Breast cancer is a disease where cells in breast tissue grow out of control and form tumors. After skin cancer, it stands as the most commonly diagnosed cancer among women in the United States, though it can affect anyone, regardless of gender. While a breast cancer diagnosis can feel overwhelming, advances in screening and treatment have led to steadily improving survival rates, offering hope to millions of people worldwide.

Who Gets Breast Cancer: Understanding the Numbers

Breast cancer touches the lives of millions of people around the globe. In 2022 alone, approximately 2.3 million women received a breast cancer diagnosis worldwide, and the disease claimed an estimated 670,000 lives that same year.^[5] The condition occurs in every country across the world, affecting women at any age after they reach puberty, though rates tend to increase as people get older.^[5]

In the United States, breast cancer ranks as the most common cancer diagnosed in women, accounting for more than one in ten new cancer diagnoses each year.^[6] It is the second most common cause of cancer death among American women, exceeded only by lung cancer.^[1] Each year, roughly 316,000 patients in the U.S. receive this diagnosis.^[6]

What many people don’t realize is that breast cancer doesn’t only affect women. Although approximately 99% of breast cancer cases occur in women, between 0.5% and 1% occur in men.^[5] Everyone is born with some breast tissue, which means anyone can potentially develop breast cancer, though it remains far more common in women.^[1]

The risk of developing breast cancer varies significantly based on where you live and your access to healthcare. In countries with very high levels of human development, one in twelve women will be diagnosed with breast cancer during their lifetime, and one in 71 will die from it. In contrast, in countries with low human development levels, while only one in 27 women receives a breast cancer diagnosis, one in 48 will die from the disease.^[5] This stark difference reflects the importance of early detection and access to quality treatment.

What Causes Breast Cancer

Breast cancer begins when cells in the breast tissue undergo changes in their genetic material and start to grow and divide uncontrollably. These abnormal cells accumulate and form tumors that can be felt as lumps or seen on imaging tests.^[5] The disease most commonly starts in the ducts, which are the small tubes that carry milk to the nipple, or in the lobules, the glands that produce milk.^[2]

Understanding how breast cancer spreads helps explain why early detection matters so much. Cancer cells that remain inside the ducts or lobules without spreading are considered non-invasive. However, when cancer cells break through the walls of ducts or lobules and grow into nearby breast tissue, the cancer becomes invasive.^[5] From there, cancer cells can travel through blood vessels and lymph vessels, which are tiny channels that carry a clear fluid called lymph throughout the body. When breast cancer spreads to other parts of the body through these pathways, doctors say the cancer has metastasized.^[2]

Despite extensive research, scientists have not identified one single cause of breast cancer. The disease appears to result from a complex interaction of genetic, hormonal, and environmental factors. In some cases, inherited genetic mutations play a role. Changes in genes called BRCA1 and BRCA2 are among the most well-known genetic factors that increase breast cancer risk, but many other genes can also contribute.^[6]

Most people who develop breast cancer, however, don’t have an obvious inherited genetic mutation. Instead, their cancer appears to arise from genetic changes that happen during their lifetime, combined with other risk factors. Because there is no single clear cause, prevention strategies focus on reducing known risk factors rather than eliminating a specific cause.^[6]

Factors That Increase Breast Cancer Risk

Several factors can increase a person’s likelihood of developing breast cancer. Being female is the strongest risk factor, as the disease is far more common in women than in men.^[5] Age also plays a significant role. As people grow older, their risk increases. Breast cancer typically affects women age 50 and older, though younger women can certainly develop the disease as well.^[17]

A person’s reproductive and hormonal history influences breast cancer risk in multiple ways. Women who started menstruating at an early age, those who entered menopause later in life, women who never had children, and those who had their first child at an older age all face somewhat higher risk.^[6] These factors all relate to lifetime exposure to the hormones estrogen and progesterone, which can fuel the growth of some breast cancers.

Family history matters too. Having close blood relatives, such as a mother, sister, or daughter, who have had breast cancer increases a person’s risk, especially if the relative was diagnosed at a young age or if multiple family members have been affected.^[6] However, it’s important to know that roughly half of all breast cancer cases occur in women who have no specific risk factors other than being female and getting older.^[5]

Lifestyle factors also contribute to breast cancer risk. Lack of physical activity, being overweight or obese (particularly after menopause), and drinking alcohol all increase risk.^[6] Even small amounts of alcohol consumption raise breast cancer risk, so limiting or avoiding alcohol is one of the few concrete steps people can take to lower their chances of developing the disease.^[22]

Previous radiation therapy to the chest area, such as treatment for another cancer during childhood or young adulthood, can increase breast cancer risk later in life.^[6] Women who have been treated with hormone replacement therapy or certain types of oral contraceptives may also face a slightly elevated risk, though the relationship between hormones and breast cancer risk is complex and depends on many factors.^[6]

Having dense breast tissue is another risk factor. Dense breasts have more connective tissue and less fatty tissue, which not only increases cancer risk slightly but also makes it harder to detect tumors on mammograms.^[6]

Recognizing the Signs and Symptoms

In many cases, especially in countries with established screening programs, breast cancer is discovered through routine mammography before any symptoms appear. However, when breast cancer does cause noticeable changes, it’s important to recognize them and seek medical attention promptly.^[6]

The most common symptom of breast cancer is a new lump or mass in the breast. These lumps may feel as small as a pea, or they may be larger and more obvious.^[1] Not all breast lumps are cancer. Many are benign, meaning they are not cancerous. But any new lump or area of thickening in the breast or underarm should be evaluated by a healthcare provider.^[17]

Changes in the size, shape, or contour of the breast can signal a problem. One breast may become noticeably different from the other, or a breast may change in appearance over time.^[17] Skin changes are also important warning signs. The skin on the breast may become dimpled, puckered, or look like the skin of an orange. It might appear red, purple, or darker than the surrounding skin, or it might become scaly or inflamed.^[1]

Changes involving the nipple deserve attention as well. A nipple may turn inward when it previously pointed outward. There may be discharge from the nipple, particularly if it occurs without squeezing the nipple or if the discharge is bloody.^[1] The nipple or breast skin may look different in texture or feel.

Some types of breast cancer cause the breast to feel warm or to appear swollen. In a rare but aggressive form called inflammatory breast cancer, the breast may become red and swollen, and the skin may look dimpled. This type grows and spreads more quickly than other forms.^[2]

Prevention: Steps You Can Take

While there is no guaranteed way to prevent breast cancer, certain lifestyle choices can help lower your risk. Maintaining a healthy weight is one important step, particularly after menopause. Being overweight or obese increases breast cancer risk, so working toward a healthy weight and staying there can help.^[22] If you need to lose weight, focus on eating fewer calories while choosing nutritious foods, and gradually increase your physical activity.

Speaking of physical activity, regular exercise is one of the most powerful tools for reducing breast cancer risk. Physical activity helps maintain a healthy weight and may also have direct effects on hormone levels and immune function that protect against cancer.^[22] Most healthy adults should aim for regular physical activity, whether that’s brisk walking, swimming, dancing, or any other form of movement they enjoy.

Limiting alcohol consumption can significantly reduce breast cancer risk. The more alcohol a person drinks, the greater their risk becomes. Even small amounts of alcohol raise the risk, so the safest choice is to avoid alcohol altogether. If you do choose to drink, limiting consumption to no more than one drink per day can help minimize risk.^[22]

For women who are able to breastfeed, nursing their babies may offer some protection against breast cancer. The longer a woman breastfeeds, the greater the protective effect appears to be.^[6]

Some women at very high risk of breast cancer because of family history or genetic mutations may consider taking medications to reduce their risk. These risk-reducing medications work differently depending on whether a woman has reached menopause, and they need to be discussed carefully with a healthcare provider who can explain the potential benefits and side effects.^[14]

Screening mammograms are not technically prevention, but they are crucial for early detection. Finding breast cancer early, when it is small and has not spread, makes treatment much more likely to be successful. Women should talk with their healthcare providers about when to start mammography screening and how often to have it done, based on their individual risk factors and preferences.^[22]

How Breast Cancer Changes the Body

Understanding what happens in the body when breast cancer develops can help people appreciate why certain symptoms occur and why treatment takes the approach it does. The breast is made up of lobes and lobules that produce milk, ducts that carry milk to the nipple, and fatty and connective tissue that gives the breast its shape and structure.^[11] Blood vessels bring oxygen and nutrients to breast tissue, while lymph vessels carry away waste products and immune cells.

When cells in the breast become cancerous, they lose the normal controls that regulate cell growth and division. Instead of growing in an orderly way and dying when they should, cancer cells continue to multiply. They accumulate and form a mass of tissue called a tumor.^[5] Not all tumors are cancerous. Benign tumors don’t spread to other parts of the body and aren’t life-threatening, though they may need to be removed if they cause problems. Malignant tumors, which are cancerous, can invade surrounding tissues and spread to distant parts of the body.

The most common type of breast cancer, called invasive ductal carcinoma, begins in the milk ducts and then breaks through the duct walls to grow into surrounding breast tissue. About 70% to 80% of breast cancers are of this type.^[2] The second most common type is invasive lobular carcinoma, which starts in the lobules and spreads to nearby breast tissue, accounting for about 10% to 15% of cases.^[2]

Some breast cancers are classified by the presence or absence of certain proteins on the cancer cells’ surface. These proteins, called receptors, can catch hormones like estrogen and progesterone from the bloodstream. When these hormones attach to the receptors, they can stimulate the cancer cells to grow.^[6] Knowing whether a breast cancer has estrogen receptors, progesterone receptors, or high levels of a protein called HER2 helps doctors choose the most effective treatment.

When breast cancer spreads beyond the breast, it typically travels first to nearby lymph nodes, particularly those in the armpit. From there, cancer cells can enter the bloodstream and travel to distant organs such as the bones, lungs, liver, or brain.^[5] This spread, called metastasis, can interfere with the function of these organs and create new challenges for treatment.

As breast cancer grows and spreads, it may cause the physical changes that people notice as symptoms. A tumor growing in the breast may create a palpable lump. Cancer that blocks lymph vessels can cause swelling or skin changes. When cancer spreads to the bones, it may cause pain. In the lungs, it might lead to shortness of breath or persistent cough. Understanding these connections helps explain why doctors ask about various symptoms and why treatment often involves more than just addressing the breast tumor itself.