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Note: This document covers only the small-cell type of lung cancer. The treatment for small cell lung cancer is very different from the treatment for non-small cell lung cancer. Much of the information for one type will not apply to the other type. If you don't know which type of lung cancer you have, ask your doctor so you can be sure you are looking at the right information.

Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?

Lung cancer is a cancer starts in the lungs. To understand lung cancer, it helps to know something about the lungs and how they work.

The lungs

The lungs are 2 sponge-like organs found in the chest. Each lung is divided into sections called lobes. The right lung has 3 lobes, while the left lung has 2 lobes. The left lung is smaller because the heart takes up more 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 windpipe (trachea). The trachea divides into tubes called the bronchi, which enter the lungs and divide into smaller bronchi. These divide into even 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 air you breathe in and pass carbon dioxide from the body into the alveoli to be breathed out when you exhale. Taking in oxygen and getting rid of carbon dioxide are your lungs’ main functions.

The thin lining around the lungs, called the pleura, helps to protect the lungs and allows them to move during breathing.

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

Start and spread of lung cancer

Lung cancers are thought to start as areas of pre-cancer changes in the lung. These changes are not a mass or tumor. They can’t be seen on an x-ray and they don’t cause symptoms.

Over time, a pre-cancer may go on to become true cancer. The cells divide to make new cells and a tumor may form. In time, the tumor becomes large enough to show up on an x-ray.

At some point, lung cancer cells can break away and spread to other parts of the body in a process called metastasis. Lung cancer can be a life-threatening disease because it often spreads this way before it is found.

The lymph system

One of the ways lung cancer can spread is through the lymph system. Lymph vessels are like veins, but they carry lymph instead of blood. Lymph is a clear fluid that contains tissue waste and cells that fight infection.

Lung cancer cells can enter lymph vessels and begin to grow in lymph nodes (small collections of immune cells) around the bronchi and in the space between the lungs. Once lung cancer cells have reached the lymph nodes, they are more likely to have spread to other organs of the body. The extent (stage) of the cancer and decisions about treatment are based in part on whether or not the cancer has spread to the nearby lymph nodes. We talk about this more in the section “Staging for small cell lung cancer.”

Types of lung cancer

There are 2 main types of lung cancer and they are treated very differently.

  • Small cell lung cancer (SCLC) – this is the less common type. It tends to grow and spread quickly, and it has almost always spread to distant parts of the body before it is found. It is also called oat cell carcinoma and small cell undifferentiated carcinoma.
  • Non-small cell lung cancer (NSCLC) – this is the most common type.

(If the cancer has features of both types, it is called mixed small cell/non-small cell cancer. This is not common.)

Along with the 2 main types of lung cancer, other kinds of cancer can start in the lungs. Sometimes tumors that aren’t cancer can start in the lungs, as well. Cancers that start in other places also often spread to the lungs.

Keep in mind that cancer that starts in other organs (such as the breast, pancreas, kidney, or skin) and spread (metastasize) to the lungs, are not lung cancers. For instance, cancer that starts in the breast and spreads to the lungs is still breast cancer, not lung cancer. Treatment for these cancers that have spread to the lungs depends on where the cancer started.

The information here only covers small cell lung cancer. Non-small cell lung cancer is covered in our document Lung Cancer (Non-Small Cell).

Last Medical Review: 09/09/2014
Last Revised: 01/19/2016