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The breast, which is mostly made of fatty tissue, contains ducts and lobes. The lobes are made of smaller sections called lobules, and they make up a network within the breast. The lobules end in milk glands, tiny bulbs that can make milk. Ducts connect the milk glands, lobules, and lobes.

Breast cancer is a cancer that begins in the cells of the breast. The majority of breast cancers begin in the ducts or lobes. There are more rare types of breast cancer but almost 75% begin in the cells that line the milk ducts and are called ductal carcinomas.

Lobular carcinoma begins in the lobules. Carcinomas of the breast can be invasive (or infiltrating), meaning the cancer has grown outside the layer of cells where it started, or in situ, meaning the cancer is confined to the layer of cells where it started. Ductal carcinoma in situ (DCIS) can be considered a pre-invasive breast cancer, because if it is left untreated it might eventually become an invasive breast cancer. Lobular carcinoma in situ (LCIS) is a breast cancer risk factor and not an actual cancer or pre-cancer.

LCIS increases the risk for breast cancer in the same breast or opposite breast from where the LCIS is found.

The average woman in the United States has a lifetime risk for breast cancer that is approximately 12.3%. The average male has a risk of <0.1%. Most breast cancer cases in women are diagnosed after the age of 50. Some hereditary cancer syndromes cause an increased risk for breast cancer.

General Risk Factors

  • Personal history of a previous breast cancer
  • Lobular carcinoma in situ (see above)
  • Atypical hyperplasia – abnormal non-cancerous cells in the breast that can be identified during a breast biopsy (tissue sample)
  • Certain benign breast conditions – some breast biopsy findings of overgrowth of cells without atypia can still slightly increase a woman’s breast cancer risk
  • Dense breast tissue – less fatty tissue of the breast (and more of other types of tissues)
  • Beginning menstruation at an early age (before age 12)
  • A late age of menopause (after age 55)
  • Previous radiation treatment to the chest area (such as for treatment of a childhood cancer)
  • Exposure to diethylstilbestrol (DES) ( a drug containing estrogen given to pregnant women in the 40s, 50s, and 60s to try to prevent miscarriages) – women who were given this drug and women whose mothers were given this drug are thought to have a slightly increased risk for breast cancer
  • Exposure to estrogen and progesterone (combined hormone therapy) after menopause
  • Use of birth control pills – some studies have suggested that the use of birth control pills (containing somewhat higher hormone dosages than many currently used birth control pills) slightly increases the risk for breast cancer while other studies have suggested that this use does not increase the risk for breast cancer
  • No history of any full-term pregnancies or having a first full-term pregnancy after age 30

As with the vast majority of cancers, there is no confirmed way to completely prevent the development of breast cancer. Maintaining a healthy weight through a healthy diet and exercise, having a history of full-term pregnancies, limiting alcohol intake, and limiting the use of combined hormone replacement therapy after menopause may help decrease the risk for breast cancer.

The drugs tamoxifen and raloxifene can reduce a woman’s risk for breast cancer. A health care provider may offer one of these medications to a woman if she is considered to have a high risk for breast cancer and is considered a good candidate for either of these medications. These drugs are associated with side effects in some women. Doctors may discuss the option of a prophylactic (risk-reducing) mastectomy with women who are found to have a high risk for breast cancer due to a genetic change associated with an increased risk for breast cancer, a strong family history of breast cancer, significant personal medical findings that increase the risk for breast cancer, or a combination of any of these. A prophylactic mastectomy is a surgery that involves the removal of non-cancerous breast tissue in attempt to prevent the formation of breast cancer.

Symptoms and Screening Methods

Symptoms of breast cancer can include lumps or thickening of skin in the breast or underarm areas, changes in the size or shape of the breast, dimpling, puckering, scaly, red, or swollen skin on any area of the breast, an inverted (turned inward) nipple, or leakage of fluid (especially if bloody) from the nipple.

Currently, the American Cancer Society (ACS) and the American Congress of OB-GYN (ACOG) recommend that women in their 20s and 30s have a clinical breast exam every one to three years. After age 40, the ACS and ACOG recommend a clinical breast exam annually as well as a screening mammogram. It can also be helpful for women of any age to practice breast awareness, being familiar with their breasts and reporting any changes to their doctor. Good breast awareness may or may not include breast self-exams. Women who are thought to have a lifetime risk for breast cancer of >20% due to a combination of personal risk factors and a family history of breast cancer are recommended to have an annual breast MRI as part of their breast cancer screening. Ultrasound can also be used to examine the breast if needed. Patients who have a family history of breast cancer or who have a genetic change that increases the risk for breast cancer may be told by their doctor that they need to begin breast cancer screening before age 40. If recommended by a doctor, findings on a mammogram, breast MRI, or breast ultrasound can be further examined by fine needle aspiration (use of a thin needle to remove tissue or fluid) or core needle biopsy (use of a wide needle to remove tissue).

Screening MethodAge to BeginFrequency
Breast awareness (recognizing the normal look and feel of the breasts and reporting any change to a health care provider) with or without a breast self-exam20As needed
Clinical breast exam20Every 1-3 years until age 40, then annually

Health care providers may recommend additional screening methods or may modify these recommendations based on a patient’s medical history or family history of breast cancer.


If breast cancer is diagnosed following any of the tests above, treatment will depend on the stage of the cancer (how far the cancer has grown or spread). Treatment can include some combination of surgery, chemotherapy and radiation therapy. Learn more about breast cancer treatments here.

Avoid the following

General Population
Familial Risk
22.1 - 25.8% One affected first-degree relative
Hereditary Risk
Up to 87%