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Note: This document covers only the non-small cell type of lung cancer. The treatment for the 2 main types of lung cancer (small cell and non-small cell) is very different. Some of the information for one type will not apply to the other type. If you are not sure which type of lung cancer you have, it is very important to ask your doctor so you can be sure you get the right information.

Lung cancer is a cancer starts in the lungs. In order to understand lung cancer, it helps to know something about the structure of 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 branches. These divide into even smaller branches called 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-cancerous 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, pre-cancers 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, 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 in 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 products 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 area 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 stage (extent) of the cancer and decisions about treatment are based in part on whether or not the cancer has spread to the nearby lymph nodes. This is covered in “Staging for non-small cell lung cancer.”

Types of lung cancer

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

  • Small cell lung cancer (SCLC)
  • Non-small cell lung cancer (NSCLC)

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

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

Non-small cell lung cancer (NSCLC)

About 9 out of 10 cases of all lung cancers are the non-small cell type. Based on how the cells look under the microscope, NSCLC is usually one of 3 sub-types:

  • Squamous cell carcinoma
  • Adenocarcinoma
  • Large cell (undifferentiated) carcinoma

Other types of lung cancer

Along with the 2 main types of lung cancer, other cancers can be found in the lungs, too. Cancers that start in other places can spread to the lungs. Sometimes tumors that aren’t cancer are found in the lungs, as well.

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

Last Medical Review: 08/18/2014
Last Revised: 02/24/2016