A carcinoid tumor starts in the hormone-producing cells of various organs. Carcinoid tumors most often develop in the gastrointestinal tract, in organs such as the stomach or intestines, or in the lungs. However, a carcinoid tumor can also develop in the pancreas, a man’s testicles, or a woman’s ovaries. More than one carcinoid tumor can occur in the same organ.
Here is a general overview of where carcinoid tumors begin: