The NCI is the U.S. Government's main agency for cancer research and information about cancer. What You Need to Know About Breast Cancer Breast cancer is the most common type of cancer among women in the United States. Cancer is a group of diseases. It occurs when cells become abnormal and divide without control or order. Every organ in the body is made up of various kinds of cells.
Cells normally divide in an orderly way to produce more cells only when they are needed.
This process helps keep the body healthy. If cells divide when new cells are not needed, they form too much tissue. The mass of extra tissue, called a tumor, can be benign or malignant. Benign tumors are not cancer. They can usually be removed, and in most cases, they don' t come back.