How Tumors Form The body is made up of many types of cells. Normally, cells grow, divide, and produce more cells as needed to keep the body healthy and functioning properly. Sometimes, however, the process goes wrong and cells become abnormal, forming more cells in an uncontrolled way. These extra cells form a mass of tissue, called a growth or tumor. Tumors can be benign, which means not cancerous, or malignant, which means cancerous. Lung cancer occurs when a tumor forms in the tissue of the lung. The Leading Cause of Cancer Death  Lung cancer is the leading cause of cancer death in men and women in the United States. Experts estimate that over 200,000 new cases of lung cancer will be diagnosed each year slightly more cases in men than women. Over 150,000 Americans die of the disease each year. Lung cancer occurs most often between the ages of 55 and 65. (Watch the videos on this page to learn more about lung cancer. To enlarge the videos, click the brackets in the lower right-hand corner of the video screen. To reduce the videos, press the Escape (Esc) button on your keyboard.) Two Major Types of Lung Cancer There are two major types of lung cancer -- non-small cell lung cancer and small cell lung cancer. Each type of lung cancer grows and spreads in different ways, and each is treated differently. - Non-small cell lung cancer is more common than small cell lung cancer.  - Small cell lung cancer grows more quickly and is more likely to spread to other organs in the body.  Non-small cell lung cancer is more common than small cell lung cancer. Small cell lung cancer grows more quickly and is more likely to spread to other organs in the body.  Learn more about non-small cell lung cancer.   Learn more about small cell lung cancer.  Lung Cancer Can Spread Lung cancer may spread to the lymph nodes or other tissues in the chest, including the lung opposite to where it originated. It may also spread to other organs of the body, such as the bones, brain, or liver. When cancer spreads from its original location in the lung to another part of the body such as the brain, it is called metastatic lung cancer, not brain cancer. Doctors sometimes call this distant disease. Smoking and Lung Cancer  Lung cancer would occur much less often if people did not smoke. The good news is that smoking is not as popular as it used to be. In 1965 about 42 percent of all adults smoked, but as of 2012, slightly less than 17 percent of people 18 and older smoked cigarettes. Also, since the 1990s there has been a steady drop in lung cancer deaths among men, mainly because fewer men are smoking, and since the turn of the century, lung cancer deaths in women have been slowly declining. Cigarette smoking rates had been dropping steadily in the 1990s and had started to level off at the start of the 21st century but the latest figures show a continued decline. The bad news is that other forms of tobacco use have shown some revival, but mainly in younger populations. The bad news is that smoking rates, which were dropping, have stopped declining in recent years. Smoking by young adults actually increased by 73 percent in the 1990s but has shown a downturn or leveling off in the past few years.