Cancer is a group of many related diseases. All cancers begin in
cells, the body's basic unit of life. Cells make up tissues, and tissues make up the organs of the body. Normally, cells grow and divide to form new cells as the body needs them. When cells grow old and die, new cells take their place.
Sometimes this orderly process goes wrong. New cells form when the body does not need them, and old cells do not die when they should. These extra cells can form a mass of tissue called a growth or
tumor.
Tumors can be benign or malignant.
Benign tumors are not cancer. Usually, doctors can remove them. Cells from benign tumors do not spread to other parts of the body. In most cases, benign tumors do not come back after they are removed. Most important, benign tumors are rarely a threat to life.
Malignant tumors are cancer. They are generally more serious. Cancer cells can invade and damage nearby tissues and organs. Also, cancer cells can break away from a malignant tumor and enter the bloodstream or the lymphatic system. That is how cancer cells spread from the original (
primary) tumor to form new tumors in other organs. The spread of cancer is called
metastasis.
Breast cancer forms in the tissues of the breast, usually in the tubes that carry milk or in the glands that make milk. Both men and women can get breast cancer, although it is rare in men. Aside from skin cancer, it is the most common type of cancer in women in the United States. Every year, more than 211,000 American women will be diagnosed with breast cancer.