Gastroenteropancreatic neuroendocrine tumors are classified based on their origin from the embryonic divisions of the alimentary tract into foregut (bronchial, stomach), midgut (small intestine, appendix, cecum) and hindgut (distal colon, rectum, genitourinary) tumours.

• Gastroentero–pancreatic neuroendocrine tumors are classified based on their origin from the embryonic divisions of the alimentary tract:

1. Foregut (bronchial, stomach)
2. Midgut (small intestine, appendix, cecum)
3. Hindgut (distal colon, rectum, genitourinary)

• Gasroenteropancreatic- neuroendocrine tumor produce a number of secretory products, resulting in a wide range of clinical symptoms.^{[1] [2][3]}
• Midgut gastrointestinal tract neuroendocrine tumors produce serotonin and other vasoactive substances that causes the manifestations of typical carcinoid syndrome.
• Lung neuroendocrine tumors produce less quantities of serotonin.
• Carcinoid syndrome is caused less commonly by lung neuroendocrine tumor and most often by tumors of large size (>5 cm).