Epidemiology and etiology of breast cancer

Epidemiology and etiology of breast cancer

Epidemiological risk factors for a disease can provide important clues as to the etiology, or cause, of a disease. The first work on the epidemiology and etiology of breast cancer was done by Janet Lane-Claypon, who published a comparative study in 1926 of 500 breast cancer cases and 500 control patients of the same background and lifestyle for the British Ministry of Health.cite book | last = Lane-Claypon | first = Janet Elizabeth | authorlink = Janet Lane-Claypon | coauthors = | title = A further report on cancer of the breast, with special reference to its associated antecedent conditions | publisher = Her Majesty’s Stationery Office (HMSO) |date=1926 | location = London, Greater London | pages = | url = | oclc = 14713036 | doi = | id = | isbn = ] Verify source|date=January 2008cite book | author = Alfredo Morabia | title = A History of Epidemiologic Methods and Concepts | publisher = Birkhauser | location = Boston | year = 2004 | pages = 301-302 | isbn = 3-7643-6818-7 | oclc = | doi = | url = http://books.google.com/books?id=E-OZbEmPSTkC&pg=PA301&lpg=PA301&dq=%22lane+claypon%22+%22further+report+*+cancer%22&source=web&ots=jSQWua-Sx0&sig=3arOIXuwGU_w_Omza11lEDj2TEo#PPA301,M1 | accessdate = 2007-12-31]

Today, breast cancer, like other forms of cancer, is considered to result from multiple environmental and hereditary factors.
# Lesions to DNA such as genetic mutations. Exposure to estrogen has been experimentally linked to the mutations that cause breast cancer.cite journal |author=Cavalieri E, Chakravarti D, Guttenplan J, "et al" |title=Catechol estrogen quinones as initiators of breast and other human cancers: implications for biomarkers of susceptibility and cancer prevention |journal=Biochim. Biophys. Acta |volume=1766 |issue=1 |pages=63–78 |year=2006 |pmid=16675129 |doi=10.1016/j.bbcan.2006.03.001] Beyond the contribution of estrogen, research has implicated viral oncogenesis and the contribution of ionizing radiation.
# Failure of immune surveillance, which usually removes malignancies at early phases of their natural history.
# Abnormal growth factor signaling in the interaction between stromal cells and epithelial cells, for example in the angiogenesis necessary to promote new blood vessel growth near new cancers.
# Inherited defects in DNA repair genes, such as "BRCA1", "BRCA2" and "p53".

Although many epidemiological risk factors have been identified, the cause of any individual breast cancer is often unknowable. In other words, epidemiological research informs the patterns of breast cancer incidence across certain populations, but not in a given individual. Approximately 5% of new breast cancers are attributable to hereditary syndromes, while no etiology is known for the other 95% of cases.cite journal | author = Madigan MP, Ziegler RG, Benichou J, Byrne C, Hoover RN | title = Proportion of breast cancer cases in the United States explained by well-established risk factors | journal = J. Natl. Cancer Inst. | volume = 87 | issue = 22 | pages = 1681–5 | year = 1995 | pmid = 7473816 | doi = | accessdate = 2007-05-26]


The risk of getting breast cancer increases with age. A woman who lives to age 90 has a lifetime risk of about 14.3%, or one in seven. [cite web | title = Individual Risk Factors| publisher = BreastCander.org | url = http://www.breastcancer.org/symptoms/understand_bc/who_gets/ind_risk.jsp | accessdate = 2007-03-11] Despite the fact that they have a proportionately higher risk of breast cancer, there is a lack of significant research on the breast cancer experiences of older women.cite journal |author=Perkins EA, Small BJ, Balducci L, Extermann M, Robb C, Haley WE |title=Individual differences in well-being in older breast cancer survivors |journal=Crit. Rev. Oncol. Hematol. |volume=62 |issue=1 |pages=74–83 |year=2007 |month=April |pmid=17240157 |pmc=1911161 |doi=10.1016/j.critrevonc.2006.11.002 |url=] However, this is beginning to change as studies, such as one published recently in "Critical Reviews in Oncology and Hematology," demonstrate that older breast cancer patients experience their illness in a significantly different manner than younger patients. For instance when compared to their younger counterparts, depression was found to occur more often in older breast cancer patients whose advanced age exacerbated the effects of their illness and treatment and resulted in functional impairment. This difference in illness experience highlights the need for more research into the psychosocial support services available to geriatric breast cancer patients.The probability of breast cancer rises with age, but breast cancer tends to be more aggressive when it occurs in younger people. One type of breast cancer that is especially aggressive and that occurs disproportionately in younger people is inflammatory breast cancer. It is initially staged as Stage IIIb or Stage IV. It also is unique because it often does not present with a lump, so it is often undetected by mammography or ultrasound. It presents with the signs and symptoms of a breast infection like mastitis, and the treatment is usually a combination of surgery, radiation, and chemotherapy.


Men have a lower risk of developing breast cancer (approximately 1.08 per 100,000 men per year), but this risk appears to be rising.cite journal | last = Giordano | first = Sharon H | authorlink = | coauthors = Cohen DS, Buzdar AU, Perkins G, Hortobagyi GN | title = Breast carcinoma in men | journal = Cancer | volume = 101 | issue = 1 | pages = 51–57 | publisher =American Cancer Society | date = May 2004 | url= http://www3.interscience.wiley.com/cgi-bin/fulltext/108565241/HTMLSTART | doi = | id = | accessdate = ] Men with gynaecomastia do not have a higher risk of developing breast cancer. [cite web |author=Ali Fawzi, MD |title=Gynecomastia |publisher=eMedicine.com |url=http://www.emedicine.com/plastic/topic125.htm | year=2006 |month=June |accessdate=2007-04-17] There may be an increased incidence of breast cancer in men with prostate cancer. The prognosis, even in stage I cases, is worse in men than in women.cite web | author = Armando E. Giuliano, MD | title =Carcinoma of the Male Breast - General Considerations | work =Breast Cancer | url=http://www.health.am/cr/more/carcinoma-of-the-male-breast/ | year = 2006 | month= May 31 | publisher=Armenian Health Network, Health.am | accessdate=2007-02-27] The treatment of men with breast cancer is similar to that in older women. Since the male breast tissue is confined to the area directly behind the nipple, treatment for males has usually been a mastectomy with axillary surgery. This may be followed by adjuvant radiotherapy, hormone therapy (such as tamoxifen), or chemotherapy.


In 5% of breast cancer cases, there is a strong inherited familial risk.cite journal |author=Malone KE, Daling JR, Thompson JD, O'Brien CA, Francisco LV, Ostrander EA |title=BRCA1 mutations and breast cancer in the general population: analyses in women before age 35 years and in women before age 45 years with first-degree family history |journal=JAMA |volume=279 |issue=12 |pages=922–9 |year=1998 |pmid=9544766 |doi=] Two autosomal dominant genes, "BRCA1" and "BRCA2", account for most of the cases of familial breast cancer. Family members who harbor mutations in these genes have a 60% to 80% risk of developing breast cancer in their lifetimes. Other associated malignancies include ovarian cancer and pancreatic cancer. If a mother or a sister was diagnosed breast cancer, the risk of a hereditary ‘’’BRCA1’’’ or ‘’’BRCA2’’’ gene mutation is about 2-fold higher than those women without a familial history. In addition to the BRCA genes associated with breast cancer, the presence of "NBR2", near breast cancer gene 1, has been discovered, and research into its contribution to breast cancer pathogenesis is ongoing.cite journal |author=Emilie Auriol, Lise-Marie Billard, Frederique Magdinier, Robert Dante |title=Specific binding of the methyl binding domain protein 2 at the "BRCA1-NBR2" locus |journal=Nucleic Acids Research |volume=33 |issue=13 |pages=4243–4254 |year=2005 |doi=10.1093/nar/gki729 |pmid=16052033] Commercial testing for ‘’’BRCA1’’’ and ‘’’BRCA2’’’ gene mutations has been available since at least 2004. Genetic testing for BRCA gene mutations is conducted exclusively by Myriad Genetics, located in Salt Lake City.


Dietary influences have been proposed and examined, and recent research suggests that low-fat diets may significantly decrease the risk of breast cancer as well as the recurrence of breast cancer. [cite journal
author = Chlebowski RT, Blackburn GL, Thomson CA, Nixon DW, Shapiro A, Hoy MK, et al.
title = Dietary fat reduction and breast cancer outcome: interim efficacy results from the Women's Intervention Nutrition Study (WINS)
journal =J Natl Cancer Inst
volume =98
issue =24
pages =1767–1776
pmid = 17179478
url =
] Another study showed no contribution of dietary fat intake on the incidence of breast cancer in over 300,000 women.cite journal |author=Hunter DJ, Spiegelman D, Adami HO, "et al" |title=Cohort studies of fat intake and the risk of breast cancer--a pooled analysis |journal=N. Engl. J. Med. |volume=334 |issue=6 |pages=356–61 |year=1996 |pmid=8538706 |doi=] A randomized controlled study of the consequences of a low-fat diet, the Women's Health Initiative, failed to show a statistically significant reduction in breast cancer incidence in the group assigned to a low-fat diet, although the authors did find evidence of a benefit in the subgoup of women who followed the low-fat diet in a strict manner.cite journal |author=Prentice RL, Caan B, Chlebowski RT, "et al" |title=Low-fat dietary pattern and risk of invasive breast cancer: the Women's Health Initiative Randomized Controlled Dietary Modification Trial |journal=JAMA |volume=295 |issue=6 |pages=629–42 |year=2006 |pmid=16467232 |doi=10.1001/jama.295.6.629] A prospective cohort study, the Nurses' Health Study II, found increased breast cancer incidence in premenopausal women only, with higher intake of animal fat, but not vegetable fat. Taken as a whole, these results point to a possible association between dietary fat intake and breast cancer incidence, though these interactions are hard to measure in large groups of women.

In a study published in the "Journal of the American Medical Association", biomedical investigators found that Brassica vegetable intake (broccoli, cauliflower, cabbage, kale and Brussels sprouts) was inversely related to breast cancer development. The relative risk among women in the highest decile of Brassica vegetable consumption (median, 1.5 servings per day) compared to the lowest decile (virtually no consumption) was 58%. That is, women who consumed around 1.5 servings of Brassica vegetables per day had 42% less risk of developing breast cancer than those who consumed virtually none. [cite journal |author=Terry P, Wolk A, Persson I, Magnusson C title = Brassica Vegetables and Breast Cancer Risk |journal=JAMA |volume=285 |issue=23 |pages=2975–2976 |year=2001
doi = 10.1001/jama.285.23.2975
title = Brassica Vegetables and Breast Cancer Risk |pmid=11410091

A significant environmental effect is likely responsible for the different rates of breast cancer incidence between countries with different dietary customs. Researchers have long measured that breast cancer rates in an immigrant population soon come to resemble the rates of the host country after a few generations. The reason for this is speculated to be immigrant uptake of the host country diet. The prototypical example of this phenomenon is the changing rate of breast cancer after the arrival of Japanese immigrants to America. [cite journal |author= Nelson N title = Migrant Studies Aid Search for Factors Linked to Breast Cancer Risk |journal=JAMA |volume=8 |issue=7 |pages=436–438 |year=2006]


Alcohol appears to increase the risk of breast cancer, with some studies finding a positive correlation between consumption levels and increased risk. One study concluded that breast cancer comprises 60 percent of alcohol-attributable cancers among women.cite journal
last = Boffetta
first = Paolo
coauthors = Hashibe, Mia; La Vecchia, Carlo; Zatonski, Witold; Rehm, Jürgen
title = The burden of cancer attributable to alcohol drinking
journal = International Journal of Cancer
volume = 119
issue = 4
pages = 884–887
publisher = Wiley-Liss, Inc
date = 2006-03-23
url = http://www3.interscience.wiley.com/cgi-bin/abstract/112550783/ABSTRACT
doi = 10.1002/ijc.21903
accessdate = 2006-10-09
] The UK's "Review of Alcohol: Association with Breast Cancer" concludes that "studies confirm previous observations that there appears to be an association between alcohol intake and increased risk of breast cancer in women. On balance, there was a weak association between the amount of alcohol consumed and the relative risk."cite web | title = Review of Alcohol: Association with Breast Cancer | publisher = U.K. Dept. of Health | url = http://www.advisorybodies.doh.gov.uk/pdfs/alcbrrev.pdf | accessdate = 2007-03-11]

The National Institute on Alcohol Abuse and Alcoholism (NIAAA) concludes that "Chronic alcohol consumption has been associated with a small (averaging 10 percent) increase in a woman's risk of breast cancer."cite journal |author=Friedenreich C, Howe G, Miller A, Jain M |title=A cohort study of alcohol consumption and risk of breast cancer |journal=Am J Epidemiol |volume=137 |issue=5 |pages=512–20 |year=1993 |pmid=8465803] cite journal |author=Longnecker M, Berlin J, Orza M, Chalmers T |title=A meta-analysis of alcohol consumption in relation to risk of breast cancer |journal=JAMA |volume=260 |issue=5 |pages=652–6 |year=1988 |pmid=3392790 |doi=10.1001/jama.260.5.652] cite journal | author=Longnecker MP | title = Alcohol consumption in relation to risk of cancers of the breast and large bowel | journal = Alcohol Health & Research World | year = 1992 | volume = 16 | issue = 3 | pages = 223–229 | url= ] cite journal |author=Nasca P, Baptiste M, Field N, Metzger B, Black M, Kwon C, Jacobson H |title=An epidemiological case-control study of breast cancer and alcohol consumption |journal=Int J Epidemiol |volume=19 |issue=3 |pages=532–8 |year=1990 |pmid=2262245 |doi=10.1093/ije/19.3.532] According to these studies, the risk appears to increase as the quantity and duration of alcohol consumption increases. Other studies, however, have found no evidence of such a link.cite journal |author=Chu S, Lee N, Wingo P, Webster L |title=Alcohol consumption and the risk of breast cancer |journal=Am J Epidemiol |volume=130 |issue=5 |pages=867–77 |year=1989 |pmid=2683749] cite journal |author=Schatzkin A, Piantadosi S, Miccozzi M, Bartee D |title=Alcohol consumption and breast cancer: a cross-national correlation study |journal=Int J Epidemiol |volume=18 |issue=1 |pages=28–31 |year=1989 |pmid=2722377 |doi=10.1093/ije/18.1.28] cite journal |author=Webster L, Layde P, Wingo P, Ory H |title=Alcohol consumption and risk of breast cancer |journal=Lancet |volume=2 |issue=8352 |pages=724–6 |year=1983 |pmid=6136850 |doi=10.1016/S0140-6736(83)92258-4]

The UK "Committee on Carcinogenicity of Chemicals in Food, Consumer Products Non-Technical Summary" concludes, "the new research estimates that a woman drinking an average of two units of alcohol per day has a lifetime risk of developing breast cancer 8% higher than a woman who drinks an average of one unit of alcohol per day."cite web | title = Consumption of Alcoholic Beverage and risk of Breast Cancer in Women | publisher = U.K. Dept. of Health | url = http://www.advisorybodies.doh.gov.uk/pdfs/alco04nontech.pdf | accessdate = 2007-03-11] The risk of breast cancer further increases with each additional drink consumed per day. The research also concludes that "approximately 6 percent (between 3.2 percent and 8.8 percent) of breast cancers reported in the UK each year could be prevented if drinking was reduced to a very low level (i.e. less than 1 unit/week)." A review article from JAMA also found that breast cancer incidence seems to increase with increasing alcohol consumption.cite journal |author=Singletary KW, Gapstur SM |title=Alcohol and breast cancer: review of epidemiologic and experimental evidence and potential mechanisms |journal=JAMA |volume=286 |issue=17 |pages=2143–51 |year=2001 |pmid=11694156 |doi=] It has been reported that "two drinks daily increase the risk of getting breast cancer by about 25 percent" (NCI), but the evidence is inconsistent. The Framingham study has carefully tracked individuals since the 1940s. Data from that research found that drinking alcohol moderately did not increase breast cancer risk (Wellness Facts). Similarly, research by the Danish National Institute for Public Health found that moderate drinking had virtually no effect on breast cancer risk.cite journal |author=Petri A, Tjønneland A, Gamborg M, Johansen D, Høidrup S, Sørensen T, Grønbaek M |title=Alcohol intake, type of beverage, and risk of breast cancer in pre- and postmenopausal women |journal=Alcohol Clin Exp Res |volume=28 |issue=7 |pages=1084–90 |year=2004 |pmid=15252295 |doi=10.1097/01.ALC.0000130812.85638.E1]

A 2007 study indicated that consuming one or two alcoholic drinks a day increases the risk of breast cancer by 10 percent compared with those who consumed less than one drink daily. Women who have three or more drinks a day increased their breast cancer risk by 30 percent. The type of drink was not a factor. [Jeremy Laurance [http://www.independent.co.uk/life-style/health-and-wellbeing/health-news/three-drinks-a-day-increases-risk-of-breast-cancer-by-a-third-403656.html Three drinks a day increases risk of breast cancer by a third] "The Independent" 27 September 2007] One study suggests that women who frequently drink red wine may have an increased risk of developing breast cancer.cite journal |author=Maggiolini M, Recchia A, Bonofiglio D, Catalano S, Vivacqua A, Carpino A, Rago V, Rossi R, Andò S |title=The red wine phenolics piceatannol and myricetin act as agonists for estrogen receptor alpha in human breast cancer cells |journal=J Mol Endocrinol |volume=35 |issue=2 |pages=269–81 |year=2005 |pmid=16216908 |doi=10.1677/jme.1.01783]

One of the largest studies of its kind has found that alcohol is a substantial risk factor for development of the most common type of breast cancer - the 70% of tumors that are classified as positive for both the estrogen and progesterone receptors (ER+/PR+). Researchers report that even moderate alcohol consumption, defined as one or two drinks per day, increased risk of developing this kind of cancer, and the more a woman drank, the higher her risk. Compared to women who did not drink at all, women who had three or more glasses of alcohol daily had as much as a 51% increased risk of ER+/PR+ breast cancer. [ [http://www.aacr.org/home/public--media/news-releases.aspx?d=1035 How What and How Much We Eat (And Drink) Affects Our Risk of Cancer] ]


Gaining weight after menopause can increase a woman's risk. A recent study found that putting on 9.9kg (22lbs) after menopause increased the risk of developing breast cancer by 18%. [BBC report [http://news.bbc.co.uk/1/hi/health/5171838.stm Weight link to breast cancer risk] ]

Hormones, pregnancy, and childbearing

Persistently increased blood levels of estrogen are associated with an increased risk of breast cancer, as are increased levels of the androgens androstenedione and testosterone (which can be directly converted by aromatase to the estrogens estrone and estradiol, respectively). Increased blood levels of progesterone are associated with a decreased risk of breast cancer in premenopausal women. [cite journal |author=Yager JD |coauthors=Davidson NE |title=Estrogen carcinogenesis in breast cancer |journal=New Engl J Med |volume=354 |issue=3 |year=2006 |pages=270–82 |pmid=16421368 |doi=10.1056/NEJMra050776] A number of circumstances which increase exposure to endogenous estrogens including not having children, delaying first childbirth, not breastfeeding, early menarche (the first menstrual period) and late menopause are suspected of increasing lifetime risk for developing breast cancer. [American Cancer Society. (2006-10-03). [http://www.cancer.org/docroot/CRI/content/CRI_2_4_2X_What_are_the_risk_factors_for_breast_cancer_5.asp What Are the Risk Factors for Breast Cancer?] Retrieved 2006-03-30.]

Hormonal contraceptives may produce a slight increase in the risk of breast cancer diagnosis among current and recent users, but this appears to be a short-term effect. In 1996 the largest collaborative reanalysis of individual data on over 150,000 women in 54 studies of breast cancer found a relative risk (RR) of 1.24 of breast cancer diagnosis among current combined oral contraceptive pill users; 10 or more years after stopping, no difference was seen. Further, the cancers diagnosed in women who had ever used hormonal contraceptives were less advanced than those in nonusers, raising the possibility that the small excess among users was due to increased detection. cite journal |author=Collaborative Group on Hormonal Factors in Breast Cancer |year=1996 |title=Breast cancer and hormonal contraceptives: collaborative reanalysis of individual data on 53,297 women with breast cancer and 100,239 women without breast cancer from 54 epidemiological studies |journal=Lancet |volume=347 |issue=9017 |pages=1713–27 |pmid=8656904 |doi=10.1016/S0140-6736(96)90806-5] cite journal |author=Collaborative Group on Hormonal Factors in Breast Cancer |year=1996 |title=Breast cancer and hormonal contraceptives: further results |journal=Contraception |volume=54 |issue=3 Suppl |pages=1S–106S |pmid=8899264 |doi=10.1016/0010-7824(96)00111-4] The relative risk of breast cancer diagnosis associated with current and recent use of hormonal contraceptives did not appear to vary with family history of breast cancer.cite web |url=http://www.cancer.gov/cancertopics/pdq/genetics/breast-and-ovarian/healthprofessional#Section_340 |title=Hormone Therapy |accessdate=2006-08-12 |author=National Cancer Institute |date=2006-08-03 |work=Genetics of Breast and Ovarian Cancer]

Data exist from both observational and randomized clinical trials regarding the association between postmenopausal hormone replacement therapy (HRT) and breast cancer. The largest meta-analysis (1997) of data from 51 observational studies, indicated a relative risk of breast cancer of 1.35 for women who had used HRT for 5 or more years after menopause. The estrogen-plus-progestin arm of the Women's Health Initiative (WHI), a randomized controlled trial, which randomized more than 16,000 postmenopausal women to receive combined hormone therapy or placebo, was halted early (2002) because health risks exceeded benefits. One of the adverse outcomes prompting closure was a significant increase in both total and invasive breast cancers (RR = 1.24) in women randomized to receive estrogen and progestin for an average of 5 years. HRT-related breast cancers had adverse prognostic characteristics (more advanced stages and larger tumors) compared with cancers occurring in the placebo group, and HRT was also associated with a substantial increase in abnormal mammograms. Short-term use of hormones for treatment of menopausal symptoms appears to confer little or no breast cancer risk.cite web |url=http://www.cancer.gov/cancertopics/pdq/genetics/breast-and-ovarian/healthprofessional#Section_340 |title=Hormone Therapy |accessdate=2006-08-12 |author=National Cancer Institute |date=2006-08-03 |work=Genetics of Breast and Ovarian Cancer]

Environmental causes


Until recently, most studies had not found an increased risk of breast cancer from active tobacco smoking. Beginning in the mid-1990s, a number of studies suggested an increased risk of breast cancer in both active smokers and those exposed to secondhand smoke compared to women who reported no exposure to secondhand smoke. By 2005 enough evidence had accumulated for the [California Environmental Protection Agency] to conclude that breathing secondhand smoke causes breast cancer in younger, primarily premenopausal women. [http://repositories.cdlib.org/context/tc/article/1194/type/pdf/viewcontent/ California Environmental Protection Agency: Air Resources Board, "Proposed Identification of Environmental Tobacco Smoke as a Toxic Air Contaminant" (June 24, 2005)] ; on January 26, 2006, the Air Resources Board, following a lengthy review and public outreach process, determined ETS to be a Toxic Air Contaminant (TAC).] The Agency concluded that the risk was increased by 70%, based on epidemiological studies and the fact that there are many mammary carcinogens in secondhand smoke. The following year (2006) the US Surgeon General [ [http://www.surgeongeneral.gov/library/secondhandsmoke/ U.S. Department of Health and Human Services. "The Health Consequences of Involuntary Exposure to Tobacco Smoke: A Report of the Surgeon General", 2006] ] identified the same risk increase and concluded that the evidence is "suggestive," one step below causal. There is some evidence that exposure to tobacco smoke is most problematic between puberty and first childbirth. The reason is that breast tissue appears most sensitive to chemical carcinogens breast cells not fully differentiated until lactation. [J. Russo, I. Russo. "Molecular Basis of Breast Cancer: Prevention and Treatment," Springer, 2003] The likely reason that the older studies of active smoking did not detect risks associated with smoking was that they compared active smokers to all nonsmokers (which includes many passive smokers). The newer studies, which exclude passive smokers from the control group, generally show elevated risks associated with active as well as passive smoking.


Women who have received high-dose ionizing radiation to the chest (for example, as treatments for other cancers) have a relative risk of breast cancer between 2.1 to 4.0. cite web |author=ACS |year=2005 |title=Breast Cancer Facts & Figures 2005-2006 |url=http://www.cancer.org/downloads/STT/CAFF2005BrFacspdf2005.pdf |accessdate=2007-04-26]

Estrogenic mimics

Although environmental exposures are not generally cited as risk factors for the disease (except for diet, pharmaceuticals and radiation), a substantial and growing body of evidence indicates that exposures to certain toxic chemicals and hormone-mimicking compounds including chemicals used in pesticides, cosmetics and cleaning products contribute to the development of breast cancer.A recent Canadian study concluded that female farm workers are three times more likely to have breast cancer. [cite news | author = Ctv.Ca News Staff | title= Breast cancer more likely in farm workers: study |url= http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/20061012/breastcancer_farming_061012/20061012?hub=Health| publisher= Ctv.Ca |date=2006-10-12 |accessdate=2007-03-11 ] The increasing prevalence of these substances in the environment may explain the increasing incidence of breast cancer, though direct evidence is sparse.


Although not well-quantified, there has long been a concern about risk associated with environmental estrogenic compounds, such as dioxins. Fact|date=June 2007

Light at night and disturbance of circadian rhythm

In 1978 Cohen et al proposed that reduced production of the hormone melatonin might increase the risk of breast cancer and citing "environmental lighting" as a possible causal factor. [cite journal |author=Cohen M, Lippman M, Chabner B |title=Role of pineal gland in aetiology and treatment of breast cancer |journal=Lancet |volume=2 |issue=8094 |pages=814–6 |year=1978 |month=October |pmid=81365 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/S0140-6736(78)92591-6] Researchers at the National Cancer Institute (NCI) and National Institute of Environmental Health Sciences conducted a study in 2005 that suggests that artificial light during the night can be a factor for breast cancer by disrupting melatonin levels.cite journal |author=Blask DE, Brainard GC, Dauchy RT, "et al" |title=Melatonin-depleted blood from premenopausal women exposed to light at night stimulates growth of human breast cancer xenografts in nude rats |journal=Cancer Res. |volume=65 |issue=23 |pages=11174–84 |year=2005 |month=December |pmid=16322268 |doi=10.1158/0008-5472.CAN-05-1945 |url=http://cancerres.aacrjournals.org/cgi/pmidlookup?view=long&pmid=16322268 |accessdate=2008-09-10]

In 2007, "shiftwork that involves circadian disruption" was listed as a probable carcinogen by the World Health Organization's International Agency for Research on Cancer. (IARC Press release No. 180). [ [http://www.iarc.fr/ENG/Press_Releases/pr180a.html IARC Press release No. 180] ] Multiple studies have documented a link between night shift work and the increased incidence of breast cancer. [cite journal |author=Schernhammer ES, Schulmeister K |title=Melatonin and cancer risk: does light at night compromise physiologic cancer protection by lowering serum melatonin levels? |journal=Br. J. Cancer |volume=90 |issue=5 |pages=941–3 |year=2004 |month=March |pmid=14997186 |doi=10.1038/sj.bjc.6601626 |url=] [cite journal |author=Hansen J |title=Increased breast cancer risk among women who work predominantly at night |journal=Epidemiology |volume=12 |issue=1 |pages=74–7 |year=2001 |month=January |pmid=11138824 |doi= |url=http://meta.wkhealth.com/pt/pt-core/template-journal/lwwgateway/media/landingpage.htm?issn=1044-3983&volume=12&issue=1&spage=74] [cite journal |author=Hansen J |title=Light at night, shiftwork, and breast cancer risk |journal=J. Natl. Cancer Inst. |volume=93 |issue=20 |pages=1513–5 |year=2001 |month=October |pmid=11604468 |doi= |url=http://jnci.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=11604468] [cite journal |author=Schernhammer ES, Laden F, Speizer FE, "et al" |title=Rotating night shifts and risk of breast cancer in women participating in the nurses' health study |journal=J. Natl. Cancer Inst. |volume=93 |issue=20 |pages=1563–8 |year=2001 |month=October |pmid=11604480 |doi= |url=http://jnci.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=11604480] A review of current knowledge of the health consequences of exposure to artificial light at night including the increased incidence of breast cancer and an explanation of the causal mechanisms has been published in the Journal of Pineal Research in 2007. [cite journal |author=Navara KJ, Nelson RJ |title=The dark side of light at night: physiological, epidemiological, and ecological consequences |journal=J. Pineal Res. |volume=43 |issue=3 |pages=215–24 |year=2007 |month=October |pmid=17803517 |doi=10.1111/j.1600-079X.2007.00473.x |url=http://www.psy.ohio-state.edu/nelson/documents/JPinealRes2007.pdf]

Viral pathogenesis research

Humans are not the only mammals susceptible to breast cancer. Some strains of mice, namely the house mouse (Mus domesticus) are prone to breast cancer which is caused by infection with the mouse mammary tumour virus (MMTV or "Bittner virus" for its discoverer Hans Bittner), by random insertional mutagenesis. This finding is taken to mean that a viral etiology of human breast cancer is at least possible, though there is no definitive evidence to support the claim that MMTV causes human breast cancer. For example, there may be critical differences between cancer pathogenesis in mice and people. The understanding of the role of MMTV or other viruses in human breast cancer is preliminary as of May 2007.

Factors with minimal or no impact on risk


The abortion-breast cancer (ABC) hypothesis (supporters call it the abortion-breast cancer link) posits induced abortion increases the risk of developing breast cancer;cite journal |author=Russo J, Russo I |title=Susceptibility of the mammary gland to carcinogenesis. II. Pregnancy interruption as a risk factor in tumor incidence |journal=Am J Pathol |volume=100 |issue=2 |pages=505–506 |year=1980 |pmid=6773421 "In contrast, abortion is associated with increased risk of carcinomas of the breast. The explanation for these epidemiologic findings is not known, but the parallelism between the DMBA-induced rat mammary carcinoma model and the human situation is striking. [...] Abortion would interrupt this process, leaving in the gland undifferentiated structures like those observed in the rat mammary gland, which could render the gland again susceptible to carcinogenesis."] it is a controversial subject and the current scientific consensus has concluded there is no significant association between first-trimester abortion and breast cancer risk.cite web |url=http://www.who.int/mediacentre/factsheets/fs240/en/index.html |title=WHO | Induced abortion does not increase breast cancer risk |accessdate=2007-12-24 |format= |work=who.int] cite web |url=http://oversight.house.gov/features/politics_and_science/example_breast_cancer.htm |title=Politics & Science - Investigating the State of Science Under the Bush Administration |accessdate=2008-04-14 |format=HTML |work=oversight.house.gov] In early pregnancy, levels of estrogen increase, leading to breast growth in preparation for lactation. The hypothesis proposes that if this process is interrupted by an abortion – before full maturity in the third trimester – then more relatively vulnerable immature cells could be left than there were prior to the pregnancy, resulting in a greater potential risk of breast cancer. The hypothesis mechanism was first proposed and explored in rat studies conducted in the 1980s.cite journal |author=Russo J, Russo I |title=Susceptibility of the mammary gland to carcinogenesis. II. Pregnancy interruption as a risk factor in tumor incidence |journal=Am J Pathol |volume=100 |issue=2 |pages=497–512 |year=1980 |pmid=6773421] cite journal |author=Russo J, Tay L, Russo I |title=Differentiation of the mammary gland and susceptibility to carcinogenesis |journal=Breast Cancer Res Treat |volume=2 |issue=1 |pages=5–73 |year=1982 |pmid=6216933 |doi=10.1007/BF01805718] cite journal |author=Russo J, Russo I |title=Biological and molecular bases of mammary carcinogenesis |journal=Lab Invest |volume=57 |issue=2 |pages=112–37 |year=1987 |pmid=3302534]


Much has been made of the possible contribution of aluminum-containing underarm antiperspirants to the incidence of breast cancer, since the most common location of a breast cancer is the upper outer quadrant of the breast. Aluminum salts, such as those used in anti-perspirants, have recently been classified as metalloestrogens. In research published in the "Journal of Applied Toxicology", Dr. Philippa D. Darbre of the University of Reading has shown that aluminum salts increase estrogen-related gene expression in human breast cancer cells grown in the laboratory. [Harding, Anne. (2006) [http://www.cancerpage.com/news/article.asp?id=9466 Aluminum Salts Could Increase Breast Cancer Risk] . Reuters Health.] [cite journal
last =Darbre
first =PD
title =Metalloestrogens: an emerging class of inorganic xenoestrogens with potential to add to the oestrogenic burden of the human breast
journal =Journal of Applied Toxicology
volume =26
issue =3
pages =191–7
publisher =John Wiley And Sons
date =2006
pmid = 16489580
url =
doi =10.1002/jat.1135
] [cite journal
last =Darbre
first =PD
title =Aluminium, antiperspirants and breast cancer
journal =Journal of Inorganic Biochemistry
volume =99
issue =9
pages =1912–9
publisher =Elsevier
date = 2005
url =
doi =10.1016/j.jinorgbio.2005.06.001
] Fortunately, this "in-vitro" association between aluminum salts and estrogen activity does not translate into an increased risk of breast cancer in humans. The lack of association between underarm deodorants and breast cancer has been the subject of a number of research articles.cite journal |author=Surendran A |title=Studies linking breast cancer to deodorants smell rotten, experts say |journal=Nat. Med. |volume=10 |issue=3 |pages=216 |year=2004 |pmid=14991030 |doi=10.1038/nm0304-216b] cite journal |author= |title=Antiperspirants don't cause breast cancer |journal=Harvard women's health watch |volume=10 |issue=5 |pages=7 |year=2003 |pmid=12543590 |doi=]

Fertility treatments

There is no persuasive connection between fertility medications and breast cancer. [cite journal |author=Potashnik G, Lerner-Geva L, Genkin L, Chetrit A, Lunenfeld E, Porath A |title=Fertility drugs and the risk of breast and ovarian cancers: results of a long-term follow-up study |journal=Fertil. Steril. |volume=71 |issue=5 |pages=853–9 |year=1999 |pmid=10231045 |doi=]


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