Note: This document covers only the non-small cell type of lung cancer. The treatment for each type of lung cancer (small cell or non-small cell) is very different. Much 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 you 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 normal structure of the lungs and how they work.

The lungs

The lungs are 2 sponge-like organs found in the chest. The right lung has 3 sections, called lobes. The left lung has 2 lobes, as shown in the picture below. The left lung is smaller because the heart takes up more room on that side of the body. The lungs bring air in and out of the body. They take in oxygen and get rid of carbon dioxide gas, a waste product.

The lining around the lungs, called the pleura, helps to protect the lungs and allows them to move during breathing. The windpipe (trachea) brings air down into the lungs. It divides into tubes called bronchi (singular, bronchus) which divide into smaller branches called bronchioles. At the end of these small branches are tiny air sacs known as alveoli.

Below the lungs, a 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

Most lung cancer starts in the lining of the bronchi, but it can also start in other parts of the lung. Lung cancer often takes many years to develop. First, there may be 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, these pre-cancerous areas may go on to become true cancer. The cancer makes chemicals that cause new blood vessels to form nearby. These new blood vessels feed the cancer cells and allow a tumor to 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 is 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 fluid 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 around the bronchi and in the area between the lungs. When lung cancer cells have reached the lymph nodes, they are more likely to have spread to other organs of the body. Staging and decisions about lung cancer treatment are based on whether or not the cancer has spread to the nearby lymph nodes. We talk about this more in the section, "After the tests: Staging."

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/large cell cancer. This is not common.

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

Non-small cell lung cancer (NSCLC)

About 8 to 9 out of 10 cases of all lung cancers are the non-small cell type. There are 3 sub-types of NSCLC. The cells in these sub-types differ in size, shape, and chemical make-up.

• Squamous cell carcinoma: About 25% to 30% of all lung cancers are this kind. They are linked to smoking and tend to be found in the middle of the lungs, near a bronchus.

• Adenocarcinoma: This type accounts for about 40% of lung cancers. It is usually found in the outer part of the lung.

• Large-cell (undifferentiated) carcinoma: About 10% to 15% of lung cancers are this type. It can start in any part of the lung. It tends to grow and spread quickly, which makes it harder to treat.

Other types of lung cancer

Along with the 2 main types of lung cancer, other tumors can be found in the lungs, too. Some of these are not cancer and others are cancer. Carcinoid tumors, for instance, are slow-growing and usually cured by surgery. We have more information about lung carcinoid tumors in our document Lung Carcinoid Tumor.

Keep in mind that cancer that starts in other organs (such as the breast, pancreas, kidney, or skin) and spreads (metastasizes) to the lungs is not the same as lung cancer. 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: 11/03/2009
Last Revised: 11/03/2009