Risk factors of breast cancer may be divided into preventable and non preventable. Their study belongs in the field of epidemiology. Breast cancer, like other forms of cancer, is considered to result from multiple environmental and hereditary risk factors.

Although many epidemiological risk factors have been identified, the cause of any individual breast cancer is most often unknowable. 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 and well-established risk factors accounted for approximately 30% of cases.

The risk of getting breast cancer increases with age. A woman is more than 100 times more likely to develop breast cancer in her 60s than in her 20s. If all women lived to age 95, about one in eight would be diagnosed with breast cancer at some point during their lives. However, the actual lifetime risk is lower than that, because 90% of women die before age 95, most commonly from heart attacks, strokes, or other forms of cancer.

The probability of breast cancer rises with age, but breast cancer tends to be more aggressive in younger people.

Sex

Men have a much lower risk of developing breast cancer than women. In developed countries, about 99% of breast cancer cases are diagnosed in women; in a few African countries, which represent the highest incidence of male breast cancer, men account for 5–15% of breast cancer cases. The rate of breast cancer in men appears to be rising somewhat.

Men diagnosed with breast cancer tend to be older than women with breast cancer. They are more likely to be diagnosed with hormone-receptor positive tumors, with about six out of seven cases being estrogen-receptor positive. The overall prognosis is worse for men than for women.

Heredity

Cancer syndrome and Hereditary breast-ovarian cancer syndrome

United Kingdom being a member of the International Cancer Genome Consortium is leading efforts to map breast cancer's complete genome.

BRCA1 and BRCA2

In 5% of breast cancer cases, there is a strong inherited familial risk.

Two autosomal dominant genes, BRCA1 and BRCA2, account for most of the cases of familial breast cancer. Women who carry a harmful BRCA mutation 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. Commercial testing for BRCA1 and BRCA2 gene mutations has been available in most developed countries since at least 2004.

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.

Other genes

Hereditary non-BRCA1 and non-BRCA2 breast tumors (and even some sporadic carcinomas) are believed to result from the expression of weakly penetrant but highly prevalent mutations in various genes. For instance, polymorphism has been identified in genes associated to the metabolism of estrogens and/or carcinogens (CYP1A1, CYP1B1, CYP17, CYP19, COMT, NAT2, GSTM1, GSTP1, GSTT, . . . ), to estrogen, androgen and vitamin D action (ESR1, AR, VDR), to co-activation of gene transcription (AIB1), to DNA damage response pathways (CHEK2, HRAS1, XRCC1, XRCC3, XRCC5). Sequence variants of these genes that are relatively common in the population may be associated with a small to moderate increased relative risk for breast cancer. Combinations of such variants could lead to multiplicative effects. Sporadic cancers likely result from the complex interplay between the expression of low penetrance gene(s) (‘‘risk variants’’) and environmental factors. However, the suspected impact of most of these variants on breast cancer risk should, in most cases, be confirmed in large populations studies. Indeed, low penetrance genes cannot be easily tracked through families, as is true for dominant high-risk genes.

Part of the hereditary non-BRCA1 and non-BRCA2 breast tumors may be associated to rare syndromes, of which breast cancer is only one component. Such syndromes result notably from mutations in TP53 (Li-Fraumeni syndrome), ATM (Ataxia Telangiectasia), STK11/LKB1(Peutz-Jeghers syndrome), PTEN (Cowden syndrome).

RAB11FIP1, TP53, PTEN and rs4973768 are also associated with increased risk of breast cancer. rs6504950 is associated with lower risk of breast cancer.

Mutations in RAD51C confer an increased risk for breast and ovarian cancer.

Prior cancers

People who have previously been diagnosed with breast, ovarian, uterine, or bowel cancer have a higher risk of developing breast cancer in the future. Mothers of children with soft-tissue sarcoma may have an increased risk of breast cancer. Men with prostate cancer may have an elevated risk of breast cancer, although the absolute risk remains low.