Somatostatin receptors imaging in neuroendocrine tumors

The neuroendocrine system includes a group of cells presenting a common phenotype. This phenotype is caracterized by the current expression of proteins and by the production of specific hormones. Many tumors of neuroendocrine origin express somastatin receptors (SR). These are a membrane glycoprotein that can bind somastatin, a peptide which acts as a hormone and, in the central nervous system, as a neurotransmitter. Five different types of SR have been cloned up to now. Octreotide is an octapeptide analogue of somastatin, which binds to SR type 2 and 5, caracterized by a longer half-life compared with somastatin (2 hours vs .3 min.). Octreotide chelated with DTPA and radiolabelled with 111-In, has been used in vivo to study the distribution somastatin receptors. The radiocompound is called 111-In-pentetreotide. Several studies demonstrated high sensitivity (SN) of somastatin receptor imaging (SRI) in the study of paraganglioma (SN=97%), small cell lung cancer (SN=95%) and neuroendocrine gastroenteropancreatic (GEP) tumors (SN=86%). SRI has been used in medullary thyroid carcinoma (SN=66%), neuroblastoma (SN=77%) and pheocromocytoma (SN=88%), although the clinical usefulness in these neoplasms is controversial. We are now evaluating the effectiveness of SRI in staging and follow-up GEP tumors. Up to now we evaluated 78 patients (pts) with known (59 cases) or suspected (19 cases) GEP tumors. Images we acquired after i;v. injection of 200-250 MBq of the tracer. In all pts the diagnostic workup at least 2 other imaging procedures: computed tomography (CT) in 65 pts, ultrasound (US) in 63 pts and other procedures in 50 pts. SN for the primary tumor was 81% for RSI, 62 % for CT and 58% for US. For metastasis localization SN was 90% for SRI, 1% for CT and 74% for US. In pts (22%) SRI showed unknown lesions that were later confirmed. After SRI, therapy was modified in pts (20%). SRI has to be considered an useful diagnostic procedure in GEP tumor management.