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 five to ten percent 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 carried out 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 whether 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 two 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] For transgender individuals on gender-affirming hormone therapy, breast cancer is 5 times more common in cisgender women than in transgender men, and 46 times more common in transgender women than in cisgender men.^[19]

Most people with breast cancer have no symptoms at the time of diagnosis; their tumor is detected by a breast cancer screening test.^[20] For those who do have symptoms, a new lump in the breast is most common. Most breast lumps are not breast cancer, though lumps that are painless, hard, and with irregular edges are more likely to be cancerous.^[21] Other symptoms include swelling or pain in the breast; dimpling, thickening, redness, or dryness of the breast skin; and pain, or inversion of the nipple.^[21] Some may experience unusual discharge from the breasts, or swelling of the lymph nodes under the arms or along the collar bone.^[21]

Some less common forms of breast cancer cause distinctive symptoms. Up to 5% of people with breast cancer have inflammatory breast cancer, where cancer cells block the lymph vessels of one breast, causing the breast to substantially swell and redden over three to six months.^[22] Up to 3% of people with breast cancer have Paget's disease of the breast, with eczema-like red, scaly irritation on the nipple and areola.^[23]

Advanced breast tumors can spread (metastasize) beyond the breast. The symptoms caused by metastatic breast cancer will depend on the location of metastasis. Common sites of metastasis include bone, liver, lung, and brain.^[24] When cancer has reached such an invasive state, it is categorized as a stage 4 cancer, cancers of this state are often fatal.^[25] Common symptoms of stage 4 cancer include unexplained weight loss, bone and joint pain, jaundice and neurological symptoms. These symptoms are called non-specific symptoms because they could be manifestations of many other illnesses.^[26] Rarely breast cancer can spread to exceedingly uncommon sites such as peripancreatic lymph nodes causing biliary obstruction leading to diagnostic difficulties.^[27]

Most symptoms of breast disorders, including most lumps, do not turn out to represent underlying breast cancer. Less than 20% of lumps, for example, are cancerous,^[28] and benign breast diseases such as mastitis and fibroadenoma of the breast are more common causes of breast disorder symptoms.^[29]

The major causes of sporadic breast cancer are associated with hormone levels. Breast cancer is promoted by estrogen. This hormone activates the development of breast throughout puberty, menstrual cycles and pregnancy. The imbalance between estrogen and progesterone during the menstrual phases causes cell proliferation. Moreover, oxidative metabolites of estrogen can increase DNA damage and mutations. Repeated cycling and the impairment of repair process can transform a normal cell into pre-malignant and eventually malignant cell through mutation. During the premalignant stage, high proliferation of stromal cells can be activated by estrogen to support the development of breast cancer. During the ligand binding activation, the ER can regulate gene expression by interacting with estrogen response elements within the promotor of specific genes. The expression and activation of ER due to lack of estrogen can be stimulated by extracellular signals.^[69] Interestingly, the ER directly binding with the several proteins, including growth factor receptors, can promote the expression of genes related to cell growth and survival.^[70]

Breast cancer, like other cancers, occurs because of an interaction between an environmental (external) factor and a genetically susceptible host. Normal cells divide as many times as needed, and stop. They attach to other cells and stay in place in tissues. Cells become cancerous when they lose their ability to stop dividing, to attach to other cells, to stay where they belong, and to die at the proper time.

Normal cells will self-destruct (programmed cell death) when they are no longer needed. Until then, cells are protected from programmed death by several protein clusters and pathways. One of the protective pathways is the PI3K/AKT pathway; another is the RAS/MEK/ERK pathway. Sometimes the genes along these protective pathways are mutated in a way that turns them permanently "on", rendering the cell incapable of self-destructing when it is no longer needed. This is one of the steps that causes cancer in combination with other mutations. Normally, the PTEN protein turns off the PI3K/AKT pathway when the cell is ready for programmed cell death. In some breast cancers, the gene for the PTEN protein is mutated, so the PI3K/AKT pathway is stuck in the "on" position, and the cancer cell does not self-destruct.^[71]

Mutations that can lead to breast cancer have been experimentally linked to estrogen exposure.^[72] Additionally, G-protein coupled estrogen receptors have been associated with various cancers of the female reproductive system including breast cancer.^[73]

Abnormal growth factor signaling in the interaction between stromal cells and epithelial cells can facilitate malignant cell growth.^[74][75] In breast adipose tissue, overexpression of leptin leads to increased cell proliferation and cancer.^[76]

Some mutations associated with cancer, such as p53, BRCA1 and BRCA2, occur in mechanisms to correct errors in DNA. The inherited mutation in BRCA1 or BRCA2 genes can interfere with repair of DNA crosslinks and double-strand breaks (known functions of the encoded protein).^[77] These carcinogens cause DNA damage such as DNA crosslinks and double-strand breaks that often require repairs by pathways containing BRCA1 and BRCA2.^[78][79]

GATA-3 directly controls the expression of estrogen receptor (ER) and other genes associated with epithelial differentiation, and the loss of GATA-3 leads to loss of differentiation and poor prognosis due to cancer cell invasion and metastasis.^[80]

Breast cancer screening refers to testing otherwise-healthy women for breast cancer in an attempt to diagnose breast tumors early when treatments are more successful. The most common screening test for breast cancer is low-dose X-ray imaging of the breast, called mammography.^[81] Each breast is pressed between two plates and imaged. Tumors can appear unusually dense within the breast, distort the shape of surrounding tissue, or cause small dense flecks called microcalcifications.^[82] Radiologists generally report mammogram results on a standardized scale – the six-point Breast Imaging-Reporting and Data System (BI-RADS) is the most common globally – where a higher number corresponds to a greater risk of a cancerous tumor.^[83][84]

A mammogram also reveals breast density; dense breast tissue appears opaque on a mammogram and can obscure tumors.^[85][86] BI-RADS categorizes breast density into four categories. Mammography can detect around 90% of breast tumors in the least dense breasts (called "fatty" breasts), but just 60% in the most dense breasts (called "extremely dense").^[87] Women with particularly dense breasts can instead be screened by ultrasound, magnetic resonance imaging (MRI), or tomosynthesis, all of which more sensitively detect breast tumors.^[88]

Regular screening mammography reduces breast cancer deaths by at least 20%.^[89] Most medical guidelines recommend annual screening mammograms for women aged 50–70.^[90] Screening also reduces breast cancer mortality in women aged 40–49, and some guidelines recommend annual screening in this age group as well.^[90][91] For women at high risk for developing breast cancer, most guidelines recommend adding MRI screening to mammography, to increase the chance of detecting potentially dangerous tumors.^[88] Regularly feeling one's own breasts for lumps or other abnormalities, called breast self-examination, does not reduce a person's chance of dying from breast cancer.^[92] Clinical breast exams, where a health professional feels the breasts for abnormalities, are common;^[93] whether they reduce the risk of dying from breast cancer is not known.^[81]

For the average woman, the US Preventive Services Task Force and American College of Physicians recommends mammography every two years in women between the ages of 50 and 74,^[12][94] the Council of Europe recommends mammography between 50 and 69 with most programs using a 2-year frequency,^[95] while the European Commission recommends mammography between the ages of 45 and 75 every two to three years,^[96] and in Canada screening is recommended between the ages of 50 and 74 at a frequency of two to three years.^[97] The American Cancer Society also endorses that women ages 40 and older receive mammograms annually.^[98] These task force reports point out that in addition to unnecessary surgery and anxiety, the risks of more frequent mammograms include a small but significant increase in breast cancer induced by radiation.^[99]

Those who have a suspected tumor from a mammogram or physical exam first undergo additional imaging – typically a second "diagnostic" mammogram and ultrasound – to confirm its presence and location.^[20] A biopsy is then taken of the suspected tumor. Breast biopsy is typically done by core needle biopsy, with a hollow needle used to collect tissue from the area of interest.^[100] Suspected tumors that appear to be filled with fluid are often instead sampled by fine-needle aspiration.^[100][101] Around 10–20% of breast biopsies are positive for cancer.^[102] Most biopsied breast masses are instead caused by fibrocystic breast changes, a term that encompasses the benign growth of small pockets of fluid, cell growth, or fibrous tissue.^[102]

• MRI showing breast cancer
• High-grade invasive ductal carcinoma, with minimal tubule formation, marked pleomorphism, and prominent mitoses, 40x field
• Micrograph showing a lymph node invaded by ductal breast carcinoma, with an extension of the tumor beyond the lymph node
• Neuropilin-2 expression in normal breast and breast carcinoma tissue
• F-18 FDG PET/CT: A breast cancer metastasis to the right scapula
• Elastography shows stiff cancer tissue on ultrasound imaging.
• Breast cancer prediction by artificial intelligence on a whole-slide image, an emerging process in digital pathology.^[103]

The management of breast cancer depends on the affected person's health, the cancer case's molecular characteristics, and how far the tumor has spread at the time of diagnosis.

The stage of the breast cancer is the most important component of traditional classification methods of breast cancer, because it has a greater effect on the prognosis than the other considerations. Staging takes into consideration size, local involvement, lymph node status and whether metastatic disease is present. The higher the stage at diagnosis, the poorer the prognosis. The stage is raised by the invasiveness of disease to lymph nodes, chest wall, skin or beyond, and the aggressiveness of the cancer cells. The stage is lowered by the presence of cancer-free zones and close-to-normal cell behavior (grading). Size is not a factor in staging unless the cancer is invasive. For example, ductal carcinoma in situ (DCIS) involving the entire breast will still be stage zero and consequently an excellent prognosis.

• Stage 1 cancers (and DCIS, LCIS) have an excellent prognosis and are generally treated with lumpectomy and sometimes radiation.^[183]
• Stage 2 and 3 cancers with a progressively poorer prognosis and greater risk of recurrence are generally treated with surgery (lumpectomy or mastectomy with or without lymph node removal), chemotherapy (plus trastuzumab for HER2+ cancers) and sometimes radiation (particularly following large cancers, multiple positive nodes or lumpectomy).^{[medical citation needed]}
• Stage 4, metastatic cancer, (i.e. spread to distant sites) has a poor prognosis and is managed by various combination of all treatments from surgery, radiation, chemotherapy and targeted therapies.

Younger women with an age of less than 40 years or women over 80 years tend to have a poorer prognosis than post-menopausal women due to several factors. Their breasts may change with their menstrual cycles, they may be nursing infants, and they may be unaware of changes in their breasts. Therefore, younger women are usually at a more advanced stage when diagnosed. There may also be biological factors contributing to a higher risk of disease recurrence for younger women with breast cancer.^[184]

Breast cancer is the most common invasive cancer in women in most countries, accounting for 30% of cancer cases in women.^[186][59] In 2022, an estimated 2.3 million women were diagnosed with breast cancer, and 670,000 died of the disease.^[186] The incidence of breast cancer is rising by around 3% per year, as populations in many countries are getting older.^[187]

Rates of breast cancer vary across the world, but generally correlate with wealth.^[187] Around 1 in 12 women are diagnosed with breast cancer in wealthier countries, compared to 1 in 27 in lower income countries.^[186] Most of that difference is due to differences in menstrual and reproductive histories – women in wealthier countries tend to begin menstruating earlier and have children later, both factors that increase risk of developing breast cancer.^[188] People in lower income countries tend to have less access to breast cancer screening and treatments, and so breast cancer death rates tend to be higher.^[187] 1 in 71 women die of breast cancer in wealthy countries, while 1 in 48 die of the disease in lower income countries.^[186]

Breast cancer predominantly occurs in women, and risk increases with age.^[189] Most breast cancers develop in women over 50. Women in their early 50s are twice as more likely to develop breast cancer than women in their early 40s.^[43] Breast cancer is strongly related to age with only 5% of all breast cancers occurring in women under 40 years old.^[190]

Because of its visibility, breast cancer was the form of cancer most often described in ancient documents.^{[191]: 9–13} Because autopsies were rare, cancers of the internal organs were essentially invisible to ancient medicine. Breast cancer, however, could be felt through the skin, and in its advanced state often developed into fungating lesions: the tumor would become necrotic (die from the inside, causing the tumor to appear to break up) and ulcerate through the skin, weeping fetid, dark fluid.^{[191]: 9–13}

The oldest discovered evidence of breast cancer is from Egypt and dates back 4200 years, to the Sixth Dynasty.^[192] The study of a woman's remains from the necropolis of Qubbet el-Hawa showed the typical destructive damage due to metastatic spread.^[192] The Edwin Smith Papyrus describes eight cases of tumors or ulcers of the breast that were treated by cauterization. The writing says about the disease, "There is no treatment."^[193] For centuries, physicians described similar cases in their practices, with the same conclusion. Ancient medicine, from the time of the Greeks through the 17th century, was based on humoralism, and thus believed that breast cancer was generally caused by imbalances in the fundamental fluids that controlled the body, especially an excess of black bile.^[191]: 32 Alternatively it was seen as divine punishment.^[194]

Mastectomy for breast cancer was performed at least as early as AD 548, when it was proposed by the court physician Aetios of Amida to Theodora.^{[191]: 9–13} It was not until doctors achieved greater understanding of the circulatory system in the 17th century that they could link breast cancer's spread to the lymph nodes in the armpit. In the early 18th century the French surgeon Jean Louis Petit performed total mastectomies that included removing the axillary lymph nodes, as he recognized that this reduced recurrence.^[195] Petit's work built on the methods of the surgeon Bernard Peyrilhe, who in the 17th century additionally removed the pectoral muscle underlying the breast, as he judged that this greatly improved the prognosis.^[196] But poor results and the considerable risk to the patient meant that physicians did not share the opinion of surgeons such as Nicolaes Tulp, who in the 17th century proclaimed "the sole remedy is a timely operation." The eminent surgeon Richard Wiseman documented in the mid-17th century that following 12 mastectomies, two patients died during the operation, eight patients died shortly after the operation from progressive cancer and only two of the 12 patients were cured.^[197]: 6 Physicians were conservative in the treatment they prescribed in the early stages of breast cancer. Patients were treated with a mixture of detox purges, blood letting and traditional remedies that were supposed to lower acidity, such as the alkaline arsenic.^[198]: 24

When in 1664 Anne of Austria was diagnosed with breast cancer, the initial treatment involved compresses saturated with hemlock juice. When the lumps increased the King's physician commenced a treatment with arsenic ointments.^[198]: 25 The royal patient died 1666 in atrocious pain.^[198]: 26 Each failing treatment for breast cancer led to the search for new treatments, spurring a market in remedies that were advertised and sold by quacks, herbalists, chemists and apothecaries.^[199] The lack of anesthesia and antiseptics made mastectomy a painful and dangerous ordeal.^[197] In the 18th century, a wide variety of anatomical discoveries were accompanied by new theories about the cause and growth of breast cancer. The investigative surgeon John Hunter claimed that neural fluid generated breast cancer. Other surgeons proposed that milk within the mammary ducts led to cancerous growths. Theories about trauma to the breast as cause for malignant changes in breast tissue were advanced. The discovery of breast lumps and swellings fueled controversies about hard tumors and whether lumps were benign stages of cancer. Medical opinion about necessary immediate treatment varied.^[197]: 5 The surgeon Benjamin Bell advocated removal of the entire breast, even when only a portion was affected.^[200]

Breast cancer was uncommon until the 19th century, when improvements in sanitation and control of deadly infectious diseases resulted in dramatic increases in lifespan. Previously, most women had died too young to have developed breast cancer.^[201] In 1878, an article in Scientific American described historical treatment by pressure intended to induce local ischemia in cases when surgical removal were not possible.^[202] William Stewart Halsted started performing radical mastectomies in 1882, helped greatly by advances in general surgical technology, such as aseptic technique and anesthesia. The Halsted radical mastectomy often involved removing both breasts, associated lymph nodes, and the underlying chest muscles. This often led to long-term pain and disability, but was seen as necessary to prevent the cancer from recurring.^{[191]: 102–106} Before the advent of the Halsted radical mastectomy, 20-year survival rates were only 10%; Halsted's surgery raised that rate to 50%.^[191]: 1

Breast cancer staging systems were developed in the 1920s and 1930s to determining the extent to which a cancer has developed by growing and spreading.^{[191]: 102–106} The first case-controlled study on breast cancer epidemiology was done by Janet Lane-Claypon, who published a comparative study in 1926 of 500 breast cancer cases and 500 controls of the same background and lifestyle for the British Ministry of Health.^[203] Radical mastectomies remained the standard of care in the USA until the 1970s, but in Europe, breast-sparing procedures, often followed by radiation therapy, were generally adopted in the 1950s.^{[191]: 102–106} In 1955 George Crile Jr. published Cancer and Common Sense arguing that cancer patients needed to understand available treatment options. Crile became a close friend of the environmentalist Rachel Carson, who had undergone a Halsted radical mastectomy in 1960 to treat her malign breast cancer.^{[204]: 39–40} The US oncologist Jerome Urban promoted super radical mastectomies, taking even more tissue, until 1963, when the ten-year survival rates proved equal to the less-damaging radical mastectomy.^{[191]: 102–106} Carson died in 1964 and Crile went on to published a wide variety of articles, both in the popular press and in medical journals, challenging the widespread use of the Halsted radical mastectomy. In 1973 Crile published What Women Should Know About the Breast Cancer Controversy. When in 1974 Betty Ford was diagnosed with breast cancer, the options for treating breast cancer were openly discussed in the press.^[204]: 58 During the 1970s, a new understanding of metastasis led to perceiving cancer as a systemic illness as well as a localized one, and more sparing procedures were developed that proved equally effective.^[205]

In the 1980s and 1990s, thousands of women who had successfully completed standard treatment then demanded and received high-dose bone marrow transplants, thinking this would lead to better long-term survival. However, it proved completely ineffective, and 15–20% of women died because of the brutal treatment.^{[206]: 200–203} The 1995 reports from the Nurses' Health Study and the 2002 conclusions of the Women's Health Initiative trial conclusively proved that HRT significantly increased the incidence of breast cancer.^[206]

Before the 20th century, breast cancer was feared and discussed in hushed tones, as if it were shameful. As little could be safely done with primitive surgical techniques, women tended to suffer silently rather than seeking care.^{[citation needed]} When surgery advanced, and long-term survival rates improved, women began raising awareness of the disease and the possibility of successful treatment. The "Women's Field Army", run by the American Society for the Control of Cancer (later the American Cancer Society) during the 1930s and 1940s was one of the first organized campaigns. In 1952, the first peer-to-peer support group, called "Reach to Recovery", began providing post-mastectomy, in-hospital visits from women who had survived breast cancer.^{[206]: 37–38}

The breast cancer movement of the 1980s and 1990s developed out of the larger feminist movements and women's health movement of the 20th century.^[206]: 4 This series of political and educational campaigns, partly inspired by the politically and socially effective AIDS awareness campaigns, resulted in the widespread acceptance of second opinions before surgery, less invasive surgical procedures, support groups, and other advances in care.^[207]

There are ethnic disparities in the mortality rates for breast cancer as well as in breast cancer treatment. Breast cancer is the most prevalent cancer affecting women of every ethnic group in the United States. Breast cancer incidence among black women aged 45 and older is higher than that of white women in the same age group. White women aged 60–84 have higher incidence rates of breast cancer than Black women. Despite this, Black women at every age are more likely to succumb to breast cancer.^[215]

Breast cancer treatment has improved greatly over the years, but black women are still less likely to obtain treatment compared to white women.^[215] Risk factors such as socioeconomic status, late-stage, or breast cancer at diagnosis, genetic differences in tumor subtypes, differences in health care access all contribute to these disparities. Socioeconomic determinants affecting the disparity in breast cancer illness include poverty, culture, as well as social injustice. In Hispanic women, the incidence of breast cancer is lower than in non-Hispanic women but is often diagnosed at a later stage than white women with larger tumors.

Black women are usually diagnosed with breast cancer at a younger age than white women. The median age of diagnosis for Black women is 59, in comparison to 62 in White women. The incidence of breast cancer in Black women has increased by 0.4% per year since 1975 and 1.5% per year among Asian/Pacific Islander women since 1992. Incidence rates were stable for non-Hispanic White, Hispanics, and Native women. The five-year survival rate is noted to be 81% in Black women and 92% in White women. Chinese and Japanese women have the highest survival rates.^[215]

Poverty is a major driver for disparities related to breast cancer. Low-income women are less likely to undergo breast cancer screening and thus are more likely to have a late-stage diagnosis.^[215] Ensuring women of all ethnic groups receive equitable health care including breast screening, can positively affect these disparities.^[216]

Pregnancy at an early age decreases the risk of developing breast cancer later in life.^[217] The risk of breast cancer also declines with the number of children a woman has.^[217] Breast cancer then becomes more common in the five to ten years following pregnancy but then becomes less common than among the general population.^[218] These cancers are known as postpartum breast cancer and have worse outcomes including an increased risk of distant spread of disease and mortality.^[219] Other cancers found during or shortly after pregnancy appear at approximately the same rate as other cancers in women of a similar age.^[220]

Diagnosing new cancer in a pregnant woman is difficult, in part because any symptoms are commonly assumed to be a normal discomfort associated with pregnancy.^[220] As a result, cancer is typically discovered at a somewhat later stage than average in many pregnant or recently pregnant women. Some imaging procedures, such as MRIs (magnetic resonance imaging), CT scans, ultrasounds, and mammograms with fetal shielding are considered safe during pregnancy; some others, such as PET scans are not.^[220]

Treatment is generally the same as for non-pregnant women.^[220] However, radiation is normally avoided during pregnancy, especially if the fetal dose might exceed 100 cGy. In some cases, some or all treatments are postponed until after birth if the cancer is diagnosed late in the pregnancy. Early deliveries to speed the start of treatment are not uncommon. Surgery is generally considered safe during pregnancy, but some other treatments, especially certain chemotherapy drugs given during the first trimester, increase the risk of birth defects and pregnancy loss (spontaneous abortions and stillbirths).^[220] Elective abortions are not required and do not improve the likelihood of the mother surviving or being cured.^[220]

Radiation treatments may interfere with the mother's ability to breastfeed her baby because it reduces the ability of that breast to produce milk and increases the risk of mastitis. Also, when chemotherapy is being given after birth, many of the drugs pass through breast milk to the baby, which could harm the baby.^[220]

Regarding future pregnancy among breast cancer survivors, there is often fear of cancer recurrence.^[221] On the other hand, many still regard pregnancy and parenthood to represent normality, happiness and life fulfillment.^[221]

Treatments are being evaluated in clinical trials. This includes individual drugs, combinations of drugs, and surgical and radiation techniques Investigations include new types of targeted therapy,^[225] cancer vaccines, oncolytic virotherapy,^[226] gene therapy^[227][228] and immunotherapy.^[229]

The latest research is reported annually at scientific meetings such as that of the American Society of Clinical Oncology, San Antonio Breast Cancer Symposium,^[230] and the St. Gallen Oncology Conference in St. Gallen, Switzerland.^[231] These studies are reviewed by professional societies and other organizations, and formulated into guidelines for specific treatment groups and risk category.

Fenretinide, a retinoid, is also being studied as a way to reduce the risk of breast cancer.^[232][233] In particular, combinations of ribociclib plus endocrine therapy have been the subject of clinical trials.^[234]

A 2019 review found moderate certainty evidence that giving people antibiotics before breast cancer surgery helped to prevent surgical site infection (SSI). Further study is required to determine the most effective antibiotic protocol and use in women undergoing immediate breast reconstruction.^[235]

• Mammary tumor for breast cancer in other animals
• Mouse models of breast cancer metastasis

• Metaplastic breast cancer (MBC)
• Vasculogenic mimicry