Nuclear medicine in the detection, staging and treatment of gastrointestinal carcinoid tumours.

Carcinoid tumours belong to the family of neuroendocrine tumours with a capacity to take up and concentrate amines and precursors as well as peptides, and can thereby be detected by nuclear medicine techniques. These rare tumours are difficult to diagnose at earlier stages because of small size and multiplicity. Computed tomography (CT) and magnetic resonance imaging (MRI) are mostly of benefit for detection of larger primary tumours (1-3 cm) and liver and lymph-node metastases. A majority of carcinoid tumours express somatostatin receptors, particularly receptor type 2, and thus somatostatin receptor scintigraphy (SRS) can be used for detection and staging of carcinoid tumours. The detection rate of carcinoid tumours has been reported to be somewhere between 80 and 100% in different studies. The scintigraphy gives a good staging of the disease and detection of unexpected tumour sites, which were not determined by conventional imaging. This method also indicates content of somatostatin receptors, which might indicate efficacy of treatment with octreotide or other somatostatin analogues. Another new non-invasive technique for detection of carcinoid tumours is positron emission tomography (PET). The biological substance for study can be labelled for radioactive imaging with radionuclears, such as (11)C, (15)O and (18)F, with emission of positrons. More than 95% of patients studied displayed high tracer uptake from PET with (11)C-5HTP (5-hydroxytryptophan), which is significantly higher compared to both computer tomography and somatostatin receptor scintigraphy. MIBG has been used for decades to visualize carcinoid tumours, because MIBG is concentrated in the endocrine cells. It was initially developed to detect phaeochromocytomas of the adrenal with reported high sensitivity (87%) and specificity as high as 99%. The method can be used when other methods fail to localize carcinoid tumours and particularly when treatment with (131)I-MIBG is being considered. Tumour-targeted treatment for malignant carcinoid tumour is still investigational, but has become of significant interest with the use of radiolabelled somatostatin analogues. Since a majority of carcinoid tumours present somatostatin receptors and can therefore be visualized in vivo by using radiolabelled somatostatin analogues, it seems logical to try to target these tumours with radioactive substances, not only for visualization but also for treatment. (111)Indium-DTPA-octreotide has been used as the first tumour-targeted treatment, with rather low response rates (in the order of 10-20%) and no significant tumour shrinkage. The second radioactive analogue which has been applied in the clinic is (90)yttrium-DOTA-Tyr3-octreotide, which has given partial and complete remissions in 20-30% of patients. The most significant side-effects have been kidney dysfunction, thrombocytopenia and liver toxicity. The most recent compound is (177)lutetium-DOTA-Tyr3-octreotate, which has been applied by the Rotterdam group and has been reported to give partial remission in about 40% of the patients. In the near future, combined treatment with both (90)yttrium and (177)lutetium coupled to a somatostatin analogue might come into clinical trials. (177)Lutetium may be more effective for smaller tumours whereas (90)yttrium may be more effective for larger tumours.