Most neuro -endocrine trumors express somatostatin receptors(SMSRs) and often with high density. These include pituitary adenomas, carcinods, islet cell carcinomas, pheochromocytomas, paragangliomas, medullary thyroid carcinomas and small cell lung carcinoma. SMS, a small neuro-peptide hormome of 14 amino acids was discovered by R. Guillemin in 1978.
SMS presents in high concentrations in the hypothalamus, cerebral cortex, brain stem, gastrointestinal tract and pancreas, SMS inhibits the release of growth hormone, insulin, glucagon and gastrin. In addition, SMS has antiproliferative effects on neuro-endocrine human tumors. There are three subtypes of human SMSRs( I , 11, m) while SMSR IV has been identified in rat pituitary and brain (type I -stomach and jejunum, type II-brain, kidney, pancreatic islet, type M-pancreatic islet?). Octreotide, a synthetic analog of SMS and a smaller peptide cf 8 amino acids, shares the same cell membrane receptor binding sites as natural SMS and has a longer biologic half-life of 2-3 hours vs 2-3 minutes due to the inclusion of D-amino acid residues.
It has been used as hormonal therapy in patients with carcinoid syndrome. Radiolabeled analogs of Octreotide has been successfully used in imaging dispersed neuro-endocrine cell tumors with SMSRs as summarized in Table 1.
The purpose of this presentation is to review radiolabeling of Octreotide, imaging protocol, organ distribution and discuss clinical cases of neuro-endocrine tumors.
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