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Lung carcinoid tumors (also known as lung carcinoids) are a type of lung cancer. Cancer starts when cells begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas. (To learn more about how cancers start and spread, see What Is Cancer?)

Lung carcinoid tumors are uncommon and tend to grow slower than other types of lung cancers.

Where lung carcinoid tumors start

Lung carcinoid tumors start in neuroendocrine cells, a special kind of cell found in the lungs. Neuroendocrine cells are also found in other areas of the body, but only cancers that form from neuroendocrine cells in the lungs are called lung carcinoid tumors.

The neuroendocrine system

Neuroendocrine cells are like nerve cells in some way, and like hormone-making endocrine cells in other ways. They make hormones like adrenaline and similar substances. Cells in this system don't form actual organs. Instead they are scattered throughout the body in organs like the lungs, stomach, and intestines.

Neuroendocrine cells in the lungs can have several different functions. They may:

• Help control air flow and blood flow in the lungs
• Help control the growth of other types of lung cells
• Detect the levels of oxygen and carbon dioxide in the air we breathe and then release chemical messages to help the lungs adjust to these changes. (People who live at higher altitudes have more lung neuroendocrine cells, probably because there is less oxygen in the air they breathe.)

The lungs

Lung carcinoid tumors start in the lungs – 2 sponge-like organs in your chest. Their main functions are to take in oxygen and get rid of carbon dioxide.

Your right lung has 3 sections, called lobes. The left lung has 2 lobes. It is smaller because the heart takes up room on that side of the body.

When you breathe in, air enters through your mouth and nose and goes into your lungs through the trachea (windpipe). The trachea divides into tubes called the bronchi (singular, bronchus), which enter the lungs and divide into smaller branches called the bronchioles. At the end of the bronchioles are tiny air sacs known as alveoli. Many tiny blood vessels run through the alveoli. They absorb oxygen from the inhaled air into your bloodstream and pass carbon dioxide (a waste product from the body) into the alveoli where it is expelled from the body when you exhale.

A thin lining called the pleura surrounds the lungs. The pleura protects your lungs and helps them slide back and forth as they expand and contract during breathing. The space inside the chest that contains the lungs is called the pleural space (or pleural cavity).

Below the lungs, a thin, dome-shaped muscle called the diaphragm separates the chest from the abdomen. When you breathe, the diaphragm moves up and down, forcing air in and out of the lungs.

Carcinoid tumors are sometimes classified by where they form in the lung. The tumor’s location is important because it can affect which symptoms a patient has and how the tumor is treated.

• Central carcinoids form in the walls of large airways (bronchi) near the center of the lungs. Most lung carcinoid tumors are central carcinoids, and nearly all are typical carcinoids. (See types of lung carcinoid tumors below.)
• Peripheral carcinoids develop in the smaller airways (bronchioles) toward the outer edges of the lungs. Most peripheral carcinoids are typical carcinoids. (See below.)

Types of lung carcinoid tumors

Lung carcinoid tumors are a type of neuroendocrine tumor. Neuroendocrine tumors are more common in the digestive system (see Gastrointestinal Carcinoid Tumors and Pancreatic Cancer), but the second most common place is in the lungs.

There are 2 types of lung carcinoid tumors:

• Typical carcinoids tend to grow slowly and rarely spread beyond the lungs. About 9 out of 10 lung carcinoids are typical carcinoids. They also do not seem to be linked with smoking. 
• Atypical carcinoids grow a little faster and are somewhat more likely to spread to other organs. They have more cells that are dividing and look more like a fast-growing tumor. They are much less common than typical carcinoids and may be found more often in people who smoke.

In addition to lung carcinoid tumors, there are other types of neuroendocrine tumors that start in the lungs: small cell lung cancer and large cell neuroendocrine carcinoma, which is a type of non-small cell lung cancer. These lung cancers are treated differently, so it's important to know exactly what type you have.