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

6-L-18F-fluorodihydroxyphenylalanine PET in neuroendocrine tumors: basic aspects and emerging clinical applications

In recent years, 6-l-18F-fluorodihydroxyphenylalanine (18F-DOPA) PET has emerged as a new diagnostic tool for the imaging of neuroendocrine tumors. This application is based on the unique property of neuroendocrine tumors to produce and secrete various substances, a process that requires the uptake of metabolic precursors, which leads to the uptake of 18F-DOPA. This nonsystematic review first describes basic aspects of 18F-DOPA imaging, including radiosynthesis, factors involved in tracer uptake, and various aspects of metabolism and imaging. Subsequently, this review provides an overview of current clinical applications in neuroendocrine tumors, including carcinoid tumors, pancreatic islet cell tumors, pheochromocytoma, paraganglioma, medullary thyroid cancer, hyperinsulinism, and various other clinical entities. The application of PET/CT in carcinoid tumors has unsurpassed sensitivity. In medullary thyroid cancer, pheochromocytoma, and hyperinsulinism, results are also excellent and contribute significantly to clinical management. In the remaining conditions, the initial experience with 18F-DOPA PET indicates that it seems to be less valuable, but further study is required.

The biological characterization of neuroendocrine tumors: the role of neuroendocrine markers

Neuroendocrine tumors (NET) may originate in different organs, from cells embryologically different but expressing common phenotypic characteristics, such as: the immuno-reactivity for markers of neuroendocrine differentiation (defined as "pan-neuroendocrine"), the capacity to secrete specific or aspecific peptide and hormones and the expression of some receptors, that are at the basis of the current diagnostic and therapeutical approach, peculiar to these tumors. NET have been conventionally distinguished in functioning, when associated with a recognized clinical endocrine syndrome, and non-functioning. However, this terminology may be misleading, since the great majority of NET may secrete neuroendocrine peptides, which can be employed as clinical markers for both diagnosis and follow-up. On the other hand, tissue immuno-reactivity for specific hormones does not always reflect secretory activity of the tumor cells. Finally, receptors and genetic markers are acquiring a relevant role in the characterization of NET, both improving knowledge of biology and physiopathology of NET, as well as in developing specific strategies to establish an early diagnosis and targeted therapies, to adopt prophylactic strategies in familial forms, and to identify more efficacious targets for therapy in the future.

Nuclear imaging of neuroendocrine tumours

The diagnosis of neuroendocrine tumours (NETs) and monitoring of therapy in many patients relies mainly on morphological imaging techniques such as computed tomography (CT), ultrasound (US) and magnetic resonance imaging (MRI). However, functional imaging modalities--such as somatostatin receptor scintigraphy (SRS)--have great impact on patient management by providing tools for better staging of the disease, visualization of occult tumour, and evaluation of eligibility for somatostatin analogue treatment. Positron emission tomography (PET) using (18)F-fluoro-deoxy-glucose (FDG) is a powerful functional modality for oncological imaging. Unfortunately, FDG is not accumulated in NETs except in the case of dedifferentiated tumours and tumours with high proliferative activity. Based on the concept of amine precursor uptake and decarboxylation (APUD), the (18)F- and (11)C-labelled amine precursors L-dihydroxyphenylalanine and 5-hydroxy-L-tryptophan (5-HTP) have been utilized for PET imaging of NETs. In comparative studies of patients with a variety of NETs, (11)C5-HTP-PET proved better than CT and SRS by visualizing additional small lesions. With carbidopa premedication orally before (11)C5-HTP-PET examination the tumour uptake could be increased and the urinary radioactivity concentration considerably reduced. This concept may also be applied to (18)F-L-DOPA-PET, a method which in a limited number of studies has gained additional diagnostic information in NET patients compared to SRS and morphological imaging. (68)Ga is available from an in-house generator and has been utilized for labelling of somatostatin analogues for PET imaging of NETs with promising results in a small number of patients. However, SRS is an established functional imaging method for patients with NETs, whereas the role for PET in the clinical routine needs further evaluation in comparative studies in larger groups of patients.

Review article: Pathogenesis and management of gastric carcinoid tumours

BACKGROUND

Gastric carcinoid tumours are rare, but are increasing in incidence.

AIM

To discuss tumour pathogenesis and outline current approaches to patient management.

METHODS

Review of published articles following a Pubmed search.

RESULTS

Although interest in gastric carcinoids has increased since it was recognized that they are associated with achlorhydria, to date there is no definite evidence that humans taking long-term acid suppressing medication are at increased risk. Type I tumours are associated with autoimmune atrophic gastritis and hypergastrinaemia, type II are associated with Zollinger-Ellison syndrome, multiple endocrine neoplasia-1 and hypergastrinaemia and sporadic type III carcinoids are gastrin-independent and carry the worst prognosis. Careful investigation of these patients is required, particularly to identify the tumour type, the source of hypergastrinaemia and the presence of metastases. Treatment can be directed at the source of hypergastrinaemia if type I or II tumours are still gastrin responsive and not growing autonomously. Type III tumours should be treated surgically.

CONCLUSIONS

Advances in our understanding of the pathogenesis of gastric carcinoids have led to recent improvements in investigation and management. Challenges remain in identifying the genetic and environmental factors, in addition to hypergastrinaemia, that are responsible for tumour development in susceptible patients.

Staging of carcinoid tumours with 18F-DOPA PET: a prospective, diagnostic accuracy study

BACKGROUND

To assess individual treatment options for patients with carcinoid tumours, accurate knowledge of tumour localisation is essential. We aimed to test the diagnostic sensitivity of 6-[fluoride-18]fluoro-levodopa ((18)F-DOPA PET), compared with conventional imaging methods, in patients with carcinoid tumours.

METHODS

In a prospective, single-centre, diagnostic accuracy study, (18)F-DOPA PET with carbidopa pretreatment was compared with somatostatin-receptor scintigraphy (SRS), CT, and combined SRS and CT in 53 patients with a metastatic carcinoid tumour. The performance of all imaging methods was analysed for individual patients, for eight body regions, and for the detection of individual lesions. PET and CT images were fused to improve localisation. To produce a composite reference standard, we used cytological and histological findings; all imaging tests, including secondary assessments for newly found lesions; follow-up; and biochemical data. Sensitivities were calculated and compared.

FINDINGS

In patient-based analysis, we recorded sensitivities of 100% (95% CI 93-100) for (18)F-DOPA-PET, 92% (82-98) for SRS, 87% (75-95) for CT, and 96% (87-100) for combined SRS and CT (p=0.45 for (18)F-DOPA PET vs combined SRS and CT). However, (18)F-DOPA PET detected more lesions, more positive regions, and more lesions per region than combined SRS and CT. In region-based analysis, sensitivity of (18)F-DOPA PET was 95% (90-98) versus 66% (57-74) for SRS, 57% (48-66) for CT, and 79% (70-86) for combined SRS and CT (p=0.0001, PET vs combined SRS and CT). In individual-lesion analysis, corresponding sensitivities were 96% (95-98), 46% (43-50), 54% (51-58), and 65% (62-69; p<0.0001 for PET vs combined SRS and CT).

INTERPRETATION

If the improved tumour localisation seen with (18)F-DOPA-PET compared with conventional imaging is confirmed in future studies, this imaging method could replace use of SRS, help improve prediction of prognosis, and be used to assess patients' response to treatment for carcinoid tumours.

Gastrointestinal carcinoids: the evolution of diagnostic strategies

Carcinoid tumors are rare, often insidious neoplasms arising from neuroendocrine cells. The majority arise in the gastrointestinal system, and are often incidentally found during investigation, although some may present as an emergency bleed or perforation. The prosaic symptoms of flushing, diarrhea, and sweating are often overlooked; thus, the diagnosis is usually much delayed and the tumor is advanced at presentation. This diagnostic delay renders effective management difficult and adversely affects outcome. This overview provides a current assessment of the evolution of the diagnostic techniques available to establish an accurate biochemical (5-hydroxyindole-3-acetic acid and chromogranin A) and topographic diagnosis (octreoscan, radio-labeled metaidobenzylguanidine, computerized tomography, magnetic resonance imaging, positron emission tomography, enteroclysis, endoscopic ultrasound, enteroscopy, capsule endoscopy, and angiography) of carcinoid tumors. The utility and shortcomings of the respective modalities available are evaluated. Although considerable advances have been made in establishing the diagnosis of carcinoid tumors and in defining the topography of metastatic disease, the major limitation is the inability to establish an early and timely diagnosis before the advent of metastatic disease.

Functional imaging of neuroendocrine tumours with PET

Several pathophysiological attributes of neuroendocrine tumours (NET) can be addressed by specific radiolabelled probes. This paper provides an overview on the different radiopharmaceuticals that have been developed for Positron Emission Tomography (PET) of neuroendocrine tumours. A review of the literature on 18F-fluordeoxyglucose (FDG), biogenic amine precursors, somatostatin analogues and hormone syntheses markers is presented. Due to the highly specific tracers that lack any clear anatomical landmarking the advantages of integrated PET/CT are obvious. Amine precursors should be employed in most gastroenteropancreatic NET, FDG should be preserved for more aggressive, less differentiated NETs. Somatostatin analogues are the most promising tracers, since they can improve dosimetry in cases in which radiopeptide therapies are planned. In conclusion, the individual diagnostic approach using PET or the integrated PET/CT should be tailored depending on the histological classification and the differentiation of the tumour.