Breast cancer

Breast cancer starts when cells in the breast grow out of control and form a lump (tumor) that can spread to other parts of the body. It can start in one or both breasts. Most breast cancers start in the milk ducts (ductal cancer). Others start in the lobules, the small glands that make milk (lobular cancer). Rarer types include inflammatory breast cancer and Paget disease of the breast, which involves the nipple.

When abnormal cells stay inside a duct and have not spread, it is called ductal carcinoma in situ (DCIS), or stage 0. Most breast cancers are invasive, which means they have grown into nearby breast tissue. From there, they can spread to lymph nodes and other organs.

Doctors also sort breast cancer by markers on the cancer cells. Hormone receptor–positive (HR-positive) cancer cells have receptors for the hormones estrogen or progesterone, which can help them grow. HER2-positive cancers make too much of a growth protein called HER2. Triple-negative cancers have neither hormone receptors nor extra HER2. These markers guide treatment. Breast cancer mostly affects women aged 45 and older. Fewer than 1 in 100 breast cancers in the United States occur in men.

Breast cancer is caused by changes in how breast cells grow and divide. Many risk factors raise the chance of these changes, but most do not directly cause cancer. Besides being female, the biggest risk factor is getting older. The U.S. National Cancer Institute says that, on average, a woman has about a 1 in 8 chance of getting breast cancer in her lifetime.

Some risks come from genes and family history. Harmful changes in genes such as BRCA1, BRCA2, PALB2, CHEK2, TP53, CDH1, PTEN, STK11 and ATM raise the risk. Some of these, such as BRCA1, BRCA2, PALB2 and PTEN changes, can raise it to 60 in 100 or more. So can chest radiation before age 30, for example as treatment for a childhood cancer.

Other risk factors include dense breast tissue, a past breast cancer or DCIS, excess body weight after menopause and combined estrogen-plus-progestin hormone therapy for menopause. Drinking alcohol and not being physically active also raise the risk. So does longer lifetime exposure to the body’s own estrogen. Examples are periods that start early, a late menopause, a first birth at an older age, never carrying a pregnancy to term and never breastfeeding. Having a risk factor does not mean a person will get breast cancer, and some people who get it have no known risk factor.

Early breast cancer often causes no symptoms, which is why screening mammograms matter. When there are signs, the most common is a lump or thick area in or near the breast or under the arm. Others include a change in breast size or shape, fluid from the nipple that is not breast milk, a nipple that changes shape, flattens or points a different way, and skin changes. Skin may become red, darker, scaly, swollen or dimpled. Breast cancer does not usually cause pain.

Breast cancer can spread (metastasize) through the blood or lymph system to the bones, liver, lungs, brain or other organs. This is called metastatic or stage 4 breast cancer. Symptoms then depend on where it has spread. They can include bone pain or broken bones, shortness of breath and a dry cough that will not go away. Belly pain, yellowing of the skin (jaundice), severe headaches, seizures or vision changes can also occur.

Most breast changes are not cancer. Breast cysts and hormone changes before a period, for example, can cause soreness or pain.

Hormone therapy is often taken for 5 years or more. Side effects depend on the treatment. The most common side effect of chemotherapy is deep tiredness (fatigue). Others include hair loss, mouth sores and nausea. Many people with breast cancer also deal with swelling of the arm from lymph node surgery (lymphedema) and worries about fertility. Men more often have a mastectomy than women, because they have less breast tissue.

Surgery, reconstruction choices and hair loss can change how people see their bodies and feel about intimacy. Emotions can run high at every stage. When treatment ends, many people feel relief mixed with fear that the cancer will come back. Tests at follow-up visits can bring on anxiety, sometimes called “scanxiety.”

After treatment, people usually have regular checkups. These can include breast exams and mammograms, and some people also have breast MRI. Care teams may give a survivorship care plan that lists past treatments and future follow-up. Some treatments can cause late effects that appear months or years later. Many people also face money worries, even with health insurance.

Breast cancer is not treated with a donor transplant, and no donor search is part of its care. When high-dose chemotherapy with stem cell rescue has been used, in trials or for a few carefully chosen patients, it has used the person’s own stem cells. They are collected before treatment and returned afterward. European transplant experts (EBMT, 2025) rate a transplant from a matched brother or sister for metastatic breast cancer as generally not recommended.

A donor can matter in a rare later situation. Years after chemotherapy or radiation for any cancer, a small number of people develop a different blood cancer, a therapy-related myeloid neoplasm. In a U.S. study of 20,063 people treated for stage 1 to 3 breast cancer from 1998 to 2007, about 5 in 1,000 had developed a marrow cancer by 10 years. A donor stem cell transplant is one treatment for these leukemias. In a European registry of 2,334 adults who had a donor transplant for them from 2006 to 2016, 643 (28%) first had breast cancer.

People who have had breast cancer sometimes ask whether they can join a donor registry. NMDP’s U.S. rules depend on the kind of cancer and treatment. They are explained in the questions below.