A tumor arising from hormone-producing cells—often slow-growing but capable of secreting excess hormones that cause symptoms.Neuroendocrine tumors (NETs) arise from enterochromaffin and other hormone-secreting cells, most commonly in the gastrointestinal tract and pancreas (GEP-NETs). They are classified by grade (Ki-67 proliferation index) and differentiation. Functional NETs secrete bioactive substances causing clinical syndromes (carcinoid syndrome from serotonin, insulinoma, gastrinoma). Somatostatin analogs (octreotide, lanreotide) are first-line therapy for symptom control and tumor growth inhibition via SSTR2 activation. Lutathera (¹⁷⁷Lu-DOTATATE) uses somatostatin receptor targeting for peptide receptor radionuclide therapy.