Differentiated Gastroenteropancreatic Neuroendocrine Tumors

Neuroendocrine tumors are a heterogeneous group of malignancy and have an increasing incidence. Different diagnostic imaging tools have been performed to evaluate neuroendocrine tumors such as computed tomography and magnetic resonance imaging. However these anatomical methods cannot give information about somatostatin receptor expression, which is extremely important in the management of neuroendocrine tumors. For this reason, somatostatin receptor imaging with radiolabelled somatostatin analogs has an increasing clinical practice. 

Neuroendocrine tumors (NETs) are characterized by the heterogeneous nature, frequently indolent course and possibility of multiple and variable anatomic sites of the primary tumor. Somatostatin is a cyclic peptide consisted of 14 amino acids and produced by neuroendocrine, immune, and inflammatory cells. The cerebral cortex, the brain stem, the hypothalamus, the pancreas and the gastrointestinal tract are physiological production sites. SSTRs are a family of G-protein-coupled receptors that comprises five subtypes (SSTR1-5) [1,2]. Well/moderately differentiated NETs are generally overexpress somatostatin receptors (SSTRs) [3]. Because of the overexpression of SSTRs the radiolabeled somatostatin analogs can be used to localize the primary tumor and its metastasis. The imaging of the SST subtype 2 (SST2) has been developed and has had clinical applications [4]. The majority of NETSs expresses SSTR types 1, 2, 3, and 5 [5]. SSTR2 is the dominant expressed subtype in pancreatic endocrine or carcinoid tumors [6].

Treatment response

Evaluation of treatment response with response evaluation criteria in solid tumors (RECIST) is difficult in well-moderately differentiated GEP-NETs due to slow growing rate. Clinical evaluation is impossible in most nonfunctioning tumors. Moreover, biochemical markers such as chromogranin A or 5-HIAA have poor sensitivity. Ga-68 somatostatin PET/BT could be more effective by giving molecular and morphological information together in the evaluation of treatment response [24].

Conclusion

Ga-68 somatostatin PET/CT is a successful imaging modality in staging-restaging of well-moderately differentiated gastroenteropancreatic neuroendocrine tumors. In addition to give functional information, it also helps to select good candidates for somatostatin analogs and peptide radionuclide treatment.