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What is breast cancer?

Breast cancers are malignant tumors that arise from the uncontrolled growth of cells in the breast. They occur primarily in the ducts that transport milk to the nipple during breast feeding (lactation) and secondarily in the lobules, the glands that produce milk.

Each breast cancer will have its own characteristics. Some are slow-growing; others can be aggressive. Some are sensitive to the hormones estrogen and progesterone, while others produce very high levels of certain proteins that make them grow. The cancer's characteristics can affect treatment choices and the potential for the cancer to recur.

Each year, more women in the United States are diagnosed with breast cancer than with any other cancer, with the exception of skin cancer. About 1 in 8 American women will develop invasive breast cancer in their lifetime. The American Cancer Society (ACS) estimates that about 270,000 new cases of invasive breast cancer will be diagnosed in women in the U.S. each year and that about 42,000 women die from the disease. Men can also develop the disease. ACS estimates that about 2,700 men are diagnosed with breast cancer each year, and about 500 men die of the disease.

The rest of this article will focus on breast cancer in women. It is recommended that men who have been diagnosed with breast cancer speak to their healthcare provider for information specific to them and see the ACS's web site about Breast Cancer in Men.

Accordion Title
About Breast Cancer
  • Risk Factors

    Breast cancer can develop at any age, but the risk of developing it increases as women get older. The majority of cases develop for reasons we do not yet understand.

    Examples of some factors that can increase risk of developing breast cancer include:

    • Family history - close relatives (mother, sister, aunt) who have had the disease
    • Being obese
    • Not getting enough exercise
    • Drinking alcohol (risk increases with higher amounts of alcohol consumed)
    • Cancer in the other breast
    • Not having children, having a first child after age 30, and/or not breastfeeding
    • Treatment with estrogen (e.g., some types of birth control, hormone replacement therapy after menopause)


    BRCA1 or BRCA2 Mutation

    • An inherited mutation in breast cancer genes, usually either BRCA1 or BRCA2. About 5% to 10% of breast cancers are hereditary, and the most common cause of hereditary breast cancer is BRCA1/BRCA2 mutation. BRCA1 and BRCA2 are two tumor suppressor genes that help prevent cancer by producing proteins that suppress abnormal cell growth. Mutations in these genes can affect their normal function, allowing uncontrolled cell growth and increasing the risk of cancer. Women with inherited BRCA1 or BRCA2 mutations have about a 72% or 69% lifetime risk of developing breast cancer, respectively.


      For those women who have a BRCA1 or BRCA2 gene mutation (sometimes called a pathogenic variant) that increases risk of breast cancer, prophylactic mastectomy is an option. Women electing this option choose to have both breasts removed before developing cancer rather than run the high risk of developing the disease later in their lifetime. Studies have shown that such surgery can reduce the risk of developing breast cancer by up to 95%. Other women elect to have a prophylactic mastectomy on their cancer-free breast after developing cancer in the other breast. A healthcare practitioner can help advise and work with a woman who is considering prophylactic mastectomy.


    A healthy lifestyle that includes regular exercise, maintaining a healthy body weight, and avoiding alcohol may help to minimize the risk of developing breast cancer. Research studies continue to identify factors that are associated with an increased or decreased risk of developing the disease, but there is no single set of actions that will cause or prevent breast cancer. Women should work with their healthcare provider to determine their personal risk factors and how to best address them. For example, a woman may choose to avoid long-term use of hormone replacement therapy (HRT).

    Women who are at high risk of developing breast cancer may be able to take medications to reduce their risk. However, these drugs have significant side effects, such as increased risk of developing blood clots, endometrial (uterine) cancer, and possibly cardiovascular disease. The decision to take the medication needs to be weighed carefully. A healthcare practitioner can help assess the risks and benefits of such treatment.

  • Signs and Symptoms

    It is important to remember that most lumps found in the breast are not cancerous but are benign and that the symptoms and signs associated with breast cancer may be due to other causes. Examples of some signs and symptoms include:

    • Mass or lump in the breast
    • Breast skin dimpling, reddening, or thickening
    • Nipple retraction
    • Breast swelling or pain
    • Nipple pain and/or discharge
    • Swelling or lumps in adjacent underarm lymph node
    • Pigmented, peeling, scaling or flaking skin in the area around the nipple
    • Changes in size, shape or appearance of the breast


    A rare form of breast cancer, inflammatory breast cancer (IBC), may not form a lump. Some of the symptoms of this condition can be similar to those of a breast infection, with warmth, tenderness, breast swelling, itching, and ridged thickened skin.

  • Early Detection and Screening

    Early breast cancer detection has a strong influence on breast cancer survival. For example, when breast cancer is found in the early, localized stage, 99% of those individuals survive for at least five years after diagnosis.

    Clinical Breast Exam and Breast Self-Awareness

    The American College of Obstetricians and Gynecologists (ACOG) and the National Comprehensive Cancer Network (NCCN) say women may be offered a clinical breast exam by a healthcare professional every 1 to 3 years for women ages 25 to 39 as part of their regular health exam and annually for women age 40 and older.

    Breast self-awareness is important, according to ACOG and NCCN. Women of all ages should discuss breast self-awareness with their healthcare provider and immediately report any change in their breasts' normal appearance and feel. These changes could include pain, a mass, nipple discharge other than breast milk, or redness.

    Mammography

    In addition to clinical breast exams, the primary early detection tool is a mammogram. The medical community recognizes the value of breast cancer screening and mammography, but there are some differences in the advice on how often it should be done or when it should be started. Most health organizations agree that women should work with their healthcare provider to assess their personal risk of developing breast cancer and to determine what is best for them. Consideration of the benefits of screening as well as the harms should be discussed. While screening can detect cancer early when it is most treatable, it may also lead to false-positive results and unnecessary anxiety and follow-up procedures, such as biopsies.

    Summary of recommendations for women with average risk:

    Women with average risk have no personal or family history of breast cancer and no other risk factors for breast cancer.

    Mammograms are generally not recommended for women younger than age 40 with no known risk factors.

    The following table summarizes recommendations from several health organizations. You may wish to discuss the options with your healthcare practitioner to decide what is right for you.

    Organization Ages Mammogram Frequency
    American Cancer Society 40-44 Should have the choice to begin screening annually
      45-54 Should be screened every year
      55 and older Switch to every two years or continue annually; continue as long as healthy and expected to live longer than 10 years
    U.S. Preventive Services Task Force, American College of Physicians, American Academy of Family Physicians Younger than 50 Individual decision whether to screen every two years
      50-74 Should be screened every two years
      75 and older Evidence is insufficient on whether benefits of screening outweigh risks
    American Congress of Obstetricians and Gynecologists 40-49 Individual decision to start screening; every 1-2 years
      50 and older Should be screened every 1-2 years
      Older than 75 Individual decision whether to continue based on health and life expectancy
    National Comprehensive Cancer Network 40 and older Should be offered annual screening; women who choose screening should be counseled about the risks and benefits. At any age, health and life expectancy should be considered when deciding whether to continue screening.
    American Medical Association 40 and older Should have access to screening mammography if they choose and their healthcare providers agree

    Women with increased risk

    Women with certain risk factors may be advised to begin screening at an earlier age and to be screened more frequently, with additional testing such as imaging scans. For instance, the ACS says that women at high risk for breast cancer may choose to have an MRI in addition to an annual mammogram beginning at age 30 and for as long as they are in good health.

  • Tests

    The goals of breast cancer testing are to:

    • Identify genetic risk in high-risk women
    • Detect and diagnose breast cancer in its earliest stages
    • Determine how far it has spread
    • Evaluate the cancer's characteristics in order to guide treatment
    • Monitor the effectiveness of treatment and monitor the person over time to detect and address any cancer recurrences


    The table below summarizes various breast cancer tests. The samples required for some of the tests may involve aspirating cells from the breast using a thin needle and a syringe (fine needle aspiration or FNA) or removing a solid piece of tissue with a larger bore needle (core needle biopsy), or by surgically removing some breast tissue or a tumor (open biopsy).

    Test Description Test Sample
    Mammogram Highly-sensitive digital X-ray technology that may detect small lumps that otherwise would not be detected through a clinical breast exam N/A
    BRCA1 / BRCA2 Genetic mutations, if present, suggest a lifetime risk around 70%. Blood
    HER2 A test for the overexpression of HER2 proteins or the amplification of the gene that codes for the protein; tumors that are positive may respond well to a medication that targets HER2, such as Herceptin®. Tissue
    Estrogen Receptor/
    Progesterone Receptor
    Increased levels suggest a good response to hormonal therapy. Tissue
    CA 15-3/ CA 27.29 These tests measure a specific cancer antigen. Elevated blood levels may indicate recurrence of cancer after initial treatment. Blood
    Breast Cancer Gene Expression Tests Genetic tests used to predict prognosis by estimating risk of recurrence, and spread (metastasis) of the cancer, as well as to guide treatment Tissue

    Laboratory Tests

    Laboratory tests for breast cancer can be broken down into groups, based on the purpose of testing.

    Genetic risk:

    BRCA1 or BRCA2 gene mutation – Women who are at high risk because of a personal or strong family history of early onset breast cancer or ovarian cancer can find out if they have a BRCA gene mutation. A mutation in either gene indicates that the person is at significantly higher lifetime risk (between 69-72%) for developing the disease. It is important to remember, however, that only about 5% to 10% of breast cancer cases occur in women with a BRCA gene mutation. Genetic counseling should be considered both before testing takes place and after receiving positive test results.

    Mutations in other genes can also increase risk of inherited breast cancer, but they are less common than BRCA mutations and they increase the risk much less than BRCA mutations. Examples include ATM, CHEK2, PTEN, STK11 and PALB2 gene mutations. Some labs offer panels of tests that can detect mutations in one or more of these genes.

    Diagnosis: cytology and surgical pathology

    When a radiologist detects a suspicious area, such as hardened tissue (calcifications) or a non-palpable mass on a mammogram, or if a lump has been found during a clinical breast exam (see Non-Laboratory Tests below), a healthcare practitioner will frequently order one of the following:

    • Fine needle aspiration (FNA)—a thin needle and a syringe are used to remove a sample of cells from a suspicious area of the breast
    • Core needle biopsy—a larger bore needle is used to remove a solid "core" of breast tissue; one or more core samples may be removed.
    • Surgical biopsy—a physician cuts out all or part of the breast lump

    In each case, a pathologist examines the breast cells microscopically for signs of cancer. This pathological examination is done to determine whether the lesion is benign or malignant.

    Malignant cells show changes or deviations from normal cells. Signs include changes in the size, shape, and appearance of cell nuclei and evidence of increased cell division. Malignant cells can also distort the normal arrangement of cells within breast tissue. Pathologists can diagnose cancer based upon the observed changes, determine how abnormal the cells appear, and see whether there is a single type of change or a mixture of changes. These results help guide breast cancer treatment.

    Needle aspiration and core needle biopsy evaluations are limited due to the size of the sample. A tissue biopsy is often needed to determine if a cancer is early stage or invasive. When a breast cancer is surgically removed (see Treatment), cells from the tumor and sometimes from adjacent tissue and lymph nodes are examined by the pathologist to help determine how far the cancer has spread.

    For more on this, see the article on Anatomic Pathology.

    Determine treatment options:

    If the pathologist's diagnosis is breast cancer, there are several tests that may be performed on the tissue cancer cells. The results of these tests provide a prognosis and help the oncologist (cancer specialist) guide the woman's treatment. The most useful of these are HER2 and estrogen and progesterone receptors.

    • HER2 is an oncogene associated with cell growth. Normal epithelial cells contain two copies of the HER2 gene and produce low levels of the HER2 protein on the surface of their cells. In about 20-30% of invasive breast cancers, the HER2 gene is amplified and its protein is over-expressed. These tumors are susceptible to treatment that specifically binds to this over-expressed protein (targeted therapy). Women with amplified HER2 gene respond well to these drugs and have a good prognosis.
    • Estrogen and progesterone receptor (ER and PR) status is important for predicting the course of the disease and helping to guide treatment. Breast cancer cells that have estrogen and/or progesterone receptors can bind estrogen and progesterone. These female hormones promote cell growth in ER- and PR-positive cancers. The higher the percentage of cancer cells that are positive, as well as the greater the intensity (the number of receptors per cell), the better the prognosis. This is because hormone-dependent cancers frequently respond well to hormonal therapy that blocks estrogen or lowers estrogen levels.


    Breast cancer cells that are negative for HER2 amplification and negative for estrogen and progesterone receptors are called "triple-negative." This type of breast cancer occurs more often in younger women and in women of African or Hispanic descent. Women with BRCA mutations may be predisposed to triple-negative breast cancer.

    Triple-negative breast cancers tend to grow and spread more quickly than other types and have a worse prognosis. Because the cells do not have amplified HER2, they will not respond to targeted treatment. Likewise, they do not have receptors for estrogen and progesterone and cannot be treated with therapy that blocks the hormones. However, they may be treated with other types of chemotherapy. New studies are currently underway to evaluate new drugs and see if treatment with these drugs alone or in combination with chemotherapy can treat triple-negative breast cancers.

    Monitor treatment:

    Cancer antigen 15-3 (CA 15-3) – this is a protein that is produced by normal breast cells. There is an increased production of CA 15-3 and the related cancer antigen 27.29 in many women with breast cancer. CA 15-3 does not cause cancer. Rather, the protein is shed by tumor cells and enters the blood, making it useful as a tumor marker to follow the course of the cancer. CA 15-3 is elevated in only about 10% of women with early localized breast cancer but is elevated in about 80% of those with metastatic breast cancer. Blood tests for CA 15-3 (or CA 27.29) may be ordered at intervals after treatment to help monitor a woman for breast cancer recurrence. They are not used as screens for breast cancer but can be used to follow it in some women once it has been diagnosed.

    Other tests:

    There are several tests available, and many others being researched, that evaluate large numbers of genetic patterns in breast cancer tumor tissue. These gene expression tests for breast cancer are relatively new, although their use is increasing. They may be used as predictive tests for the recurrence of breast cancer and therapy outcomes. The American Society of Clinical Oncology (ASCO) mentions several of them in its 2017 "Use of Biomarkers to Guide Decisions on Adjuvant Systemic Therapy for Women With Early-Stage Invasive Breast Cancer" and its 2015 "Use of Biomarkers to Guide Decisions on Systemic Therapy for Women With Metastatic Breast Cancer" guidelines. Two of the most commonly ordered are MammaPrint and Oncotype DX Breast. For details, see the article on Breast Cancer Gene Expression Tests.

    There are additional tests that may be used in some breast cancer cases, such as DNA ploidy, Ki-67, or other markers. (For more on these, read the American Cancer Society’s article on Breast Cancer: Ploidy and Cell Proliferation.) Some medical centers use these tests for additional information in evaluating women with breast cancer. However, most authorities believe that HER2, estrogen and progesterone receptor status are the most important to evaluate first.

    Non-Laboratory Tests

    In addition to laboratory tests, there are non-laboratory tests that are equally important. These include the following:

    • Mammography is widely recommended as a screening tool. A screening mammogram uses X-ray technology to produce an image of the breasts and can reveal breast cancer up to two years before a lump is large enough to be felt during a clinical or self-exam.
    • Ultrasound or magnetic resonance imaging (MRI)—women whose breast tissue is too dense to show tumors clearly on the X-ray film used for a standard mammogram may benefit from ultrasound exams or MRI.
    • Newer technologies, such as digital mammography and computer-aided detection, may yield a clearer image than a mammogram in some cases.


    For more information on mammography and other imaging technologies, visit the National Cancer Institute's web site or RadiologyInfo.org.

  • Stages

    Breast cancer may be divided into several stages based on factors such as the size of the tumor and the extent to which the cancer has spread in the body. Generally, the lower the number of the stage, the less the cancer has spread. Determining the stage of a cancer can aid in treatment decisions and establishing a prognosis, i.e., predicting the course of the disease and the chances of remission and/or recurrence. For more detail, see the section on "Biopsies and examination of tissues" in the Anatomic Pathology feature article.

    "Anatomic Stage Groups"

    Stage Size of the tumor Location
    Stage 0   Confined within the breast ducts (ductal carcinoma in situ, DCIS), within the lobules (lobular carcinoma in situ, LCIS), or within the nipples (Paget disease of the nipple)
    Stage I    
    Stage IA Less than 2 cm (3/4 inch) across Tumor has spread beyond the ducts but is still confined within the breast tissue
    Stage IB Less than 2 cm or no primary tumor Cancer has spread to the lymph nodes but is very small (0.2 to 2 millimeters across)
    Stage II  
    Stage IIA Less than 2 cm across or no tumor Spread to one to three lymph nodes in the armpit (axilla)
      Between 2 and 5 cm (3/4 to 2 inches) across No spread to the lymph nodes
    Stage IIB 2 to 5 cm across Spread to the lymph nodes
      Larger than 5 cm across No spread to the lymph nodes
    Stage III  
    Stage IIIA Any size Spread to lymph nodes in the armpit that are attached to each other or other structures and may have spread to lymph nodes near the breast bone
    Stage IIIB Any size

    Spread to chest wall or skin of the breast; may have spread to fewer than 10 lymph nodes in the armpit and may have spread to near the breast bone

    Inflammatory breast cancer, a rare type of cancer that may not form a lump, usually falls into this category because it is aggressive.

    Stage IIIC Any size Spread to 10 or more lymph nodes in the armpit and to lymph nodes either near the breast bone or above the collar bone
    Stage IV Any size Spread to distant organs such as bone or liver
    Recurrent Any size Breast cancer that was undetected after treatment but is now detectable in any area of the body
  • Treatment

    Most breast cancers are treated by surgically removing the cancer, or as much of the cancer as possible, and then using one or more other therapies to kill or control any remaining cancer cells. A lumpectomy removes the cancerous tissue while leaving the remaining breast tissue intact. A mastectomy is a more extensive surgical procedure but can vary in the amount of breast and surrounding tissue removed. While mastectomy was once the preferred treatment even in early stage breast cancer, more choices have become available.

    Lumpectomy followed by radiation has been demonstrated to be as effective as a mastectomy in treating many early stage breast cancers. In performing either a lumpectomy or mastectomy, a healthcare practitioner may remove some or all of the lymph nodes under the arm and submit them for pathological examination to determine if the cancer has spread.

    The use of radiation, chemotherapy, and other drug therapies depends on the woman, on the cancer's characteristics, and on how far the cancer has spread. Drug therapies may include tamoxifen, aromatase inhibitors, and other anti-estrogen drugs that target hormone-sensitive cancers. Those who over-express the HER2 protein may benefit from therapy with drugs that target HER2. These drugs can substantially improve survival in individuals with HER2-positive invasive breast cancer.

    There is a great deal of new research being performed in the field of breast cancer treatment, and a healthcare provider is the best source of information about treatment options. New drugs with fewer side effects than existing therapies are being developed. There are also promising gene-targeting drugs and vaccines, some of which are already being used on a limited, or trial, basis. See the links in Related Content for more information on treatment.

View Sources

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