What are lung carcinoid tumors?

Lung carcinoid tumors are an uncommon type of tumor that starts in the lungs. They tend to grow slower than other types of lung cancers. They are made up of special kinds of cells called neuroendocrine cells.

To understand lung carcinoid tumors, it helps to know something about the normal structure and function of the lungs, as well as the neuroendocrine system.

The lungs

The lungs are 2 sponge-like organs in your chest cavity. Your right lung has 3 sections, called lobes. The left lung has 2 lobes. It is smaller because the heart takes up more room on that side of the body. The lungs bring air in and out, taking in oxygen and getting rid of carbon dioxide gas, a waste product 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 divide into smaller branches called the bronchioles. At the end of the bronchioles are tiny air sacs known as alveoli.

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 cavity.

The diffuse neuroendocrine system

Carcinoid tumors start from cells of the diffuse neuroendocrine system. This system is made up of cells that are like nerve cells in certain ways and like hormone-making endocrine cells in other ways. These cells do not form an actual organ like the adrenal or thyroid glands. Instead, they are scattered throughout the body in organs like the lungs, stomach, and intestines.

Neuroendocrine cells make hormones like adrenaline and similar substances. In the lungs, this may help control air flow and blood flow and may help control the growth of other types of lung cells. Neuroendocrine cells may 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.

Types of lung neuroendocrine tumors

Like most cells in your body, lung neuroendocrine cells sometimes go through certain changes that cause them to grow too much and form tumors. These are known as neuroendocrine tumors or neuroendocrine cancers. Carcinoid tumors are one type of neuroendocrine tumor.

Neuroendocrine tumors can develop anywhere in the body. Neuroendocrine tumors that begin in the digestive system, another common site for these tumors, are discussed in our documents, Gastrointestinal Carcinoid Tumors and Pancreatic Cancer.

This document focuses only on carcinoid tumors that start in the lungs.

There are 4 types of neuroendocrine lung tumors. Starting with the fastest growing, they are:

• Small cell lung cancer
• Large cell neuroendocrine carcinoma
• Atypical carcinoid tumor
• Typical carcinoid tumor

Small cell lung cancer

Small cell lung cancer (SCLC) is one of the fastest growing and spreading of all cancers. It is discussed in our document, Lung Cancer (Small Cell).

Large cell neuroendocrine carcinoma

Large cell neuroendocrine carcinoma (LCNEC) is a rare cancer. It is a type of large cell carcinoma, which is actually a type of non-small cell lung cancer (NSCLC). Although it shares some features with SCLC (including a tendency to grow quickly), it is typically treated as a type of NSCLC. For treatment information, see Lung Cancer (Non-Small Cell).

Typical and atypical carcinoid tumors

The other 2 types of lung neuroendocrine tumors are carcinoids. The rest of this document will only cover these 2 types of tumors.

Typical and atypical carcinoid tumors look different under the microscope.

• Typical carcinoids tend to grow slowly and only rarely spread beyond the lungs. About 9 out of 10 lung carcinoids are typical carcinoids.
• Atypical carcinoids grow a little faster and are somewhat more likely to spread to other organs. Seen under a microscope, they have more cells in the process of dividing and look more like a fast-growing tumor. They are much less common than typical carcinoids.

Carcinoids are sometimes also classified by where they form in the lung.

• 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 of these are also typical carcinoids.
• Peripheral carcinoids develop in the narrower airways (bronchioles) toward the outer edges of the lungs. These are more likely than central carcinoids to be atypical, although most peripheral carcinoids are still typical carcinoids.

This distinction is important because the tumor’s location affects which symptoms a patient may have (see the section called “How are lung carcinoid tumors diagnosed?”) and may also affect how the tumor is treated.