Non-small cell lung cancer is a disease in which malignant (cancer) cells form in the tissues of the lung.

The lungs are a pair of cone-shaped breathing organs in the chest. The lungs bring oxygen into the body as you breathe in. They release carbon dioxide, a waste product of the body’s cells, as you breathe out. Each lung has sections called lobes. The left lung has two lobes. The right lung is slightly larger and has three lobes. Two tubes called bronchi lead from the trachea (windpipe) to the right and left lungs. The bronchi are sometimes also involved in lung cancer. Tiny air sacs called alveoli and small tubes called bronchioles make up the inside of the lungs.

Anatomy of the respiratory system, showing the trachea and both lungs and their lobes and airways. Lymph nodes and the diaphragm are also shown. Oxygen is inhaled into the lungs and passes through the thin membranes of the alveoli and into the bloodstream (see inset).

A thin membrane called the pleura covers the outside of each lung and lines the inside wall of the chest cavity. This creates a sac called the pleural cavity. The pleural cavity normally contains a small amount of fluid that helps the lungs move smoothly in the chest when you breathe.

There are two main types of lung cancer: non-small cell lung cancer and small cell lung cancer. (See the PDQ summary on Small Cell Lung Cancer Treatment for more information.)

There are several types of non-small cell lung cancer.

Each type of non-small cell lung cancer has different kinds of cancer cells. The cancer cells of each type grow and spread in different ways. The types of non-small cell lung cancer are named for the kinds of cells found in the cancer and how the cells look under a microscope:

• Squamous cell carcinoma: Cancer that begins in squamous cells, which are thin, flat cells that look like fish scales. This is also called epidermoid carcinoma.
• Large cell carcinoma: Cancer that may begin in several types of large cells.
• Adenocarcinoma: Cancer that begins in the cells that line the alveoli and make substances such as mucus.

Other less common types of non-small cell lung cancer are: pleomorphic, carcinoid tumor, salivary gland carcinoma, and unclassified carcinoma.

Smoking can increase the risk of developing non-small cell lung cancer.

Smoking cigarettes or cigars is the most common cause of lung cancer. The more years a person smokes, the greater the risk. If a person has stopped smoking, the risk becomes lower as the years pass, but is never completely gone.

Anything that increases a person's chance of developing a disease is called a risk factor. Having a risk factor does not mean that you will get cancer; not having risk factors doesn’t mean that you will not get cancer. People who think they may be at risk should discuss this with their doctor. Risk factors for lung cancer include the following:

• Smoking cigarettes or cigars, now or in the past.
• Being exposed to second-hand smoke.
• Being treated with radiation therapy to the breast or chest.
• Being exposed to asbestos, radon, chromium, arsenic, soot, or tar.
• Living where there is air pollution.