Breast cancer is a disease in which cells in the breast grow out of control and form a tumor. It is the most common cancer diagnosed in women, with an estimated 321,910 new cases expected in the United States in 2026 alone. About 13 percent of women will be diagnosed with breast cancer at some point in their lifetime.
How Breast Cancer Develops
Breast cancer begins when the DNA inside a breast cell becomes damaged or mutated. Normally, cells have built-in repair systems that fix these errors. But when the damage accumulates beyond what the cell can repair, the cell starts dividing without the usual controls. These abnormal cells multiply, eventually forming a mass of tissue, or tumor.
In the earliest stage, the abnormal cells stay contained within the milk ducts or lobules (the small glands that produce milk). This is called “in situ” cancer, meaning it hasn’t spread. Over time, though, these cells can acquire the ability to break through the walls of the duct or lobule and invade surrounding breast tissue. Once they do, the cancer can also enter the bloodstream or lymphatic system and travel to other parts of the body, a process called metastasis.
Two things accelerate this process: genetic vulnerability and environmental exposure. Some people inherit gene mutations that make their cells more prone to errors in the first place. Then factors like prolonged estrogen exposure, certain chemicals, or other environmental triggers cause additional mutations on top of that inherited risk. The cancer cells also develop ways to hide from the immune system and recruit nearby cells to help fuel their growth.
Types of Breast Cancer
Breast cancers are classified by where they start and whether they’ve spread beyond that starting point.
Ductal carcinoma in situ (DCIS) is the earliest, non-invasive form. The abnormal cells are confined to the milk ducts and haven’t broken into surrounding tissue. DCIS is highly treatable, though it can progress to invasive cancer if left alone.
Invasive ductal carcinoma (IDC) is the most common type, accounting for the majority of breast cancer diagnoses. It starts in the milk ducts but has broken through into the surrounding breast tissue. From there, it has the potential to spread to lymph nodes and other organs.
Invasive lobular carcinoma (ILC) is the second most common type, making up roughly 10 percent of all breast cancers. It starts in the milk-producing lobules. ILC behaves differently from ductal cancer in important ways: its cells tend to spread in a single-file pattern through the tissue rather than forming a distinct lump, which can make it harder to detect on mammograms. ILC is also less likely to respond to chemotherapy, but it tends to be very responsive to hormonal treatments because the vast majority of these tumors are fueled by estrogen.
Molecular Subtypes and Why They Matter
Beyond the physical type, every breast cancer is tested for specific receptors on its cells. These receptors determine which treatments will work and significantly influence prognosis.
- Estrogen receptor positive (ER-positive): The cancer cells have receptors that respond to estrogen. Hormone-blocking treatments can slow or stop growth. This is the most common subtype.
- Progesterone receptor positive (PR-positive): The cancer cells respond to progesterone. These tumors also benefit from hormone-blocking therapy and often overlap with ER-positive cancers.
- HER2-positive: The cancer cells produce excessive amounts of a protein called HER2, which drives aggressive growth. Targeted drugs that block HER2 have dramatically improved outcomes for this subtype.
- Triple-negative: The cancer cells lack estrogen receptors, progesterone receptors, and excess HER2 protein. This means hormonal therapies and HER2-targeted drugs are ineffective, leaving chemotherapy and newer immunotherapy options as the primary treatments. Triple-negative breast cancer tends to grow faster and has fewer targeted treatment options, though research in this area is advancing rapidly.
Risk Factors
Some breast cancer risk factors are beyond your control. Age is the single biggest one: most breast cancers are diagnosed after age 50. Inherited mutations in the BRCA1 and BRCA2 genes substantially raise risk for both breast and ovarian cancer. Having a first-degree relative (mother, sister, or daughter) with breast cancer also increases your risk, especially if that relative was diagnosed young. A family history of ovarian cancer or breast cancer in a male relative raises risk as well.
Other risk factors are tied to lifestyle and environment. Physical inactivity, being overweight after menopause, and drinking alcohol all increase risk. Alcohol in particular shows a dose-response relationship: the more you drink, the higher your risk. Hormone replacement therapy that combines estrogen and progesterone, taken for more than five years during menopause, also raises risk. Reproductive factors play a role too: having your first pregnancy after age 30, never having a full-term pregnancy, and not breastfeeding are all associated with higher risk. Smoking, exposure to cancer-causing chemicals, and working night shifts (which disrupts hormones) may also contribute.
Symptoms and Warning Signs
The most recognized sign of breast cancer is a new lump in the breast or underarm area, but many breast cancers present with other changes first. Watch for thickening or swelling in part of the breast, dimpling or irritation of the skin, redness or flaky skin around the nipple, or a nipple that suddenly pulls inward. Nipple discharge that isn’t breast milk, particularly if it contains blood, is another warning sign. Any unexplained change in the size or shape of one breast, or persistent pain in any area of the breast, is worth getting checked.
It’s worth noting that many of these symptoms can also be caused by benign conditions like cysts or infections. But because early detection makes such a significant difference in outcomes, any new or unusual breast change should be evaluated promptly.
How Breast Cancer Is Diagnosed
Diagnosis typically follows a step-by-step process that starts with imaging and ends with a tissue sample. If a screening mammogram shows something abnormal, the next step is usually a diagnostic mammogram, which takes more detailed X-ray images of the suspicious area. An ultrasound, which uses sound waves to create images, can help distinguish between a solid mass and a fluid-filled cyst. In some cases, an MRI is used to get a more detailed picture, particularly for women with dense breast tissue or a high genetic risk.
None of these imaging tools can confirm cancer on their own. A biopsy is the only way to make a definitive diagnosis. During a biopsy, a small sample of tissue is removed from the suspicious area and examined under a microscope. There are several types: a fine-needle aspiration uses a thin needle to draw out cells, a core biopsy uses a slightly larger needle to remove a small cylinder of tissue, and an open biopsy involves a minor surgical incision. The biopsy not only confirms whether cancer is present but also provides the receptor and molecular information that guides treatment decisions.
Stages of Breast Cancer
Once breast cancer is diagnosed, it is assigned a stage from 0 to IV based on three key factors: the size of the tumor, whether cancer has reached nearby lymph nodes, and whether it has spread to distant organs.
Stage 0 is non-invasive cancer that remains contained within the ducts or lobules (such as DCIS). Stage I cancers are small, typically 2 centimeters or less, and either haven’t reached the lymph nodes or involve only tiny deposits. Stage II tumors are larger or have spread to a small number of nearby lymph nodes. Stage III is locally advanced cancer, meaning it has grown into the chest wall or skin or involves many lymph nodes, but hasn’t traveled to distant organs. Stage IV means the cancer has metastasized to other parts of the body, most commonly the bones, lungs, liver, or brain.
The stage at diagnosis is one of the strongest predictors of survival. Breast cancer caught while still localized to the breast has a five-year survival rate above 99 percent. When it has spread to nearby lymph nodes, that rate drops to roughly 86 percent. For cancer that has metastasized to distant organs, the five-year survival rate is around 32 percent, though newer treatments are improving that number.
Treatment Options
Treatment plans depend on the cancer’s stage, type, and molecular subtype. Most people receive a combination of therapies.
Surgery is typically the first step. A lumpectomy removes only the tumor and a margin of surrounding tissue, preserving the rest of the breast. The procedure takes one to two hours, and most women go home the same day. A mastectomy removes the entire breast and is recommended when the tumor is large relative to the breast, when there are multiple tumors, or when genetic risk makes recurrence likely. Some women choose to have breast reconstruction during the same operation. In both cases, the surgeon usually removes nearby lymph nodes to check whether the cancer has spread.
Chemotherapy targets cancer cells that may have traveled beyond the breast. It is sometimes given before surgery to shrink a large tumor, making it possible to do a lumpectomy instead of a mastectomy. After surgery, chemotherapy lowers the chance of the cancer returning. Radiation therapy, which uses targeted energy beams to kill remaining cancer cells in the breast or chest wall, is standard after a lumpectomy and sometimes after a mastectomy.
For hormone receptor-positive cancers, hormone therapy blocks the body’s estrogen or progesterone from fueling cancer growth. These medications are typically taken for five to ten years after initial treatment. For HER2-positive cancers, targeted drugs that specifically attack the HER2 protein are highly effective and are used alongside chemotherapy. Triple-negative breast cancers, which lack these treatment targets, are increasingly treated with immunotherapy drugs that help the immune system recognize and attack cancer cells, often combined with chemotherapy.
Breast Cancer in Men
Though far less common, men can develop breast cancer too. Men have a small amount of breast tissue, and cancer can form in it the same way it does in women. Male breast cancer accounts for less than 1 percent of all breast cancer cases, but it tends to be diagnosed at a later stage because most men aren’t aware it’s possible. A lump near the nipple, changes to the skin over the chest, or nipple discharge are the most common signs. Men with BRCA2 mutations or a strong family history of breast cancer are at the highest risk.