A range of factors are associated with increased breast cancer risk in women. These risk factors are divided into two categories: modifiable or lifestyle risk factors, which can be prevented; and non-modifiable risk factors.
Modifiable risk factors
It has been estimated that 22.7% of breast cancers in Australia can potentially be prevented by addressing the major modifiable risk factors for breast cancer such as overweight and obesity, physical inactivity and alcohol consumption .
Overweight and obesity, and physical inactivity
Overweight and obesity
An Australian analysis estimated that 6.6% of breast cancers diagnosed among women aged 0-74 years in 2013 could be attributed to overweight and obesity. 
The World Cancer Research Fund (WCRF) found probable evidence that overweight and obesity increase the risk of postmenopausal breast cancer. WCRF estimates that 16% of breast cancers in the UK and 17% of breast cancers in the USA are attributable to body fatness. In Australia it was estimated that 8% of postmenopausal breast cancers diagnosed in 2010 were attributable to overweight and obesity.
While high body mass index is associated with increased risk of postmenopausal breast cancer, there is an inverse correlation for pre-menopausal women. WCRF has found probable evidence that body fatness decreases the risk of premenopausal breast cancer. However, there is an association between obesity and increased risk of triple-negative breast cancer (a subtype of breast tumour that does not have any of the three receptors commonly found on breast cancer cells) in premenopausal women.
WCRF states that there is probable evidence that physical activity decreases the risk of postmenopausal breast cancer and vigorous physical activity decreases the risk of premenopausal and postmenopausal breast cancer. WCRF estimates that low levels of physical activity account for 12% of breast cancers in the UK, and 17% in the USA. In Australia it has been estimated that 8% of postmenopausal breast cancers diagnosed in 2010 were attributable to insufficient physical activity. The relationship between physical activity and breast cancer risk is dose-responsive, with breast cancer risk decreasing with increasing levels of physical activity.
See the Overweight and obesity, physical inactivity and nutrition chapter of the National Cancer Prevention Policy for more information.
There is limited, non-conclusive evidence for a link between a range of dietary factors and breast cancer risk. For more information, including information relating to breast cancer recurrence, see Cancer Council Australia’s Nutrition and physical activity position statements.
Alcohol consumption is associated with an increased risk of breast cancer. WCRF has found convincing evidence that alcohol consumption increases the risk of breast cancer in postmenopausal women and probable evidence for premenopausal women.
The link between alcohol consumption and breast cancer risk is dose dependent. While even light levels of alcohol consumption are associated with breast cancer, the risk increases with the amount consumed.
WCRF estimates that alcohol consumption accounts for 22% of breast cancers in the UK and 11% in the USA. More conservative estimates suggest that 5–6.4% of breast cancers (in Europe and the UK) are attributable to alcohol use. The UK-based Million Women study found that alcohol consumption was associated with a 12% increased risk of breast cancer. The study estimated that, for every additional drink regularly consumed per day, breast cancer incidence increased by 11 cases per 1000 women up to the age of 75.
In Australia it has been estimated that 6.4% of breast cancers diagnosed among women aged 0-74 years in 2013 could be attributed to alcohol consumption.
See the Alcohol chapter of the National Cancer Prevention Policy for more information.
Use of combination hormone replacement therapy (HRT) is associated with increased breast cancer risk. Combination HRT (containing oestrogen and progesterone) is associated with a greater risk of breast cancer than oestrogen-only HRT. The associated risk increases with therapy duration. The increased risk appears to be limited to the period during which women receive HRT therapy with the risk decreasing within two years of therapy ceasing. There is little to no reported increase in risk for oestrogen-alone HRT.
There is some evidence that use of oral contraceptives is associated with a small increase in breast cancer risk, with longer term users at higher risk. See the Cancer Council Australia position statement on Combined oral contraceptives and cancer risk for more information.
In utero exposure to diethylstilbestrol (DES) is associated with increased breast cancer risk. DES is a synthetic form of oestrogen that was used to prevent spontaneous abortion and premature delivery between the early 1940s and early 1970s. Women exposed to DES in utero have a higher risk of breast cancer, particularly after the age of 40.
Evidence for a link between tobacco use and breast cancer is inconsistent. The International Agency for Research on Cancer states that there is limited evidence for a link between tobacco smoking and breast cancer.
Recent analysis suggests that women who smoke are 24% more likely to develop breast cancer than women who do not smoke, with the highest risk in women who begin smoking before menarche, or 11 years or more before their first full-term pregnancy. However, the increase in risk is only seen in current and former drinkers of alcohol. Tobacco and alcohol have been shown to have a synergistic effect in causing other cancer types.
See the Tobacco control chapter of the National Cancer Prevention Policy for more information.
Non-modifiable risk factors
Sex and age
Breast cancer is over 100 times more common in women than in men. Breast cancer incidence in Australia is 1.2 per 100,000 in males, compared with 140.9 per 100,000 in females. The risk of developing breast cancer by the age of 85 is one in eight for women, compared with one in 721 for men.
Breast cancer risk in women increases with increasing age. In Australia, the risk of being diagnosed with breast cancer for a woman in her 30s is approximately one in 250, compared with approximately one in 30 for a woman in her 70s. More than three quarters (79%) of breast cancers in Australia are diagnosed in women over the age of 50.
While eight out of nine women who develop breast cancer do not have an affected first-degree relative (mother, sister or daughter), family history of breast cancer is associated with an increased risk of the disease.
Breast cancer risk increases with an increasing number of first-degree relatives (parents, siblings or children) with breast cancer. Compared with women with no affected first-degree relatives, the increased likelihood of developing breast cancer is 1.8 for women with one affected relative, 2.9 for women with two affected and 3.9 for women with three affected relatives.
The absolute risk of breast cancer for Australian women with one affected first-degree relative is one in six and for those with two affected first-degree relatives is one in four, compared with one in 13 for those with no blood relatives affected with breast cancer.
The two genes most commonly involved in breast cancer susceptibility are BRCA1 and BRCA2. These genes carry rare inherited mutations associated with increased breast cancer risk. BRCA1 and BRCA2 mutations are associated with about 5–10% of all breast cancers.
BRCA1 and BRCA2 mutations are estimated to increase the risk of breast cancer by 47% and 32% by the age of 75 years, respectively. The associated increased breast cancer risk declines with age for women with BRCA1 mutations, but not for women with BRCA2 mutations.
Certain BRCA1 and BRCA2 mutations are associated with up to a thirty-fold increase in breast cancer risk. For those with a family history of breast cancer, the breast cancer risk conferred by BRCA mutations appears to be higher.
There are a number of other genetic conditions associated with increased breast cancer risk, including Li-Fraumeni syndrome, Cowden syndrome and Peutz-Jeghers syndrome .
Place of residence
Breast cancer incidence and mortality rates vary widely among different populations globally. Incidence rates are highest in affluent regions, such as North America, western and northern Europe, Australia and New Zealand. Regional patterns in mortality rates are generally similar to the incidence patterns.
Within Australia, incidence rates tend to decrease with increasing remoteness. This may in part be due to lower participation rates in population-based mammographic screening in remote regions, leading to lower detection rates.
Table 1 outlines breast cancer incidence rates in Australian states and territories. Incidence rates are similar across most Australian states. However, the Northern Territory has a significantly lower rate and ACT has a significantly higher rate of breast cancer. The lower rate in the Northern Territory may in part be due to the lower participation by Aboriginal and Torres Strait Islander women in population-based mammographic screening.
Table 1. State-based breast cancer incidence rates 2004–2008, new cases per 100,000 women
Personal history of breast cancer and other pre-invasive breast conditions is associated with increased breast cancer risk. Breast carcinoma in situ (when breast cells have the appearance of invasive cancer, but do not invade adjacent tissue) and benign breast disease (also known as fibrocystic disease) are both associated with increased risk of invasive breast cancer.
Mammographic breast density (a measurement of dense epithelial or connective tissue in the breast) is a highly heritable risk factor for breast cancer. Women with high breast density have a four- to six-fold increased risk of breast cancer, compared with women with the lowest level of breast density. Breast density is partly modifiable and is impacted by age, parity, weight and diet. Breast density is reduced with menopause and by tamoxifen.
High endogenous oestrogen levels are associated with increased breast cancer risk, particularly for postmenopausal women.
Early menarche and late menopause are associated with prolonged exposure to endogenous oestrogens. Early menarche is associated with a slight increase in risk of breast cancer, particularly for women who had their first menstrual period before the age of 12 years. Women with shorter menstrual cycles and later menopause are also at increased risk of breast cancer.
Reproductive history and breastfeeding also appear to influence breast cancer risk, albeit to a lesser extent than other risk factors. The risk of breast cancer is lower in women who have children compared with those who have no children or have their first child at 30 years or older. Breast cancer risk decreases by around 7% with each birth following the first. Breastfeeding appears to lower breast cancer risk. Breast cancer risk is decreased in women who have ever breastfed, but the protective effect is stronger in women who breastfeed for greater than 12 months compared with shorter durations.
For information on exogenous oestrogens and breast cancer risk, see the Modifiable risk factors section.
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