Carcinoid tumors are rare, often slow-growing tumors that form from neuroendocrine cells found throughout the body. Carcinoid tumors most commonly arise in the gastrointestinal system, including the appendix, small intestine, or rectum, but may occur in other sites, including the lungs.
In most cases, there is no clear cause of carcinoid tumors. There does not appear to be a strong association between cases of these tumors and typical cancer risk factors such as smoking and exposure to environmental toxins. In rare cases, carcinoid tumors can be linked to hereditary conditions such as multiple endocrine neoplasia types 1 and 2 and Von-Hippel Lindau disease.