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Carcinoid tumors, also called neuroendocrine tumors, or NETs, are a type of submucosal nodule that can be found anywhere along the digestive tract, but are most frequently found in the rectum and a section of the small intestine called the ileum. Many have a high potential for being malignant, or cancerous. They grow out of a type of neuroendorcrine cell found in the digestive tract, called enterochromaffin cells, which help regulate gastrointestinal functions.

There are three types of carcinoid tumors:

Type 1
A majority of carcinoid tumors found in the GI tract are Type 1 tumors. These tumors often appear as multiple polyp-like lesions and are located in the stomach. Type 1 carcinoid tumors are related to multiple dynamic functions of the stomach related to acid and gastrin production. Patients may have an underlying condition called autoimmune atrophic gastritis, with or without pernicious anemia. This is where the immune system attacks the acid-producing parietal cells in the stomach, which affects the red blood cells and causes chronic inflammation and atrophy of the lining of the stomach. This causes certain cells to overgrow and lead to the development of Type 1 carcinoids. Smaller Type 1 carcinoids are generally benign, but can become malignant, particularly when greater than one centimeter in size.

Type 2
Type 2 carcinoid tumors are the least common type of gastrointestinal carcinoid. It is thought that they develop as a result of a Zollinger-Ellison syndrome, a condition in which certain tumors, called gastrinomas, cause the stomach to produce too much gastrin-type acid. Many people who have Type 2 carcinoids also have a disorder of the endocrine system called multiple endocrine neoplasia type 1, or MEN-1. Type 2 carcinoids are generally slow-growing and considered benign.

Type 3
Type 3 carcinoids, which account for approximately 20 percent of gastrointestinal NETs, are the most aggressive type. They are known as sporadic NETs because they are not associated with any other conditions. In more than half of cases, Type 3 carcinoids are malignant, or cancerous, and metastasize to other tissues and organs.

Symptoms of carcinoids found in the ileum can include abdominal pain and obstruction of the small intestine. In cases where the carcinoid is malignant, or cancerous, it may metastasize, or spread, to the lymph nodes and liver. Rectal carcinoids may not cause any symptoms, but they can cause rectal bleeding or pain. Carcinoids that spread to the liver can lead to symptoms of carcinoid syndrome, such as diarrhea, flushing, diaphoresis, tremors, or swelling. Treatment of these tumors usually involves surgical removal of the carcinoid and surrounding tissues.