More than 200,000 Americans are diagnosed with lung cancer each year. Most lung cancers occur in people who smoke or in those who have smoked in the past. Tobacco smoke damages cells in the lungs, causing the cells to grow abnormally. The risk of lung cancer from smoking is higher for people who smoke heavily and/or who have smoked for a long time. A common way to measure how much a person has smoked over his or her lifetime is in pack years. A pack year is the number of packs of cigarettes smoked per day multiplied by the number of years a person has smoked. For example, smoking one pack a day for 30 years or two packs a day for 15 years both equal 30 pack years.
Recently, results from the National Lung Screening Trial (NLST)  showed that there were 20% fewer lung cancer deaths in people who received screening with a low-dose CT scan than with a chest x-ray. A CT scan creates a three-dimensional picture of the inside of the body with an x-ray machine. A computer then combines these images into a detailed, cross-sectional view that shows any abnormalities or tumors. Sometimes, a contrast medium (a special dye) is injected into a patient's vein to provide better detail in the images.