Neuroendocrine tumor is an uncommon cancer that begins in specialized cells called neuroendocrine cells. Neuroendocrine cells have traits similar to those of nerve cells and hormone-producing cells. Neuroendocrine cells release hormones into the blood that then regulate specific body functions, such as blood pressure, heart rate, digestion, breathing, and blood sugar.
When neuroendocrine cells change and grow uncontrollably, NETs can develop in any of these organs. Sometimes, a doctor cannot tell a tumor’s primary site, i.e., the place in which it first started to grow. In such a case, the tumor is said to be of “unknown primary”.
The most common sites for NETs to form are the digestive system and the lungs. The gastro-intestinal tract accounts for 54.1% cases. Gastro-intestinal tract NETs are usually found in the large bowel and appendix (23.3%), small intestine (20.6%), pancreas (6.5%) and stomach (3.7%). NETs can also develop in the lungs (25.9%) and adrenal glands.