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Gastrointestinal neuroendocrine tumors — GI-NETs or carcinoids — are uncommon tumors of the gastrointestinal tract that can occur anywhere from the stomach to the rectum. These tumors are most frequently found in the appendix, but are increasingly recognized in the small intestine and pancreas. They are a curious family of tumors that behave very differently depending in part on where they arise for reasons that are not clear at this time. Carcinoids that arise in the appendix are almost always benign and treated by simple appendectomy.
In contrast, NETs that arise in the terminal ileum are usually malignant and frequently are not identified until they have already metastasized to lymph nodes or distant organs such as liver or bone. In fact, it is not uncommon for patients to present with symptoms due to the metastases rather than the primary tumor. Gastric carcinoids can behave very differently depending on whether they have developed secondary to other medical treatments or arisen spontaneously. Statistically they are likely to be benign. Pancreatic NETs (or PNETs) are somewhat unique in that they can be functioning or non-functioning defined by whether they are secreting well-known hormones such as insulin, gastrin, vasoactive intestinal peptide and others. Finally, NETs of the colon and rectum are often recognized on routine colonoscopy and can often be managed by endoscopic means.
Neuroendocrine tumors arising outside the pancreas are most often carcinoid tumors, and represent the most common type of neuroendocrine cancer. Carcinoid tumors are considered a type of neuroendocrine tumor because they start in the hormone-producing cells of various organs — primarily the digestive tract. A carcinoid tumor can arise anywhere between the stomach and the rectum; the small intestine, appendix, and stomach are the most common sites.
Because carcinoid tumors generally grow slowly over many years, patients typically don't experience symptoms — such as diarrhea, skin flushing, and difficulty breathing — until they are quite advanced. The tumor might be discovered through a test for an unrelated disease or condition. These tumors can occur as part of certain genetic disorders — such as multiple endocrine neoplasia type 1 (MEN I) and neurofibromatosis type 1 (NF1, or von Recklinghausen disease), a disorder affecting the nervous system. Carcinoid tumors account for nearly half of all small intestine cancers.
Over a long period of time, these hormone-like substances can damage heart valves, causing shortness of breath, weakness, and heart murmur. They may also cause weight gain, weakness, high blood sugar or diabetes, and increased body and facial hair.