Neuroendocrine tumors are a highly diverse group of tumors formed by neuroendocrine cells. Because these cells are found throughout the body, neuroendocrine tumors can arise in a variety of organs, including those of the gastrointestinal system, lungs, thymus, and skin. Another type of tumor that arises in the adrenal glands — called pheochromocytoma — is sometimes classified as a neuroendocrine tumor.

Carcinoid tumors are by far the most common type of neuroendocrine tumor found in the gastrointestinal system. Other, more rare types of gastrointestinal neuroendocrine tumors include small cell carcinoma, large cell carcinoma, and pancreatic neuroendocrine tumors, which arise in the islet cells of the pancreas.

Doctors may diagnose and treat neuroendocrine tumors according to the types of cells involved, the appearance of the tumor cells under the microscope, and their location in the body.