Best Practice & Research Clinical Endocrinology & Metabolism
7Nuclear medicine in the detection, staging and treatment of gastrointestinal carcinoid tumours
Section snippets
Somatostatin receptor scintigraphy (SRS)
Somatostatin receptors are structurally related membrane glycoproteins. At the moment, five subtypes of human somatostatin receptors have been cloned.8, 9 The somatostatin analogue octreotide binds with high affinity to receptor subtypes 2 and 5. The expression of somatostatin receptors 2 and 5 is present in 70–90% of carcinoid tumours.10 Therefore, the radioactively labelled somatostatin analogue can allow visualization and staging of carcinoid tumours expressing somatostatin receptors 2 and 5.
Positron emission tomography (PET)
PET is a non-invasive technique for measurement of regional tracer accumulation and quantification. A biological substance for study can be labelled for radioactive imaging with radionuclides such as 11C, 15O and 18F with emission of positrons. The positrons generate photons, which are detected by a special camera. The half-lives of standard positron emitters are as follows: 18F, 110 minutes; 11C, 20 minutes; and 15O, 2 minutes.18 The use of PET in clinical oncology has significantly increased
123I- or 131I-meta-iodobenzylguanidine (MIBG)
MIBG has been used for many years to visualize carcinoid tumours as it is concentrated in endocrine cells.6 The method was initially developed to detect phaeochromocytomas of the adrenal with reported high sensitivity (87%) and specificity as high as 99%. The uptake of MIBG shares the same mechanism as norepinephrine. MIBG scintigraphy for carcinoid tumours has shown lower sensitivity than somatostatin receptor scintigraphy (50%).22 The method can be used when other methods have failed to
Tumour-targeted treatment
A new treatment modality for malignant carcinoid tumour is the use of radiolabelled somatostatin analogues. The majority of carcinoid tumours possess somatostatin receptors and can therefore be visualized in vivo by using radiolabelled somatostatin analogue 111indium-DTPA-octreotide (Octreoscan®). Therefore, it is logical to target these tumours with radioactive substances, not only for visualization but also for treatment.
Summary
Carcinoid tumours provide problems in diagnosing and localizing small tumours. Although computed tomography and magnetic resonance imaging techniques have significantly improved during the last year, small primary tumours in the gastrointestinal tract remain difficult to detect. Therefore, new techniques such as somatostatin receptor scintigraphy and PET-scanning have significantly improved the localization of carcinoid tumours. The staging of the disease is very important for selection of the
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