Breast cancer starts when cells in breast tissue begin to grow abnormally and multiply without stopping

Breast tissue contains different types of cells—some that make milk, some that carry milk to the nipple, and some that provide structure and support. Cancer begins when one of these cells develops a mutation in its DNA that tells it to divide repeatedly. Unlike healthy cells, which follow a built-in limit on how many times they can divide, cancer cells ignore that limit. Over time, these abnormal cells can form a mass large enough to feel as a lump or to show up on imaging.

Most breast cancers start in the cells that line the milk ducts (ductal carcinoma) or in the cells that produce milk (lobular carcinoma). These two types account for the majority of cases. The cancer may stay contained within the duct or lobe for a time—a stage called in situ—or it may break through the wall and begin to invade surrounding tissue. Once it invades, it becomes invasive breast cancer and carries a higher risk of spreading to lymph nodes or distant organs.

Key Takeaways

  • Breast cancer develops when cells in breast tissue acquire mutations that cause them to divide uncontrollably, forming a mass of abnormal cells.
  • Ductal carcinoma (starting in milk ducts) and lobular carcinoma (starting in milk-producing cells) are the two most common types.
  • In situ cancers are confined to the duct or lobe, while invasive cancers have broken through tissue walls and can spread to lymph nodes or other organs.
  • Risk factors include age, family history, certain genetic mutations, hormone exposure, and lifestyle factors, though having a risk factor does not mean cancer will develop.
  • Early detection through screening or noticing changes in breast appearance or sensation can lead to treatment when the cancer is smaller and more localized.

How breast tissue is organized and where cancer can start

The breast is made up of lobes—clusters of milk-producing glands—connected by ducts that carry milk to the nipple. Surrounding these structures is fatty tissue, connective tissue, and blood vessels. Lymph vessels run through the breast and connect to lymph nodes in the armpit, collarbone, and chest. Understanding this anatomy helps explain why doctors check lymph nodes and why cancer location matters for treatment.

Cancer can start in any of these tissues, but the vast majority begins in the epithelial cells—the inner lining of ducts and lobes. A small number of breast cancers start in the fatty or connective tissue (phyllodes tumors or sarcomas) or in the cells that line blood vessels. The location where cancer begins affects how it behaves and how it is treated, which is why pathologists examine tissue samples to identify exactly where the cancer originated.

The difference between in situ and invasive cancer

In situ means "in place." Ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS) are cancers that have not yet broken through the wall of the duct or lobe. The abnormal cells are still contained within their original location. DCIS is considered a higher-risk condition because cells are actively multiplying and can progress to invasive cancer; LCIS is considered a marker of increased risk rather than cancer itself, though it signals that invasive cancer is more likely to develop elsewhere in either breast.

Once cancer breaks through the duct or lobe wall and invades the surrounding tissue, it becomes invasive ductal carcinoma (IDC) or invasive lobular carcinoma (ILC). At this point, the cancer has access to blood vessels and lymph vessels, which means it can travel to lymph nodes and potentially to distant organs. Invasive cancers are staged based on size, lymph node involvement, and whether they have spread to other parts of the body—information that guides treatment decisions.

What causes the mutations that lead to breast cancer

Cancer develops when a cell accumulates mutations in genes that normally control cell division and death. Some people inherit mutations in genes like BRCA1 or BRCA2 that significantly raise their lifetime risk of breast cancer. Most breast cancers, however, develop from mutations that occur during a person's lifetime—not inherited mutations—often from a combination of factors.

Age is the strongest risk factor: breast cancer is more common as women get older, because cells have had more time to accumulate mutations. Hormone exposure matters too; longer exposure to estrogen and progesterone (from starting menstruation early, having fewer pregnancies, or using hormone therapy) increases risk. Alcohol use, obesity, and smoking are associated with higher risk. Radiation exposure, particularly to the chest during childhood or young adulthood, also raises risk. Family history of breast cancer, especially in a close relative diagnosed before age 50, signals that genetic factors may be at play. Having one or more risk factors does not mean cancer will develop—many people with risk factors never get breast cancer, and some without known risk factors do.

How breast cancer grows and spreads

Cancer cells divide much faster than normal breast cells, but growth is not instantaneous. A tumor typically takes months to years to reach a size that can be felt or seen on imaging. As the tumor grows, it may trigger the formation of new blood vessels to feed it—a process called angiogenesis. These new vessels also provide a route for cancer cells to enter the bloodstream.

Lymph nodes are often the first place breast cancer spreads. Cancer cells can travel through lymph vessels to nodes in the armpit (axillary nodes), above the collarbone (supraclavicular nodes), or inside the chest (internal mammary nodes). If cancer cells reach the bloodstream, they can travel to distant organs: the bones, lungs, liver, or brain. This spread is called metastasis. The more lymph nodes involved and the farther the cancer has spread, the more advanced the disease and the more intensive the treatment typically becomes.

Subtypes of breast cancer based on hormone and growth receptors

Not all breast cancers behave the same way. Pathologists test tumor tissue for the presence of three key receptors: estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). These receptors act like switches that tell cancer cells to grow. Knowing which receptors are present shapes treatment strategy.

Hormone receptor-positive (HR+) cancers have ER and/or PR receptors and grow in response to estrogen or progesterone. These cancers often grow more slowly and may respond to hormone-blocking drugs. HER2-positive cancers have high levels of the HER2 protein and tend to grow faster; they respond to drugs that target HER2. Triple-negative cancers lack all three receptors and do not respond to hormone therapy or HER2-targeted drugs, making chemotherapy the primary option. Inflammatory breast cancer and Paget's disease of the nipple are rare subtypes with distinct characteristics. Understanding the subtype helps doctors predict how the cancer will behave and which treatments are most likely to work.

Symptoms and how breast cancer is detected

Early breast cancer often causes no symptoms. That is why screening—mammography in particular—can find cancer before a person notices anything. When symptoms do appear, they may include a lump or thickening in the breast or armpit, dimpling or puckering of the skin, redness or scaling of the nipple or breast skin, nipple discharge, or breast pain. Changes in breast size or shape can also signal cancer, though many benign conditions cause similar changes.

Screening mammography is offered to women starting at age 40 or 50, depending on guidelines and individual risk. When a mammogram or clinical exam finds something suspicious, further imaging (ultrasound or MRI) or a biopsy may follow. A biopsy—removal of a small sample of tissue for examination under a microscope—is the only way to confirm whether cells are cancerous. Once cancer is confirmed, additional tests determine the stage, subtype, and whether it has spread, all of which inform treatment planning.

Frequently Asked Questions

Can men get breast cancer?

Yes, though it is much less common. Men have breast tissue and can develop ductal carcinoma or other types of breast cancer. Male breast cancer accounts for about 1% of all breast cancers. Risk factors include family history, BRCA mutations, obesity, and liver disease. Detection is often delayed because men and their doctors may not suspect breast cancer, so it is sometimes found at a later stage.

If I have a lump, does that mean I have cancer?

No. Most breast lumps are benign—fibroadenomas, cysts, or areas of dense tissue are common and not cancer. Only a biopsy can determine whether a lump is cancerous. If you notice a new lump or change in your breast, contact your doctor for evaluation, but a lump alone does not indicate cancer.

Does breast cancer always require surgery?

Most invasive breast cancers are treated with surgery to remove the tumor, but the extent varies. Some cancers are treated with chemotherapy first to shrink the tumor before surgery. In rare cases of advanced cancer, surgery may not be the first step. Your doctor will discuss the options based on the cancer's stage, location, and subtype.

What is the difference between breast cancer and breast disease?

Breast disease is a broad term for any condition affecting breast tissue—cysts, infections, hormonal changes, or benign tumors. Cancer is one type of breast disease, but most breast diseases are not cancer. A doctor can help determine whether a change in your breast is benign or requires further investigation.

Can I lower my risk of breast cancer?

Some risk factors like age and family history cannot be changed, but others can. Limiting alcohol, maintaining a healthy weight, staying physically active, and avoiding hormone therapy when possible are associated with lower risk. Breastfeeding, if you are able, also lowers risk. For people with a strong family history or BRCA mutations, screening starting at a younger age or preventive medications may be options to discuss with a doctor.