Breast cancer is a cancer that develops from breast tissue.^[7] Signs of breast cancer may include a lump in the breast, a change in breast shape, dimpling of the skin, milk rejection, fluid coming from the nipple, a newly inverted nipple, or a red or scaly patch of skin.^[1] In those with distant spread of the disease, there may be bone pain, swollen lymph nodes, shortness of breath, or yellow skin.^[8]

Risk factors for developing breast cancer include obesity, a lack of physical exercise, alcohol consumption, hormone replacement therapy during menopause, ionizing radiation, an early age at first menstruation, having children late in life or not at all, older age, having a prior history of breast cancer, and a family history of breast cancer.^[1][2][9] About 5–10% of cases are the result of an inherited genetic predisposition,^[1] including BRCA mutations among others.^[1] Breast cancer most commonly develops in cells from the lining of milk ducts and the lobules that supply these ducts with milk.^[1] Cancers developing from the ducts are known as ductal carcinomas, while those developing from lobules are known as lobular carcinomas.^[1] There are more than 18 other sub-types of breast cancer.^[2] Some, such as ductal carcinoma in situ, develop from pre-invasive lesions.^[2] The diagnosis of breast cancer is confirmed by taking a biopsy of the concerning tissue.^[1] Once the diagnosis is made, further tests are done to determine if the cancer has spread beyond the breast and which treatments are most likely to be effective.^[1]

The balance of benefits versus harms of breast cancer screening is controversial. A 2013 Cochrane review found that it was unclear if mammographic screening does more harm than good, in that a large proportion of women who test positive turn out not to have the disease.^[10] A 2009 review for the US Preventive Services Task Force found evidence of benefit in those 40 to 70 years of age,^[11] and the organization recommends screening every two years in women 50 to 74 years of age.^[12] The medications tamoxifen or raloxifene may be used in an effort to prevent breast cancer in those who are at high risk of developing it.^[2] Surgical removal of both breasts is another preventive measure in some high risk women.^[2] In those who have been diagnosed with cancer, a number of treatments may be used, including surgery, radiation therapy, chemotherapy, hormonal therapy, and targeted therapy.^[1] Types of surgery vary from breast-conserving surgery to mastectomy.^[13][14] Breast reconstruction may take place at the time of surgery or at a later date.^[14] In those in whom the cancer has spread to other parts of the body, treatments are mostly aimed at improving quality of life and comfort.^[14]

Outcomes for breast cancer vary depending on the cancer type, the extent of disease, and the person's age.^[14] The five-year survival rates in England and the United States are between 80 and 90%.^[15][4][5] In developing countries, five-year survival rates are lower.^[2] Worldwide, breast cancer is the leading type of cancer in women, accounting for 25% of all cases.^[16] In 2018, it resulted in 2 million new cases and 627,000 deaths.^[17] It is more common in developed countries^[2] and is more than 100 times more common in women than in men.^[15][18]