Lung cancer is a cancer starts in the lungs. In order to understand lung cancer, it helps to know about the normal structure and function of the lungs.

The Lungs

The lungs are two 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, taking in oxygen and getting 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.

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 and make chemicals that cause new blood vessels to form nearby. These new blood vessels nourish the cancer cells and allow a tumor to form. Finally, 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.

One of the ways lung cancer can spread is through the lymphatic system. Lymphatic vessels are like veins but carry lymph fluid instead of blood. Lymph is a clear fluid that contains tissue waste products and immune system cells. Lung cancer cells can enter lymphatic vessels and begin to grow in lymph nodes around the bronchi and in the 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 too. Staging and decisions about lung cancer treatment are based on whether or not the cancer has spread to the nearby lymph nodes. These topics are discussed later in the section, "How Is Non-Small Cell Lung Cancer Staged?"

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.

The information here only covers non-small cell lung cancer. Small cell lung cancer is covered in a separate document.

Other types of tumors can grow in the lungs, too. Some of these are not cancer and others are cancerous. Carcinoid tumors, for example, are slow-growing and usually cured by surgery. The American Cancer Society has more information about lung carcinoid tumors. To learn about these tumors, see our document Lung Carcinoid Tumor.

Non-Small Cell Lung Cancer (NSCLC)

About 85% to 90% of all lung cancers are of 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. 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 breast and spreads to the lungs is still breast cancer, not lung cancer.