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Approved by the Cancer.Net Editorial Board, 05/2022

About tumors

A tumor begins when the DNA of healthy cells is damaged, causing the cells to change and grow out of control, forming a mass. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body if it is not found early and treated. A benign tumor means the tumor can grow but will not spread. A benign tumor usually can be removed without it causing much harm.

About neuroendocrine tumors

A neuroendocrine cancer, often referred as a neuroendocrine tumor (NET) or neuroendocrine neoplasm, begins in the specialized cells of the body’s neuroendocrine system. These cells have traits of both hormone-producing endocrine cells and nerve cells. They are found throughout the body’s organs and help control many of the body’s functions. Hormones are chemical substances that are carried through the bloodstream to have a specific effect on the activity of other organs or cells in the body. Most NETs take years to develop and grow slowly. However, some NETs can be fast-growing. See the Grades section to learn more.

NETs can begin in any part of the body, including:

  • Gastrointestinal (GI) tract. NETs develop most commonly in the GI tract, specifically in the large intestine (20%), small intestine (19%), and appendix (4%). The GI tract plays a central role in digesting foods and liquid and in processing waste. GI tract NETs used to be called carcinoid tumors. Learn more about NET of the GI tract.
  • Lung. The lung is the second most common location of NETs. About 30% of NETs occur in the bronchial system, which carries air to the lungs. Lung NETs also used to be called carcinoid tumors. Learn more about NET of the lung.
  • Pancreas. Approximately 7% of NETs can develop in the pancreas, a pear-shaped gland located in the abdomen between the stomach and the spine. Pancreas NETs used to be called islet cell tumors. Learn more about NET of the pancreas.

NETs can also begin in other organs. In about 15% of cases, a primary site cannot be found. Sometimes, NETs may develop in or on the adrenal glands. These rare types of NETs are called pheochromocytoma and paraganglioma, and they are discussed in another section of this website. Other types of tumors that begin in hormone-producing cells are also described in their own sections on Cancer.Net, including thyroid canceradrenal gland tumors, and pituitary gland tumors.

Classification of NETs

NETs may be classified, or described, in many different ways, including by where they are located, what the tumor cells look like under a microscope, and how fast the cells are growing and dividing (see Stages and Grades sections of this guide).

In addition, NETs are described as “functional” or “non-functional.”

  • Functional NETs make and release hormones and other substances that cause symptoms. Carcinoid syndrome is a sign of a “functional” NET in which a substance called serotonin is released that can cause diarrhea and facial flushing (see Symptoms and Signs).
  • Non-functional NETs either do not release substances or do not release enough substances to cause symptoms. About 60% of NETs are non-functional. Because of a lack of symptoms, NETs are usually diagnosed (see Diagnosis) at a more advanced stage.