Gastrointestinal Neuroendocrine Tumors

Gastrointestinal neuroendocrine tumors (GI-NETs) are a diverse group of neoplasms that arise from neuroendocrine cells throughout the digestive tract. While these tumors can develop in various locations, the midgut is the most common site, particularly in the ileum and appendix. GI-NETs typically grow slowly; however, their clinical behavior is influenced by several factors, including differentiation status, histological grade, and the Ki-67 proliferation index. Poorly differentiated tumors, known as neuroendocrine carcinomas, are associated with more aggressive disease progression and poorer outcomes. Some GI-NETs are functional, producing bioactive hormones or peptides that can lead to specific clinical syndromes. Depending on their origin, these tumors can cause local symptoms, eg, intestinal obstruction, acute appendicitis, or bleeding. Metastases commonly occur in regional lymph nodes and the liver, leading to additional symptoms. Carcinoid syndrome, characterized by systemic symptoms due to circulating tumor products, may occur with midgut tumors that have significant liver metastases. Accurate diagnosis and staging of GI-NETs rely on cross-sectional imaging and, when appropriate, biochemical testing for hormone hypersecretion. Surgical resection is the primary treatment approach, especially when complete or near-complete removal of the tumor is possible. In situations where surgery is not feasible, other strategies, including medical management with somatostatin analogs, targeted therapies, or peptide receptor radionuclide therapy, are used for symptom relief and disease control. This discussion focuses specifically on GI-NETs originating in the midgut and hindgut, while foregut and thoracic NETs are considered separately.