Breast cancer is a disease in which cells in the breast grow out of control, forming tumors that can invade surrounding tissue and spread to other parts of the body. It is the most commonly diagnosed cancer worldwide, with 2.3 million new cases and 670,000 deaths globally in 2022. While the word “cancer” can feel overwhelming, survival rates have improved dramatically, and understanding the basics helps you make sense of screening results, diagnoses, and treatment options.
How Breast Cancer Develops
Healthy breast cells follow strict instructions encoded in their DNA: grow, divide, stop, and die on schedule. Cancer begins when genetic changes disrupt that cycle. Some mutations activate genes that push cells to multiply faster than normal. Others disable genes responsible for repairing damaged DNA or triggering cell death. The result is a population of cells that keeps growing when it shouldn’t, eventually forming a mass of abnormal tissue.
Two well-known protective genes, BRCA1 and BRCA2, normally help repair broken DNA. When either gene carries a harmful mutation, the cell’s repair machinery becomes less efficient, and damaged cells are more likely to survive and multiply. Women who inherit a BRCA1 or BRCA2 mutation face a lifetime breast cancer risk above 60%, compared with about 13% for women in the general population. Men can carry these mutations too: those with a BRCA2 change have a 1.8% to 7.1% chance of developing breast cancer by age 70, versus roughly 0.1% for men overall.
Not all breast cancers are inherited. The majority arise from mutations that accumulate over a person’s lifetime due to aging, hormonal exposure, or other factors that aren’t fully understood. Whether inherited or acquired, the underlying principle is the same: cells escape the normal controls on growth and begin expanding unchecked.
Where It Starts: Types of Breast Cancer
Breast tissue is made up of milk-producing glands (lobules) connected by small tubes (ducts) that carry milk to the nipple. Most breast cancers start in one of these two structures, and the location determines the type.
Invasive ductal carcinoma is by far the most common, originating in the cells lining the milk ducts and eventually breaking through into surrounding breast tissue. Invasive lobular carcinoma, the second most common type, accounts for about 10% of all breast cancers. It starts in the lobules and tends to grow in a different pattern. Rather than forming a distinct hard lump, lobular cancer cells often spread in single-file lines through the tissue, which can make them harder to detect on a mammogram.
A less common but important form is inflammatory breast cancer, representing 1% to 5% of cases. It behaves very differently from other types. Cancer cells block the tiny lymph vessels in the skin of the breast, causing redness, swelling, and a dimpled texture that resembles orange peel. Symptoms come on rapidly, often over weeks. There is usually no distinct lump, and standard mammograms frequently miss it. Inflammatory breast cancer is already at least stage III at diagnosis and requires aggressive, longer-duration treatment.
Molecular Subtypes and Why They Matter
Beyond where a tumor starts, doctors classify breast cancer by what’s happening on the surface of its cells. Lab tests check for three key receptors: estrogen, progesterone, and a protein called HER2. The combination determines which treatments will work.
Cancers that test positive for estrogen or progesterone receptors (often called “hormone receptor-positive”) rely on those hormones to grow. Therapies that block hormone activity in the body can slow or stop these tumors and are among the most effective breast cancer treatments available. HER2-positive cancers produce excess amounts of the HER2 protein, which drives faster growth. Targeted drugs that interfere with HER2 have dramatically improved outcomes for this subtype.
Triple-negative breast cancer tests negative for all three receptors. Because the most common targeted therapies don’t apply, treatment relies on chemotherapy and newer approaches like immunotherapy. Triple-negative cancers tend to be more aggressive and are more common in younger women and Black women.
Signs and Symptoms
The most familiar sign is a new lump or thickening in the breast or underarm area. But breast cancer can announce itself in other ways that are easy to overlook. Skin changes include dimpling that looks like orange peel, persistent redness, or areas of thickened skin. The nipple may turn inward (become inverted) when it wasn’t before, develop a scaly rash, or produce discharge, sometimes bloody, in someone who isn’t pregnant or breastfeeding. A noticeable change in breast size or shape, especially if it develops on one side only, also warrants attention.
Some breast cancers cause no symptoms at all in their early stages, which is why routine screening matters. Inflammatory breast cancer is a notable exception to the “lump” rule: it typically causes rapid swelling, warmth, and skin discoloration rather than a palpable mass, and its symptoms can be mistaken for an infection.
How Breast Cancer Is Diagnosed
Diagnosis usually follows a step-by-step process that narrows down what’s going on. A screening mammogram (breast X-ray) is often the first tool, capable of detecting abnormalities before you can feel them. If something suspicious appears, additional imaging follows. Ultrasound can help distinguish a fluid-filled cyst from a solid mass, and MRI provides detailed images when more information is needed.
Imaging alone cannot confirm cancer. A biopsy, in which a small sample of tissue is removed and examined under a microscope, is the only way to make a definitive diagnosis. The most common approach is a core needle biopsy, where a hollow needle extracts several tiny tissue samples, each about the size of a grain of rice. The area is numbed first, and imaging (usually ultrasound) guides the needle to the right spot. For lumps that can be felt easily, a fine-needle aspiration may be used first to draw out cells or fluid with a very thin needle. In some cases, a stereotactic biopsy uses mammogram images taken from multiple angles to pinpoint the exact location of an abnormality that can’t be felt.
The biopsy sample tells pathologists not just whether cancer is present, but what type it is, how fast it’s growing, and which receptors are on the cell surfaces. All of that information shapes the treatment plan.
Stages of Breast Cancer
Staging describes how far the cancer has spread and is one of the strongest predictors of outcome. The system considers three factors: the size of the tumor, whether cancer has reached nearby lymph nodes, and whether it has traveled to distant organs.
Stage 0 refers to non-invasive cancer that remains contained within the ducts or lobules and hasn’t broken into surrounding tissue. Stages I through III describe progressively larger tumors or greater lymph node involvement, but the cancer is still confined to the breast and nearby regions. Stage IV means the cancer has spread (metastasized) to distant parts of the body, most commonly the bones, liver, lungs, or brain.
Survival rates reflect these stages clearly. When breast cancer is detected while still localized (confined to the breast), the five-year relative survival rate is effectively 100%. For regional disease, where cancer has spread to nearby lymph nodes, it drops to about 87%. Distant (stage IV) breast cancer has a five-year survival rate of roughly 33%, though newer treatments continue to extend life for many people in this category.
Risk Factors Beyond Genetics
Inherited gene mutations like BRCA1 and BRCA2 get a lot of attention, but they account for a relatively small share of all breast cancers. Several other factors influence risk. Age is the biggest one: the likelihood of developing breast cancer increases significantly after 50. Having dense breast tissue, which contains more connective tissue relative to fatty tissue, makes cancers harder to spot on mammograms and slightly raises risk on its own.
Hormonal exposure plays a role too. Starting menstruation before age 12, entering menopause after 55, or using certain types of hormone replacement therapy all extend the window of estrogen exposure over a lifetime. Having your first pregnancy after 30 or never having a full-term pregnancy also modestly increases risk. Lifestyle factors, including alcohol consumption, physical inactivity, and obesity after menopause, contribute as well. None of these factors guarantee a diagnosis, and many people with breast cancer have no identifiable risk factors at all.
What Early Detection Looks Like
Screening mammograms remain the most effective tool for catching breast cancer before symptoms appear. Most guidelines recommend that women at average risk begin regular mammograms between ages 40 and 50, with screening continuing every one to two years. Women with a strong family history, known genetic mutations, or other high-risk factors may start earlier and add breast MRI to their screening routine.
The gap between localized and distant survival rates underscores why early detection matters so much. Finding breast cancer when it’s still confined to the breast transforms it from a potentially life-threatening disease into one with an excellent prognosis. Paying attention to changes in your breasts between screenings, even subtle ones like skin texture changes or nipple alterations, adds another layer of protection.