Lung cancer is not just one disease but rather a group of diseases. All forms of cancer cause cells in the body to change and grow out of control. Most types of cancer cells form a lump or mass called a tumor. Cells from the tumor can break away and travel to other parts of the body where they can continue to grow.

This spreading process is called metastasis. When cancer spreads, it is still named after the part of the body where it started. For example, if breast cancer spreads to the lungs, it is still breast cancer, not lung cancer.

Another word for cancerous is malignant, so a cancerous tumor is referred to as malignant. But not all tumors are cancer. A tumor that is not cancer is called benign. Benign tumors do not grow and spread the way cancer does. They are usually not a threat to life.

A few cancers, such as blood cancers (leukemia), do not form a tumor. Most cancers are named after the part of the body where the cancer first starts. Lung cancer begins in the lungs. The lungs are two sponge-like organs in the chest. The right lung has three sections, called lobes. The left lung has two lobes. It is smaller because the heart takes up more room on that side of the body.

The lungs bring air in and out of the body, taking in oxygen and getting rid of carbon dioxide gas, a waste product. The lining around the lungs, called the pleura, helps to protect the lungs and allows them to move during breathing. The windpipe (trachea) brings air down into the lungs. It divides into tubes called bronchi, which divide into smaller branches called bronchioles. At the end of these small branches are tiny air sacs known as alveoli.

Most cancers of the lungs start in the lining of the bronchi but they can also begin in other areas such as the trachea, bronchioles, or alveoli. Lung cancer often takes many years to develop. Once the cancer occurs, cancer cells can break away and spread to other parts of the body.

Cancer of the lung is a life- threatening disease because it often spreads in this way before it is found. It is the leading cause of cancer death for both men and women. During the year 2000 there will be about 164,100 new cases of lung cancer in this country. About 156,900 people will die of this disease: about 89,300 men and 67,600 women.

More people die of cancer of the lung than of colon, breast, and prostate cancers combined. Lung cancer is fairly rare in people under the age of 40. The average age of people found to have cancer of the lungs is 60. If the cancer is found and treated by surgery early, before it has spread to lymph nodes or other organs, the five-year survival rate is about 42%.

However, few cancers of the lung are found at this early stage. The five-year survival rate for all stages of lung cancer combined was 14% in 1995, the last year for which we have national data. A risk factor is something that increases a person's chance of getting a disease. Some risk factors, like smoking, can be controlled. Others, such as a person's age, can't be changed.

Smoking is by far the leading risk factor for lung cancer. More than 8 out of 10 diagnosed cancers of the lungs are thought to result from smoking. The longer a person has been smoking, and the more packs per day smoked, the greater the risk. If a person stops smoking before cancer develops, the lung tissue slowly returns to normal. Stopping smoking at any age lowers the risk of lung cancer.