Gastroenteropancreatic neuroendocrine tumors (GEP-NET) are rare and complex neoplasms. The term comprises pre-neoplastic lesions, well-differentiated tumors, and poorly differentiated carcinomas - all of them originating from neuroendocrine cells -, as well as mixed neoplasms of variable malignancy. They may develop anywhere along the gastrointestinal tract and differ in the release of biologically active substances. Accordingly, the clinical presentation is highly heterogeneous, ranging from local mass effects to carcinoid syndromes.
The clinical presentation largely depends on the site of the tumor and the functionality of tumor cells. Most GEP-NET are non-functional, with affected individuals remaining asymptomatic for prolonged periods of time. The majority of GEP-NET are slowly growing neoplasms, but mass effects may eventually be exerted by the primary tumor or by metastases. Non-specific gastrointestinal disorders such as abdominal pain, nausea, vomiting, diarrhea, and constipation are most commonly reported to this end, and they may be misrelated to irritable bowel syndrome  . In the long term, patients tend to lose their appetite and weight. Gastrointestinal hemorrhages may lead to melena, hematochezia, and anemia, but these are rare occurrences. Masses interfering with bile secretion into the duodenum may induce jaundice. Larger GEP-NET may be palpable .
Those tumors that do release mediators may provoke early complaints comprising the so-called carcinoid syndrome . These are due to the systemic effects of the substances released by the tumor cells, which vary according to their secretion profile. Diarrhea, hot flushes, bronchoconstriction, and palpitations are most frequently described. Palpitations are suggestive of cardiac damage, and endomyocardial fibrosis and/or tricuspid and pulmonary insufficiency - commonly referred to as "carcinoid heart disease" - may indeed have fatal consequences  .
Diagnostic imaging is the basis of GEP-NET detection. Indeed, these tumors are regularly identified during imaging procedures realized for unrelated reasons. Endoscopy, endoscopic ultrasonography, abdominal ultrasound, contrast-enhanced computed tomography or magnetic resonance imaging, and octreotide scintigraphy are most commonly employed to this end. The latter techniques are very useful for tumor staging, too. While GEP-NET initially metastasize to the regional lymph nodes, the liver, lungs, and bones should be examined for distant metastases .
Tissue samples have to be obtained to confirm the diagnosis of GEP-NET and to proceed with tumor grading. The microscopic examination of hematoxylin and eosin-stained sections allows for the morphological characterization of tumor cells and the evaluation of the tumor's demarcation to healthy tissue. Furthermore, the mitotic index may be determined in this step. Immunohistochemical analyses should then be carried out to assess the expression of proliferation marker Ki67 and to check for pan-neuroendocrine markers like chromogranin A, neuron-specific enolase, and synaptophysin .
According to the WHO classification of digestive neuroendocrine neoplasms, either one of three grades may be assigned to the tumor: Well-differentiated tumors with low expression of Ki67 and few mitotic figures are classified as G1, GEP-NET of intermediate differentiation are classified as G2, and G3 corresponds to poorly differentiated carcinomas with Ki67 and mitotic indices of >20%. Finally, mixed tumors with neuroendocrine and non-neuroendocrine components as well as hyperplastic or dysplastic pre-neoplastic lesions are defined separately .
Of note, the mediators released by functional GEP-NET, or the corresponding metabolites, may be detected in blood and urine samples. For instance, serotonin may be produced by GEP-NET and may be metabolized to 5-hydroxyindoleacetic acid. Increased levels of urinary 5-hydroxyindoleacetic acid have been stated to be diagnostic for neuroendocrine tumors. While all patients diagnosed with GEP-NET should undergo a thorough cardiological examination, this is particularly true when functional tumors release serotonin .
Management strategies include surgery, radiotherapy, and cytotoxic chemotherapy, as well as the administration of somatostatin analogs and novel biological agents such as sunitinib and everolimus  :
The prognosis largely depends on the tumor grade and stage at the time of diagnosis:
Most GEP-NET are sporadic, but increased familial incidence has been observed and the disease has been related to hereditary syndromes. In this context, multiple endocrine neoplasia type 1, neurofibromatosis type 1, and von Hippel-Lindau disease shall be mentioned as predisposing conditions . Beyond that, distinct genetic polymorphisms have been speculated to modulate the individual risk for GEP-NET  , but reliable data have yet to be provided. Similarly, little is known about the possible influence of environmental factors .
Reported incidence rates have multiplied in the last decades: In the 1970s, the annual incidence of neuroendocrine tumors had been estimated at 1 in 100,000 inhabitants, but in 2004, this number had risen to 5 in 100,000  . The apparent increase in incidence is likely due to the extensive implementation of preventive gastroenterology procedures and investigations , but the comprehensive use of proton pump inhibitors has also been discussed as a possible cause . Patients may live for years with GEP-NET, which is why the prevalence is as high as 35 in 100,000 .
Females and males are affected equally, and the average patient presents during their seventh decade of life. The latter applies to all kinds of GEP-NET, the majority of which develop in the foregut and small intestine, except for neuroendocrine tumors of the appendix. These are more commonly diagnosed in mid-adulthood. Furthermore, early onset is characteristic of syndromic tumors  .
GEP-NET may be functional and release a variety of peptides and amines, e.g., serotonin, histamine, gastrin, insulin, glucagon, adrenocorticotropic hormone, and growth hormone, among others . Tumor cells are not part of the regulatory feedback loops, and the aforementioned mediators are usually released excessively. They may exert systemic effects or interfere with the function of determined organs, but the clinical presentation does not allow for conclusions on the localization of the primary tumor. They may suggest, however, the presence of hepatic metastases: While mediators released in the bowel may be metabolized in the liver, those secreted in the liver directly pass into the systemic circulation .
Beyond that, GEP-NET are known to induce mesenteric fibrosis. Interestingly, the fibroblastic reaction to be observed in the mesenterium is independent of the release of mediators as described above. It may lead to an intermittent partial obstruction of the bowel and the impingement or encasement of mesenteric vessels, and has been reported in patients with functional and non-functional GEP-NET . Mesenteric metastases may be involved in the pathogenesis of this condition .
The etiology and pathogenesis of GEP-NET are incompletely understood, so recommendations can hardly be given to prevent the disease. In general, treatment should be initiated as early as possible to reduce morbidity and mortality. If the patient is eligible for surgery and the tumor is considered resectable, every possible effort should be made to remove it entirely. Otherwise, local and systemic complications may be prevented following treatment strategies as described above. The risk of a carcinoid crisis should be taken into account when manipulating the tumor and/or using anesthetics, and the appropriate measures should be taken to inhibit the release of bioactive substances by the tumor cells .
GEP-NET is a very broad term comprising distinct neoplasms to be found along the gastrointestinal tract. GEP-NET have traditionally been divided according to tumor locations :
GEP-NET originate from neuroendocrine cells, i.e., tumor cells may be able to produce and release mediators and to induce characteristic hormonal syndromes. Because GEP-NET have formerly been referred to as carcinoids, the respective syndromes are usually called carcinoid syndromes. However, GEP-NET may be non-functional, and the clinical presentation may be limited to local mass effects. What's more, the relatively high incidence of GEP-NET in autopsy specimens suggests that many of these tumors never cause clinical disorders .
Gastroenteropancreatic neuroendocrine tumors (GEP-NET) is a generic term referring to a heterogeneous group of neoplasms originating from neuroendocrine cells in the digestive system. Neuroendocrine cells are able to release mediators and hormones, such as serotonin, histamine, gastrin, insulin, and glucagon. Accordingly, GEP-NET may secrete these biologically active substances into the circulation. Patients may then develop symptoms like diarrhea, hot flushes, dyspnea, cough, and palpitations. These symptoms don't allow for conclusions regarding the localization and size of the causative tumor, which are usually determined employing diagnostic imaging techniques. Indeed, imaging procedures carried out fo unrelated reasons may reveal the presence of non-functional GEP-NET: These tumors don't release any of the aforementioned substances and tend to grow slowly. They may cause non-specific gastrointestinal disorders such as abdominal pain, nausea, vomiting, diarrhea, and constipation, but they tend to go unnoticed for prolonged periods of time.
Regardless of the functionality of the tumor, surgery is the only potentially curative therapeutic strategy. The complete removal of the tumor may not be feasible if metastases are encountered, and patients diagnosed with advanced-stage GEP-NET may be considered for surgical debulking, molecular radiotherapy, or chemotherapy to decrease the tumor burden. Additionally, medical therapy may help to inhibit tumor growth and the secretion of mediators and hormones. The patient's quality of life may be increased this way.
In sum, the outcome largely depends on the tumor's properties - GEP-NET may be benign or malignant - and the tumor stage at the time of diagnosis. Patients with benign GEP-NET who are diagnosed early have a favorable prognosis; those who are found to suffer from metastatic, malignant GEP-NET have a poor prognosis.