use specific nuclear imaging, such as somatostatin receptor scintigraphy (SRS), metaiodobenzylguanidine scans and PET scanning. NETs express somatostatin receptors on tumor cells and can, thus, be visualized by ¹¹¹In-gadolinium-diethylenetriame pentaacetic acid-octreotide (OctreoScan^®), which is currently the most common scanning technique for NETs. Every patient with a NET should be subjected to SRS. Technetium-labeled somatostatin analogs are currently growing in importance. Metaiodobenzylguanidine scanning was previously the only method for detection and follow-up of NETs, but is nowadays more or less replaced by octreotide scanning. During the last decade, PET scanning has been developed for detection and follow-up of patients with NETs. It has clearly demonstrated the highest sensitivity and specificity in the range of 85-95%. It detects smaller tumors down to 3 mm, compared with SRS, which has a size limit of approximately 1 cm. ⁶⁸Ga-DOTA-octreotide will, in the future, replace SRS owing to its higher sensitivity and specificity, and also reduce the time for investigation. It will also offer the possibility to evaluate the number of somatostatin receptors in a specific tumor. In the future, PET scanning will be more readily available and less expensive, and it will be possible to study tumor biology, vascularization and gene expression in NETs with the development of new tracers.

In vivo imaging of molecular targets and their function in endocrinology

Imaging is one of the fastest growing fields of study. New technologies and multimodal approaches are increasing the application of imaging to determine molecular targets and functional processes in vivo. The identification of a specific target, transporter, or biological process using imaging has introduced major breakthroughs to the field of endocrinology primarily utilizing computed tomography, magnetic resonance imaging, ultrasonography, positron emission tomography, single-photon emission computed tomography, and optical imaging. This review provides a general background to the specific developments in imaging that pertains to in vivo function and target identification in endocrine-based diseases.

18F-dihydroxyphenylalanine PET in patients with biochemical evidence of medullary thyroid cancer: relation to tumor differentiation

Curative treatment for recurrent medullary thyroid cancer (MTC), diagnosed by rising serum calcitonin, is surgery, but tumor localization is difficult. Therefore, the value of 18F-dihydroxyphenylalanine PET (18F-DOPA PET), 18F-FDG PET, (99m)Tc-V-di-mercaptosulfuricacid (DMSA-V) scintigraphy, and MRI or CT was studied.

METHODS

Twenty-one patients with biochemical recurrent or residual MTC underwent 18F-DOPA PET, 18F-FDG PET, DMSA-V scintigraphy, and MRI or CT. Patient- and lesion-based sensitivities were calculated using a composite reference consisting of all imaging modalities.

RESULTS

In 76% of all patients with MTC, one or more imaging modalities was positive for MTC lesions. In 6 of 8 patients with a calcitonin level of <500 ng/L, imaging results were negative. In 15 patients with positive imaging results, 18F-DOPA PET detected 13 (sensitivity, 62%; with 4.6 lesions per patient [lpp]). Morphologic imaging (n = 19) was positive in 7 (sensitivity, 37%; 4.7 lpp), DMSA-V (n = 18) in 5 (sensitivity, 28%; 1.1 lpp), and 18F-FDG PET (n = 17) in 4 (sensitivity, 24%; 1.6 lpp). In a lesion-based analysis, 18F-DOPA PET detected 95 of 134 lesions (sensitivity, 71%), morphologic imaging detected 80 of 126 (sensitivity, 64%), DMSA-V detected 20 of 108 (sensitivity, 19%), and 18F-FDG PET detected 48 of 102 (sensitivity, 30%). In 2 of 3 patients with a calcitonin/carcinoembryonic antigen (CEA) doubling time of < or =12 mo, 18F-FDG PET performed better than 18FDOPA PET; in the third patient, 18F-FDG PET was not performed.

CONCLUSION

MTC lesions are best detectable when serum calcitonin was >500 ng/L. 18F-DOPA PET is superior to 18F-FDG PET, DMSA-V, and morphologic imaging. With short calcitonin doubling times (< or =12 mo), 18F-FDG PET may be superior.

Improved staging of patients with carcinoid and islet cell tumors with 18F-dihydroxy-phenyl-alanine and 11C-5-hydroxy-tryptophan positron emission tomography

PURPOSE

To evaluate and compare diagnostic sensitivity of positron emission tomography (PET) scanning in carcinoid and islet cell tumor patients with a serotonin and a catecholamine precursor as tracers.

PATIENTS AND METHODS

Carcinoid (n = 24) or pancreatic islet cell tumor (n = 23) patients with at least one lesion on conventional imaging including somatostatin receptor scintigraphy (SRS) and computed tomography (CT) scan underwent (11)C-5-hydroxytryptophan ((11)C-5-HTP) PET and 6-[F-18]fluoro-L-dihydroxy-phenylalanine ((18)F-DOPA) PET. PET findings were compared with a composite reference standard derived from all available imaging along with clinical and cytologic/histologic information.

RESULTS

In carcinoid tumor patients, per-patient analysis showed sensitivities for (11)C-5-HTP PET, (18)F-DOPA PET, SRS, and CT of 100%, 96%, 86%, 96%, respectively, and in islet cell tumors of 100%, 89%, 78%, 87%, respectively. In carcinoid patients, per-lesion analysis revealed sensitivities for (11)C-5-HTP PET, (11)C-5-HTP PET/CT, (18)F-DOPA PET, (18)F-DOPA PET/CT, SRS, SRS/CT, and CT alone of, respectively, 78%, 89%, 87%, 98%, 49%, 73%, and 63% and in islet cell tumors of 67%, 96%, 41%, 80%, 46%, 77%, and 68%, respectively. In all carcinoid patients (18)F-DOPA PET and (11)C-5-HTP PET detected more lesions than SRS (P < .001). (11)C-5-HTP PET was superior to (18)F-DOPA PET in islet cell tumors (P < .0001). In all cases, CT improved the sensitivity of the nuclear scans.

CONCLUSION

(18)F-DOPA PET/CT is the optimal imaging modality for staging in carcinoid patients and (11)C-5-HTP PET/CT in islet cell tumor patients.