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Pathology and Classification
Gastrointestinal neuroendocrine tumors are tumors derived from diffuse neuroendocrine system of GIT; composed of amine- & acid-producing cells with different hormonal profiles, depending on site of origin. These tumors are now recognized principally by their histologic staining patterns due to shared cellular proteins. Currently, immunocytochemical localization of chromogranins is used. Foregut tumors generally are argentaffin-negative but argyrophilic. Midgut carcinoids are argentaffin-positive. Hindgut carcinoids are argentaffin-negative. Nonfunctional pancreatic neuroendocrine tumors release no products that cause a specific clinical syndrome.
Prognosis and Mutations
A TNM classification has been proposed for staging that is based on the level of tumor invasion and tumor size. The depth of invasion, rapid rate of growth, histologic features and necrosis, and presence of cytokeratin are all important prognostic factors for the development of metastatic disease. MEN 1 is the most important genetic syndromes associated with increased incidence of NETs. MEN 1 is an autosomal dominant disorder due to a defect in a 10-exon gene on 11q13. Three phacomatoses associated with NETs are von Hippel–Lindau disease, von Recklinghausen’s disease, and tuberous sclerosis.
Types of Carcinoids
Appendiceal NETs occur usually in the appendiceal tip. Invasion of mesoappendix, poor differentiation, positive resection margins are some prognostic factors. Small intestinal NETs are frequently multiple i.e. 9–18% occur in the jeunum, 70–80% are present in the ileum, and 70% occur in the ileocecal valve. In small intestinal NETs, there is a male predominance (1.5:1), and race affects frequency. Rectal carcinoid comprises 27% of all GI-NETs and occur between 4 and 13 cm above dentate line. Gastric NETS originates from gastric enterochromaffin-like cells (ECL).
Symptoms of carcinoid syndrome are flushing and diarrhoea, deep red erythema of upper body, lacrimation and salivation, valvular constriction. It occurs only when sufficient concentrations of products secreted by the tumor reach the systemic circulation. Tachykinins are also responsible for diarrhoea as it is stored in GI-NETs. Carcinoid crisis are provoked by anesthesia, chemotherapy, radiologic examinations, vessel catheterization.
Diagnosis and Treatment
The diagnosis of carcinoid syndrome relies on measurement of urinary or plasma serotonin or its metabolites in the urine. Platelet serotonin levels are more sensitive than urinary 5-HIAA. Treatment includes avoiding conditions that precipitate flushing, dietary supplementation with nicotinamide. If patients still have symptoms, serotonin receptor antagonists or somatostatin analogues are the drugs of choice. Transthoracic echocardiography remains a key element in establishing the diagnosis of carcinoid heart diseaese. Hepatic artery embolization alone or with chemotherapy has also been used to control the symptoms of carcinoid syndrome.