Breast cancer is the uncontrolled growth of abnormal cells in breast tissue. It develops when cells in the milk ducts or milk-producing glands accumulate genetic damage that disrupts normal growth controls, allowing them to multiply without stopping and, in many cases, spread to other parts of the body. Globally, about 2.3 million women are diagnosed each year, making it the most common cancer in women worldwide.
How Breast Cancer Starts
Healthy breast cells follow a tightly regulated life cycle: they grow, divide, and eventually die on schedule. Breast cancer begins when mutations disrupt the genes that control this cycle. The damaged cells lose the ability to repair their own DNA or to self-destruct when something goes wrong. Instead, they keep dividing, passing those errors along to each new generation of cells until a mass of abnormal tissue, a tumor, forms.
Two of the most well-known genetic mutations linked to breast cancer are BRCA1 and BRCA2. These genes normally help repair damaged DNA. When they’re mutated, cells continue to multiply even when their DNA is broken, creating a snowball effect of genetic instability. Other mutations in genes like TP53 and PTEN also play a role by disrupting the checkpoints cells use to catch and correct errors before dividing.
Estrogen, a hormone naturally present in breast tissue, can accelerate the process. Higher estrogen levels push cells to divide more rapidly, giving them less time to fix mistakes. Over years, these accumulated errors can tip a normal cell into cancerous behavior.
Non-Invasive vs. Invasive Cancer
Not all breast cancers behave the same way. Non-invasive breast cancer stays contained within the ducts or lobules where it started. It hasn’t broken through the walls of those structures into surrounding tissue. This is sometimes called “stage 0” or carcinoma in situ, and it carries a much lower risk of becoming life-threatening.
Invasive breast cancer, by contrast, has pushed through the duct or lobule wall and into nearby breast tissue. Once it reaches surrounding tissue, it has the potential to enter the lymph system or bloodstream and travel to distant organs like the bones, lungs, liver, or brain. This process is called metastasis, and it’s what makes cancer dangerous.
The Most Common Types
About 8 in 10 invasive breast cancers are invasive ductal carcinoma (IDC), which begins in the cells lining a milk duct and then breaks through the duct wall. It’s the type most people are referring to when they say “breast cancer.”
Invasive lobular carcinoma (ILC) accounts for roughly 1 in 10 cases. It starts in the milk-producing glands (lobules) and tends to be harder to detect on mammograms and physical exams. About 1 in 5 women diagnosed with ILC have cancer in both breasts at the time of diagnosis.
Receptor Status and Why It Matters
When breast cancer is diagnosed, one of the first things tested is what’s driving the cancer cells to grow. Pathologists check for three key biomarkers: estrogen receptors, progesterone receptors, and a protein called HER2. These results shape the entire treatment plan.
If cancer cells have estrogen or progesterone receptors, the cancer is considered “hormone receptor-positive,” and treatments that block those hormones can slow or stop its growth. If the cells produce excess HER2 protein, therapies that target HER2 are effective. Triple-negative breast cancer tests negative for all three biomarkers, which means hormone-blocking drugs and HER2-targeted therapies won’t work. This makes triple-negative breast cancer harder to treat, though chemotherapy and immunotherapy are options.
Warning Signs to Recognize
The CDC lists several physical changes that can signal breast cancer:
- A new lump in the breast or armpit
- Thickening or swelling of part of the breast
- Dimpling or irritation of the skin
- Redness or flaky skin on the nipple or breast
- Nipple pulling inward or nipple pain
- Discharge from the nipple other than breast milk, including blood
- Any change in breast size or shape
- Pain in any area of the breast
Many of these symptoms have non-cancerous explanations, but any persistent change is worth getting checked.
How Breast Cancer Is Found and Diagnosed
The U.S. Preventive Services Task Force recommends mammograms every two years for women ages 40 through 74 at average risk. Mammography remains the primary screening tool for catching breast cancer before symptoms appear.
If imaging reveals something suspicious, the next step is a biopsy, where a small sample of tissue is removed and examined under a microscope. The most common approach is a core needle biopsy, which uses a slightly larger needle to collect tissue and typically takes 20 to 60 minutes under local anesthesia. Fine-needle aspiration is quicker (10 to 15 minutes) and uses a thinner needle, but collects less tissue. In some cases, a surgical biopsy is needed, where a surgeon removes part or all of the abnormal area under general anesthesia. Only a biopsy can confirm whether cancer is present.
Staging: How Far It Has Spread
Once breast cancer is confirmed, doctors determine its stage using three factors: the size and extent of the primary tumor (T), whether cancer has reached nearby lymph nodes (N), and whether it has spread to distant parts of the body (M). Together, these factors place the cancer on a scale from stage 0 (non-invasive, confined to the duct or lobule) through stage IV (cancer has spread to distant organs).
Staging matters because it directly predicts outcomes and guides treatment decisions. The five-year relative survival rate for localized breast cancer (cancer that hasn’t spread beyond the breast) is 100%. When cancer has spread to nearby lymph nodes, that number is 87.5%. For distant-stage breast cancer that has reached other organs, the five-year survival rate drops to 33.8%. These numbers, drawn from SEER data covering 2016 through 2022, underscore why early detection is so consequential.
Treatment Options
Breast cancer treatment falls into two broad categories. Local treatments target the tumor itself: surgery (either a lumpectomy to remove the tumor or a mastectomy to remove the breast) and radiation therapy to destroy remaining cancer cells in the area. Systemic treatments travel through the entire body to find and kill cancer cells that may have spread. These include chemotherapy, hormone therapy for hormone receptor-positive cancers, targeted therapy for HER2-positive cancers, and immunotherapy, which is used primarily for triple-negative breast cancer.
Not everyone receives every type of treatment. A small, early-stage hormone receptor-positive cancer might be treated with surgery and hormone therapy alone. A more aggressive or advanced cancer might require a combination of surgery, chemotherapy, radiation, and targeted drugs. Treatment plans are built around the cancer’s stage, receptor status, and how quickly it’s growing.
Who Gets Breast Cancer
Breast cancer risk varies significantly by geography and development level. In countries with very high development, 1 in 12 women will be diagnosed in their lifetime. In lower-income countries, the rate is about 1 in 27, though women in those countries are more likely to die from it (1 in 48 compared to 1 in 71) because of later diagnosis and less access to treatment.
While breast cancer overwhelmingly affects women, it can also occur in men, though this is rare. The major risk factors include age, family history (particularly BRCA1/BRCA2 mutations), prolonged estrogen exposure from early menstruation or late menopause, obesity, and alcohol use. Having risk factors doesn’t mean you’ll develop breast cancer, and many women diagnosed have no obvious risk factors at all.