Breast cancer is one of the most common cancers that affects women and people assigned female at birth (AFAB). It happens when cancerous cells in your breasts multiply and become tumors. About 80% of breast cancer cases are invasive, meaning a tumor may spread from your breast to other areas of your body.

Breast cancer typically affects women age 50 and older, but it can also affect women and people AFAB who are younger than 50. Men and people assigned male at birth (AMAB) may also develop breast cancer. 

Breast cancer types

Healthcare providers determine cancer types and subtypes so they can tailor treatment to be as effective as possible with the fewest possible side effects. Common types of breast cancer include:

• Invasive (infiltrating) ductal carcinoma (IDC): This cancer starts in your milk ducts and spreads to nearby breast tissue. It’s the most common type of breast cancer in the United States.
• Lobular breast cancer: This breast cancer starts in the milk-producing glands (lobules) in your breast and often spreads to nearby breast tissue. It’s the second most common breast cancer in the United States.
• Ductal carcinoma in situ (DCIS): Like IDC, this breast cancer starts in your milk ducts. The difference is DCIS doesn’t spread beyond your milk ducts.

Less common breast cancer types include:

• Triple-negative breast cancer (TNBC): This invasive cancer is aggressive and spreads more quickly than other breast cancers. 
• Inflammatory breast cancer (IBC): This rare, fast-growing cancer looks like a rash on your breast. IBC is rare in the United States.
• Paget’s disease of the breast: This rare cancer affects the skin of your nipple and may look like a rash. Less than 4% of all breast cancers are Paget’s disease of the breast.

Breast cancer subtypes

Healthcare providers classify breast cancer subtypes by receptor cell status. Receptors are protein molecules in or on cells’ surfaces. They can attract or attach to certain substances in your blood, including hormones like estrogen and progesterone. Estrogen and progesterone help cancerous cells to grow. Finding out if cancerous cells have estrogen or progesterone receptors helps healthcare providers plan breast cancer treatment.

Subtypes include:

• ER-positive (ER+) breast cancers have estrogen receptors.
• PR-positive (PR+) breast cancers have progesterone receptors.
• HR-positive (HR+) breast cancers have estrogen and progesterone receptors.
• HR-negative (HR-) breast cancers don’t have estrogen or progesterone receptors.
• HER2-positive (HER2+) breast cancers, which have higher than normal levels of the HER2 protein. This protein helps cancer cells to grow. About 15% to 20% of all breast cancers are HER2-positive.

What are breast cancer symptoms?

The condition can affect your breasts in different ways. Some breast cancer symptoms are very distinctive. Others may simply seem like areas of your breast that look very different from any other area. Breast cancer may not cause noticeable symptoms either. But when it does, symptoms may include:

• A change in the size, shape or contour of your breast.
• A mass or lump, which may feel as small as a pea.
• A lump or thickening in or near your breast or in your underarm that persists through your menstrual cycle.
• A change in the look or feel of your skin on your breast or nipple. Your skin may look dimpled, puckered, scaly or inflamed. It may look red, purple or darker than other parts of your breast. 
• A marble-like hardened area under your skin.
• A blood-stained or clear fluid discharge from your nipple.

What causes breast cancer?

Experts know breast cancer happens when breast cells mutate and become cancerous cells that divide and multiply to create tumors. They aren’t sure what triggers that change. However, research shows there are several risk factors that may increase your chances of developing breast cancer. These include:

• Age: Being 55 or older.
• Sex: Women and people AFAB are much more likely to develop the condition than men and people AMAB.
• Family history: If your parents, siblings, children or other close relatives have breast cancer, you’re at risk of developing the disease.
• Genetics: Up to 15% of people with breast cancer develop the disease because they have inherited genetic mutations. The most common genetic mutations involve the BRCA1 and BRCA2 genes. 
• Smoking: Tobacco use has been linked to many different types of cancer, including breast cancer.
• Drinking beverages containing alcohol: Research shows that drinking beverages containing alcohol may increase breast cancer risk.
• Having obesity.
• Radiation exposure: If you’ve had prior radiation therapy — especially to your head, neck or chest — you’re more likely to develop breast cancer.
• Hormone replacement therapy: People who use hormone replacement therapy (HRT) have a higher risk of being diagnosed with the condition.

What are the complications of breast cancer?

The most significant complication is metastatic breast cancer — breast cancer that spreads to other areas of your body, including your brain, bones, liver and lungs. Studies show about 1 in 3 women and people AFAB who have early-stage cancer later develop metastatic breast cancer.