What Is Breast Cancer?

Breast cancer is one of the most common cancers in women. In the U.S., a woman has a 1 in 8 chance of developing the disease at some point in her life. What’s going on in the breast long before a lump forms?

Breast cancer starts when abnormal cells in the breast grow and multiply uncontrollably.

Healthy cells in our bodies follow a routine: They grow and divide, making copies of themselves as needed to replace old or abnormal cells. And just as they receive signals to grow, they receive signals to die off when damaged.   

But cancer cells, which are caused by a mistake in genetic material, don’t behave like healthy cells. These abnormal cells not only survive, but also divide and multiply — despite the fact the body doesn’t need them. This creates more and more abnormal cells like them, which then form a tumor.

A tumor can be non-cancerous (benign) or cancerous (malignant). Non-cancerous tumors are made up of cells that look very similar to normal cells, grow slowly, and don’t invade nearby tissues or spread to other parts of the body. Cancerous tumors, if left unchecked, can eventually spread beyond the original tumor to other parts of the body.

Breast cancer can start in any area of the breast, but it’s commonly found in the breast’s milk-producing glands (called the lobules) or in the passages that drain milk from the lobules to the nipple (called the ducts). Breast cancer can also start in the fatty and fibrous connective tissues of the breast (called the stromal tissues) or in the nipple. 

As breast cancer cells multiply, they can invade nearby healthy breast tissue and make their way into the underarm lymph nodes. Lymph nodes are small organs that filter out foreign substances in the body. If cancer cells get into the lymph nodes, they can travel in the lymph fluid to other parts of the body. 

There are many different types of breast cancer, determined by where the breast cancer started, whether it spread, and by other characteristics of the tumor.

A lump is one of the most common signs and symptoms of breast cancer, but symptoms vary and can also include swelling, skin dimpling, and breast or nipple pain. Many breast cancers have no symptoms. 

The best way to find breast cancer as early as possible — especially for people who have no symptoms — is by regularly examining your own breasts and getting mammograms. Multiple U.S. medical organizations recommend that people assigned female at birth begin receiving mammograms starting at age 40. Early detection makes it easier for doctors to treat breast cancer. 

Regular breast cancer screening and testing is also essential if you’ve been diagnosed with the disease. There are several tests that allow you and your cancer care team to see if cancer has spread outside the breast, monitor how well treatments are working, and watch for signs of recurrence.

If you are diagnosed with breast cancer, you may hear doctors and other members of your healthcare team refer to breast cancer stages. Your pathology report will also likely include the cancer stage. Grouping cancers into stages helps doctors to come up with a treatment plan and identify clinical trials for you.

Doctors determine the breast cancer stage by testing breast and lymph tissue. The stage is usually expressed as a number on a scale of 0 through IV — with stage 0 describing non-invasive cancers that remain within their original location and stage IV describing invasive cancers that have spread outside the breast to other parts of the body.

Stages are not the only thing that your care team will look at when recommending treatments; the tumor size, hormone receptor status, HER2 status, and other characteristics are just as important.

There are many factors linked to breast cancer risk. Some of these risk factors may be within your control, while others are not. For example, women with close relatives who have been diagnosed with breast cancer have a higher risk of developing the disease. Getting older also increases the risk of developing breast cancer. 

Risk factors you can control include alcohol use, obesity, and smoking. Eating healthy foods, avoiding alcohol, and exercising more may help reduce your risk of breast cancer. 

Over the past 40 years, breast cancer screenings and advances in treatment have led to declines in deaths from breast cancer. However, there’s evidence that not all groups of people are benefiting from this progress. For example, despite having lower incidence of breast cancer compared with white women, Black women and American Indian and Alaska Natives are more likely to die from the disease. 

One of the goals of Breast Cancer Awareness Month, held every October, is to educate the public about the importance of screening, the need for more research, and the experience of the many people impacted by breast cancer.

— Last updated on August 7, 2025 at 4:59 PM