How do tumors develop?

The body is made up of many types of cells. All cancers begin in the cells which make up the human body. To understand cancer, it is helpful to understand what happens when normal cells become cancer cells.

Different cell types make up the different types of body tissues. For example, there are bone cells in bone or nerve cells in the brain. Most liver tissue is made up of liver cells (hepatocytes).

Normal cells grow and multiply in a controlled manner that produces more cells as they are needed to keep the body healthy. When cells become old or damaged, they die and are replaced with new cells.

Sometimes the orderly process of cell growth and division goes wrong.

The genetic material (DNA) of a cell can become damaged or changed, producing mutations that affect normal cell growth and division. When this happens, cells do not die when they should and new cells form when the body does not need them. The extra cells may form a mass of tissue called a tumor. The term "cancer" is used for the many diseases that occur when the cells of the body multiply in an uncontrolled manner, grow to solid tissue lumps and invade other tissues.

Normal cell division showing apoptosis (cell suicide) and cancer cell division.

Once tumors start growing into functional structures such as airways, brain or kidney they can affect organ function and finally, cause death.

These changes can be triggered, for example, by toxins such as cigarette smoke or some chemicals. Although some of the risk factors are known, there is still a lot to be learned about why and how cancer develops.

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