If you or a loved one has been diagnosed with breast cancer, it's important to understand some basics: What is breast cancer and how does it happen?

In this section, you can learn about how breast cancer develops, how many people get breast cancer, and what factors can increase risk for getting breast cancer. You also can learn more about signs and symptoms to watch for and how to manage any fears you may have about breast cancer.

What is Breast Cancer

Breast cancer is an uncontrolled growth of breast cells. To better understand breast cancer, it helps to understand how any cancer can develop.

Cancer occurs as a result of mutations, or abnormal changes, in the genes responsible for regulating the growth of cells and keeping them healthy. The genes are in each cell’s nucleus, which acts as the “control room” of each cell. Normally, the cells in our bodies replace themselves through an orderly process of cell growth: healthy new cells take over as old ones die out. But over time, mutations can “turn on” certain genes and “turn off” others in a cell. That changed cell gains the ability to keep dividing without control or order, producing more cells just like it and forming a tumor.

A tumor can be benign (not dangerous to health) or malignant (has the potential to be dangerous). Benign tumors are not considered cancerous: their cells are close to normal in appearance, they grow slowly, and they do not invade nearby tissues or spread to other parts of the body. Malignant tumors are cancerous. Left unchecked, malignant cells eventually can spread beyond the original tumor to other parts of the body.

Symptoms of Breast Cancer

Initially, breast cancer may not cause any symptoms. A lump may be too small for you to feel or to cause any unusual changes you can notice on your own. Often, an abnormal area turns up on a screening mammogram (x-ray of the breast), which leads to further testing.

According to the American Cancer Society, any of the following unusual changes in the breast can be a symptom of breast cancer:

• Swelling of all or part of the breast
• Skin irritation or dimpling
• Breast pain
• Nipple pain or the nipple turning inward
• Redness, scaliness, or thickening of the nipple or breast skin
• A nipple discharge other than breast milk
• A lump in the underarm area

Breast Cancer Risk and Risk Factors

By now you may be familiar with the statistic that says 1 in 8 women will develop breast cancer. Many people misinterpret this to mean that, on any given day, they and the women they know have a 1-in-8 risk of developing the disease. That’s simply not true.

People tend to have very different ways of viewing risk. For you, a 1-in-8 lifetime risk may seem like a high likelihood of getting breast cancer. Or you may turn this around and reason that there is a 7-in-8, or 87.5%, chance you will never get breast cancer, even if you live to age 80. How you view risk often depends on your individual situation — for example, whether you or many women you know have had breast cancer, or you have reason to believe you are at higher-than-normal risk for the disease — and your usual way of looking at the world.