Breast cancer is a disease in which cells in the breast grow out of control, forming a tumor that can invade surrounding tissue and, in some cases, spread to other parts of the body. It is the most common cancer in women worldwide, though it also affects men: about 1 out of every 100 breast cancers diagnosed in the United States is found in a man. When caught early and still confined to the breast, the five-year survival rate is nearly 100%.
How Breast Cancer Develops
Normal breast cells grow, divide, and die on a predictable schedule, regulated by signaling pathways that control how quickly cells multiply, when they stop, and when they self-destruct. Breast cancer begins when genetic changes disrupt these controls. Some mutations switch on genes that force cells to keep dividing (called oncogenes), while others disable genes meant to pump the brakes on growth (tumor suppressors). The result is a population of cells that multiplies without the usual checks.
Over time, these abnormal cells can also reshape the tissue around them, converting their local environment into one that supports further tumor growth. If left unchecked, cancer cells gain the ability to break away from the original tumor and travel through blood or lymph vessels to distant organs, most commonly the bones, lungs, liver, or brain.
Types of Breast Cancer
Most breast cancers begin in one of two structures: the ducts that carry milk to the nipple or the lobules that produce milk. The most common type, invasive ductal carcinoma, starts in the ducts and then grows into the surrounding breast tissue. Invasive lobular carcinoma is the second most common type, making up roughly 10% of all cases. Its cells look and behave differently: they are small, tend not to stick together, and infiltrate tissue in single-file lines rather than forming a solid mass. This pattern can make lobular cancers harder to detect on a mammogram.
A non-invasive form called ductal carcinoma in situ (DCIS) is sometimes found during screening. In DCIS, abnormal cells line the inside of a duct but have not yet broken through into surrounding tissue. It is considered a precursor that may progress to invasive cancer if untreated.
Molecular Subtypes
Beyond where a cancer starts, doctors also classify it by the receptors on its cells, because these receptors determine which treatments will work. The three main groupings are:
- Hormone receptor-positive (HR+): The most common subtype. These cancer cells have receptors for estrogen, progesterone, or both, which fuel their growth. Therapies that block these hormones are highly effective, and the five-year survival rate for localized HR+/HER2- cancer is 100%.
- HER2-positive: These cells overproduce a protein called HER2 that promotes rapid growth. Targeted drugs that block HER2 have dramatically improved outcomes. Localized HER2-positive cancers have five-year survival rates above 97%.
- Triple-negative: These cells lack hormone receptors and do not overproduce HER2, which means they don’t respond to hormone therapy or HER2-targeted drugs. Triple-negative breast cancer tends to be more aggressive. The five-year survival rate for localized cases is about 92%, dropping to roughly 15% when the cancer has spread to distant sites.
Warning Signs and Symptoms
The most recognized symptom is a new lump in the breast or underarm, but breast cancer can show up in several other ways. The CDC lists these warning signs:
- Thickening or swelling of part of the breast
- Dimpling or irritation of the skin, sometimes resembling an orange peel
- Redness or flaky skin on the nipple or breast
- Pulling in of the nipple or nipple pain
- Discharge from the nipple other than breast milk, including blood
- Any change in the size or shape of the breast
- Pain in any area of the breast
In men, the most common sign is a lump or swelling, though redness, skin dimpling, nipple discharge, and nipple retraction also occur. Because men rarely expect breast cancer, it is often diagnosed at a later stage.
Risk Factors
Age is the single biggest risk factor. Most breast cancers are diagnosed after age 50. Family history matters too: having a close relative (parent, sibling, or child) who had breast cancer roughly doubles your risk. Inherited mutations in the BRCA1 or BRCA2 genes carry the highest genetic risk. More than 60% of women who inherit a harmful change in either gene will develop breast cancer during their lifetime. In men, these same mutations also raise the risk of breast cancer, high-grade prostate cancer, and pancreatic cancer.
Other factors that increase risk include previous radiation therapy to the chest, obesity (particularly after menopause), long-term use of hormone replacement therapy, and heavy alcohol consumption. Some conditions specific to men, like Klinefelter syndrome or liver cirrhosis, increase breast cancer risk by raising estrogen levels.
How Breast Cancer Is Staged
Staging tells you how far the cancer has progressed and directly shapes treatment decisions. Doctors use three measurements: the size of the tumor, whether cancer has reached nearby lymph nodes, and whether it has spread to distant organs.
Tumor size is grouped into categories. A tumor 20 millimeters (about ¾ inch) or smaller is classified as T1. Between 20 and 50 millimeters is T2. Larger than 50 millimeters is T3. A tumor that has grown into the chest wall or skin is T4. For lymph nodes, the scale runs from N0 (no cancer in the lymph nodes) through N3 (cancer in 10 or more underarm lymph nodes or in lymph nodes below the collarbone). Metastasis is simply M0 (no spread) or M1 (cancer found in distant organs).
These measurements combine into an overall stage. The practical takeaway is the distinction between localized cancer (still confined to the breast), regional cancer (spread to nearby lymph nodes), and distant cancer (spread to other organs). Five-year survival rates across all subtypes are 100% for localized, 87% for regional, and about 33% for distant disease.
Screening and Diagnosis
The U.S. Preventive Services Task Force recommends that all women begin routine mammograms at age 40 and continue every other year through age 74. People with higher risk factors, such as BRCA mutations or a strong family history, may start screening earlier or use additional imaging like breast MRI.
If a mammogram or physical exam finds 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 extract a small cylinder of tissue. Fine-needle aspiration uses a thinner needle and is sometimes used for fluid-filled cysts or lymph nodes. In some cases, a surgical biopsy removes the entire suspicious area along with a margin of normal tissue. For lesions that are hard to feel, a thin wire may be placed in the breast beforehand to guide the surgeon to the right spot.
The biopsy determines not only whether cancer is present but also its type, grade, and receptor status, all of which shape the treatment plan.
Treatment Overview
Treatment depends on the cancer’s stage, subtype, and your overall health. Most people with early-stage breast cancer have surgery to remove the tumor, either a lumpectomy (removing the tumor and a small margin of surrounding tissue) or a mastectomy (removing the entire breast). Radiation therapy often follows lumpectomy to destroy any remaining cancer cells in the area.
For hormone receptor-positive cancers, hormone-blocking therapy typically continues for five to ten years after surgery to reduce the chance of recurrence. HER2-positive cancers are treated with targeted drugs that block the HER2 protein, usually given alongside chemotherapy. Triple-negative breast cancers rely more heavily on chemotherapy, though newer immunotherapy options have improved outcomes for some patients.
Advanced or metastatic breast cancer is treated as a chronic condition. The goal shifts from cure to controlling the disease and maintaining quality of life for as long as possible, using combinations of the therapies above tailored to the cancer’s molecular profile.