Cancer begins when cells in a part of the body start to grow out of control. There are many kinds of cancer, but they all start because of out-of-control growth of abnormal cells.

How a normal cell becomes cancer

Normal body cells grow, divide, and die in an orderly fashion. During the early years of a person's life, normal cells divide faster to allow the person to grow. After the person becomes an adult, most cells divide only to replace worn-out or dying cells or to repair injuries.

Because cancer cells continue to grow and divide, they are different from normal cells. Instead of dying, cancer cells outlive normal cells and keep forming new abnormal cells. Another difference between cancer cells and normal cells is that cancer cells can invade (grow into) other tissues. Being able to grow out of control and to invade other tissues makes a cell a cancer cell.

Cells become cancer cells because of damage to DNA. DNA is in every cell and directs all its actions. Most of the time, when DNA gets damaged the cell can fix it. If the cell can’t repair the damage, the cell dies. In cancer cells the damaged DNA is not repaired, but the cell doesn’t die like it should. Instead, this cell goes on making new cells even though the body does not need them. These new cells will all have the same DNA damage as the first cell does.

People can inherit damaged DNA, but most of the time DNA damage is caused by something we are exposed to in our environment. Sometimes the cause of the DNA damage is something obvious, like cigarette smoking. But many times no clear cause is found.

A cancer cell has many mistakes in its DNA -- having damage in just one spot does not cause cancer. Even when someone inherits damaged DNA, more mistakes in their DNA are needed before a cancer will develop. Staying away from things that are known to damage DNA (like smoking) as a part of a healthy life style lowers the chance that more DNA damage will take place. This can reduce the risk of cancer -- even in people who have an inherited tendency to get cancer.

How cancers grow and spread

In most cases the cancer cells form a tumor. Some cancers, like leukemia, do not form tumors. Instead, these cancer cells involve the blood and blood-forming organs and circulate through other tissues where they grow. But sometimes the extra cells in these blood cancers may also form a mass of tissue called a tumor.

Cancer cells often travel to other parts of the body, where they begin to grow and replace normal tissue. This process is called metastasis. It happens when the cancer cells get into the bloodstream or lymph vessels of our body.

But no matter where a cancer may spread, it is always named for the place where it started. For example, breast cancer that has spread to the liver is still called breast cancer, not liver cancer. Prostate cancer that has spread to the bone is metastatic prostate cancer, not bone cancer.

Not all tumors are cancerous. Tumors that aren't cancer are called benign. Benign tumors can cause problems -- they can grow very large and press on healthy organs and tissues. But they cannot grow into (invade) other tissues. Because they can't invade, they also can't spread to other parts of the body (metastasize). These tumors are almost never life threatening.

How cancers differ

Different types of cancer can behave very differently. For example, lung cancer and breast cancer are very different diseases. They grow at different rates and respond to different treatments. That is why people with cancer need treatment that is aimed at their particular kind of cancer.

How common is cancer

Cancer is the second leading cause of death in the United States. Nearly half of all men and a little over one third of all women in the United States will develop cancer during their lifetimes.

Today, millions of people are living with cancer or have had cancer. The risk of developing most types of cancer can be reduced by changes in a person's lifestyle, for example, by quitting smoking and eating a better diet. Often, the sooner a cancer is found and treatment begins, the better are the chances for living for many years.

Last Medical Review: 02/24/2009
Last Revised: 02/24/2009