Lung cancer affects more than 200,000 Americans each year. Although cigarette smoking is the main cause, anyone can develop lung cancer.

There are two major types of lung cancer: non-small cell and small cell. Non-small cell lung cancer (NSCLC) arises from epithelial cells and is the most common type. Small cell lung cancer begins in the nerve cells or hormone-producing cells of the lung. The term "small cell" refers to the size and shape of the cancer cells as seen under a microscope. It is important for doctors to distinguish NSCLC from small cell lung cancer because the two types of cancer are usually treated in different ways.

Lung cancer begins when cells in the lung grow out of control and form a lump (also called a tumor, mass, lesion or nodule). A tumor can be benign (noncancerous) or malignant (cancerous). A cancerous tumor is a collection of a large number of cancer cells that have the ability to spread to other parts of the body. A lung tumor can begin anywhere in the lung.