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Lung cancer is a disease in which cancer cells grow in the lungs.

Lung cancer occurs when cells in the lung divide without control or order. Normally, cells divide in a regulated manner. If cells keep dividing uncontrollably when new cells are not needed, a mass of tissue forms, called a growth or tumor. The term cancer refers to malignant tumors, which can invade nearby tissues and can spread to other parts of the body. A benign tumor does not invade or spread.

Lung cancers that start in the lungs are divided into two types:

• Non-small cell lung cancer—This type generally grows and spreads more slowly. Non-small cell lung cancer accounts for the majority of the lung cancer cases that are diagnosed each year in the United States. The most common types of non-small cell lung cancer are squamous cell cancer and adenocarcinoma.
• Small cell lung cancer—This type generally grows more quickly. It is more likely to spread to other parts of the body.

Most cases of lung cancer are associated with tobacco use, such as:

• First- or second-hand cigarette smoke
• Pipe or cigar smoking, chewing tobacco

While most cases of lung cancer are due to smoking, a percentage of people with lung cancer have never smoked, nor have they been exposed to secondhand smoke.

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