Breast cancer occurs when cells in the breast begin to divide and grow in an uncontrolled way. After skin cancer, breast cancer is the most common type of cancer in women worldwide. In the U.S., a woman is diagnosed with breast cancer every two minutes.

What Is Breast Cancer?

Like every tissue in the body, the breast is made up of individual cells. Normal breast cells divide in a regular, ordered fashion, forming new cells that are exact copies to replace old ones. Certain genes in each cell are responsible for telling cells when to divide and when to stop dividing. Other genes identify and fix problems in DNA that is copied wrong, or cause cells with bad DNA to self-destruct rather than keep multiplying. If a genetic mutation causes one or more of these genes to turn off in some cells, they can divide at a faster rate without regulation or order, becoming more and more mutated. When these disordered cells begin to invade nearby tissues or break off and migrate to other locations, they are called cancer.

There are many different types of breast cancer based on where in the breast it begins, its genetic characteristics, and other factors. Some types of breast cancer tend to grow more quickly and spread more aggressively than other types. When cancer is diagnosed, the doctor evaluates the stage and grade of the cancer, terms that describe how fast it is likely to grow and how far it has spread. Determining the type, stage, and grade of breast cancer helps the oncologist predict the pattern of its growth and recommend the most effective treatments.

The History of Breast Cancer

Breast cancer has been observed and studied for thousands of years. In the ancient Egyptian medical text known as the Edwin Smith Surgical Papyrus, written more than 1,600 years BCE, several cases of breast cancer are recorded. The ancient Greeks studied cancers extensively. Renowned Greek physician Hippocrates used the term "karkinos" (crab) to describe cancer — a tumor surrounded by blood vessels reminded him of a crab that had dug down into the sand. Today, we used the related term “carcinoma” to refer to a malignant tumor of the skin or organ lining. In 400 BCE, Hippocrates categorized progressive stages of breast cancer. Hippocrates believed that imbalances in the humors, or bodily fluids — defined as blood, phlegm, and yellow and black bile — caused all disease, including breast cancer. His theories dominated medical thinking for hundreds of years.

Another persistent breast cancer theory was formulated during the 18th century. Bernardino Ramazzini, a Renaissance-era doctor in Italy, observed high rates of breast cancer among nuns. He theorized that breast cancer was caused by pent-up sexual desire. Centuries later, scientists would realize that childlessness — not celibacy — increases a woman’s risk of breast cancer.

During the 18th and 19th centuries, breast cancer was scientifically studied by many doctors across Europe and America. Surgery became the dominant avenue for breast cancer study and treatment. Scottish surgeon John Hunter, sometimes called the father of exploratory surgery, found that if a tumor had not invaded nearby tissue, it could be successfully removed. This discovery made the operating room the forefront of breast cancer study and treatment. Anesthesia became available in 1846, and sterilization of instruments and the use of sterile gloves were also standardized around that time.

Hunter was also the first to discover another bodily fluid — lymph — and theorize its involvement in breast cancer. In 1878, British doctor Thomas Beatson discovered that the mammary glands of rabbits stopped producing milk after removing their ovaries. Doctors began to use oophorectomy (surgical removal of the ovaries) in advanced cases of breast cancer.

William Stewart Halsted, professor of surgery at Johns Hopkins University, developed the radical mastectomy for breast cancer in 1894. The surgery disfigured a woman, removing her breasts, lymph nodes, and underlying muscles in one piece. By 1932, English surgeon David Patey developed the modified radical mastectomy, which preserved the pectoralis muscle and was less disfiguring. In the 1970s, modern clinical trials found that less extensive surgery is an equally effective treatment for most women with breast cancer. Today radical mastectomy is performed less often; most surgeons focus on breast conservation and reconstruction.

In the 1930s, English surgeon Geoffrey Keynes pioneered breast-sparing surgery followed by radiation therapy. Radiation would become more accurate in the late 20th century as advances in physics and computers made it possible to precisely target radiation.

Before the early 1900s, breast cancer diagnosis often required exploratory surgery to take tissue samples (biopsy) for testing. American surgeon J. Collins Warren pioneered the use of needle biopsy in the diagnosis of breast cancer, reducing the need for exploratory surgery. In late 20th century and early 21st century, ultrasound (sonography), computed tomography (CT) scans, magnetic resonance imaging (MRI), and positron emission tomography (PET) scans would make exploratory operations less necessary.

1953 was a turning point for breast cancer treatment. Roy Hertz and Min Chiu Li, researchers at the U.S. National Cancer Institute (NCI), became the first to achieve the complete cure of a solid tumor. The cure was chemotherapy with a drug called Methotrexate. Chemotherapy is used widely in breast cancer treatment today, often after surgery to destroy any remaining cancer cells in the body. Using chemotherapy after surgery is called adjuvant therapy. Adjuvant therapy was tested first in breast cancer and found to be effective in treating other types of cancer.

The growth of breast cancer was long suspected by doctors to be dependent on hormones. American oncologist Elwood Jensen found proof of this theory in 1967, when he discovered estrogen receptors (ER) and progesterone receptors (PR) on breast cancer cells. Hormones bind to the receptors on a cell, traveling to the nucleus where gene regulation takes place. Jensen found that Tamoxifen blocks some of the effects of estrogen, slowing the growth of the cancer.

The 1990s began a new era in breast cancer research as scientists learned how specific gene mutations, including BRCA1 and BRCA2, were powerful risk factors in the development of breast and ovarian cancer in women. Targeting aggressive forms of breast cancer in women with certain genetic mutations, doctors began using hormone-blocking drugs such as Tamoxifen and the use of prophylactic (preventative) surgery to reduce their risk for developing cancer.

Many new types of treatment have been developed in the past 25 years. Antibodies are proteins used by the immune system to identify and neutralize dangerous invaders such as bacteria and viruses. In the 1990s, researchers began developing genetically modified antibodies that would target hormone receptors on breast cancer cells and interfere with the cells’ growth. Also known as biologics, Herceptin (trastuzumab) and Perjeta (pertuzumab) are monoclonal antibodies currently in use to fight breast cancer. Many more monoclonal antibodies and new classes of drugs are currently being studied.

How Common Is Breast Cancer?

Around the world, breast cancer is the most common type of cancer in women. In the U.S., about 12 percent of women, or one in eight, will have invasive breast cancer during her life. Hereditary and environmental factors may raise or lower that risk. There are more than 3 million women living with breast cancer — currently under treatment or previously treated — in the U.S. today.

Age is a factor in breast cancer risk. A woman’s risk for developing breast cancer increases with age. Only 5 percent of breast cancers are diagnosed in women under 40. Breast cancer is most prevalent in women over 70.

How Is Breast Cancer Diagnosed?

Breast cancer may initially be detected during a regular manual (hands-on) breast exam or during a screening with a mammogram. To diagnose breast cancer, the doctor will perform a needle biopsy to collect a small tissue sample. Cells from the sample will be examined in a laboratory. The doctor may also order imaging tests such as mammogram, ultrasound, or magnetic resonance imaging (MRI) to better understand the shape and structure of the cancer. Learn more about the breast cancer diagnostic process.

Breast Cancer Symptoms

Early on, breast cancer may not cause any detectable symptoms. The first symptom of breast cancer may be a change in the way your breast looks or feels, such as tenderness or a lump. Breast cancer can also cause skin irritation and dimpling of the skin. Nipple changes such as discharge, pain, redness, scaly skin, or retraction can indicate breast cancer. Read more about breast cancer symptoms.

Breast Cancer Treatments

Most breast cancer is treated with surgery. There are many different surgical approaches depending on the type and stage of the breast cancer as well as the woman’s preference. Some women undergo breast reconstruction surgery. Radiation therapy may be used after surgery to decrease the risk of the cancer returning. Chemotherapy may be done before surgery to shrink a tumor, after surgery to help ensure that no cancer cells remain, or as a main therapy in people whose cancer has spread. Hormone therapy drugs and targeted therapy drugs may be effective in certain types of breast cancer. Learn more about breast cancer treatments.

Breast cancer treatments can cause serious side effects in nearly any system of the body. Different treatments cause different side effects, and most women will not experience every possible side effect. Some side effects are more common, while others are rare. Some of the most common side effects of breast cancer treatments include hair loss, fatigue, early menopause, lymphedema (swelling), and gastrointestinal (digestive system) problems such as nausea and vomiting. Read more about side effects of breast cancer treatments.

What Is the Prognosis for Breast Cancer?

The prognosis for an individual case of breast cancer depends on many factors, including the type of breast cancer, the stage at which the breast cancer is diagnosed, and which treatments are used. Death rates from breast cancer have been decreasing for years. Today, 90 percent of women are alive five years after their breast cancer diagnosis, and 83 percent are alive 10 years after diagnosis. Breast cancer is the third leading cause of death in U.S. women after heart disease and lung cancer.

Condition Guide

• Breast Cancer: Stage and Grade
• Breast Cancer Symptoms
• Breast Cancer – The Path to Diagnosis
• Breast Cancer Treatment
• Side Effects of Breast Cancer Treatments
• Types of Breast Cancer
• What Causes Breast Cancer?