Lung cancer occurs when cells in the lung start to grow rapidly in an uncontrolled manner. Lung cancer can start anywhere in the lungs and affect any part of the respiratory system.

Most lung cancers are caused by tobacco smoke. Harmful substances, called carcinogens, in tobacco smoke damage the cells in the lungs. Exposure to other harmful substances, such as arsenic, asbestos, radioactive dust, and radon, also can increase the risk for lung cancer.

Early lung cancer rarely causes symptoms. In its advanced stage, the cancer cells interfere with normal lung function. Respiratory problems may be the first symptom of lung cancer.

There are two main types of lung cancer: non–small cell lung cancer and small cell lung cancer.

• Non–small cell lung cancer generally grows and spreads more slowly than small cell lung cancer.
• Small cell lung cancer, which used to be called oat cell cancer, grows very rapidly and can spread to other organs in the body by the time it is diagnosed. Small cell lung cancer is more strongly linked to smoking.

A combination of therapies, such as surgery, chemotherapy, and radiation, may be used to treat lung cancer.