Prevention

Links between lifestyle and breast cancer

It has been estimated that more than 22% 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 use.⁽¹⁾

Overweight and obesity, physical activity, and alcohol

An Australian analysis estimated that 6.6% of breast cancers diagnosed in 2013 among women aged up to 74 years could be attributed to overweight and obesity.⁽¹⁾ In 2017-18, 67% of women aged 55-64, and 73% of women aged 65-74 years were living with overweight or obesity.⁽²⁾ The World Cancer Research Fund’s (WCRF) Continuous Update Project report concluded that there is strong evidence that adult body fatness increases the risk of post-menopausal breast cancer.⁽³⁾ However, the WCRF has found probable evidence that body fatness in younger women aged 18-30 decreases the risk of pre-menopausal breast cancer.⁽³⁾

Physical activity and breast cancer risk has a dose-response relationship.^{(4, 5)} Higher levels of physical activity are also inversely associated with both pre- and post-menopausal breast cancer, but this evidence is less consistent in pre-menopausal women.⁽⁶⁾ The population attributable fraction (PAF) for breast cancers attributable to insufficient physical activity was 11.1% in 2015.⁽⁷⁾

The WCRF has found strong evidence that alcohol use increases the risk of breast cancer in both pre- and post-menopausal women.⁽³⁾ Alcohol use and breast cancer risk has a dose-response relationship, particularly for long term use.⁽⁸⁾ Multiple studies have demonstrated that alcohol use is associated with a 7-10% increase in breast cancer risk for each standard alcoholic drink per day.^(9-12) In 2017-18, 10% of women aged 45-64, and 9% of women aged 65-74 years were in exceedence of the lifetime alcohol risk guidelines.⁽¹³⁾ In Australia, it has been estimated that 6.4% of breast cancers diagnosed among women aged up to 74 years could be attributed to alcohol use.⁽¹⁾

Further information on the risk factors of overweight, obesity, physical activity and alcohol, and their impact on the cancer burden in Australia can be found in the relevant policy documents on Cancer Council’s website.

Exogenous oestrogens

Hormone replacement therapy (HRT) is associated with increased breast cancer risk, in particular combination HRT (containing oestrogen and progestogen).^(14-16) In 2019, the Collaborative Group on Hormonal Factors in Breast Cancer found one year use of HRT increased the risk of breast canccer among post-menopausal women, with the risk increasing the longer HRT was used.⁽¹⁷⁾ It is estimated that the use of combined HRT for five years would cause an additional 15 to 20 breast cancer cases per 1,000 women.⁽¹⁷⁾ Whilst the risk decreases within two years of HRT therapy ceasing⁽¹⁴⁾, its magnitude is dependent on the duration of previous use, with low excess risk after less than one year of HRT use.⁽¹⁷⁾ There has been little to no reported increase in risk for oestrogen-only HRT.^{(16, 17)}

There is some evidence that use of some oral contraceptives (combined oral contraceptives) is associated with a small increase in breast cancer risk, with longer term users at higher risk.^(18-21) For further information on the link between oral contraceptives and cancer, please visit Cancer Council’s oral contraceptives position statement.

Tobacco

The International Agency for Research on Cancer (IARC) states that there is accumulating evidence that tobacco use may be associated with breast cancer in women.⁽²²⁾ Analysis of 102,927 women from the prospective Breast Cancer Now Generations cohort study found that women who had ever smoked were 14% more likely to go on to develop breast cancer than those who had not, with the highest risk among women who began smoking at adolescent or peri-menarcheal ages.⁽²³⁾ The risk of breast cancer associated with smoking was found to be greatest for women with a family history of the disease.⁽²³⁾

See Cancer Council’s tobacco policy for more information on the link between tobacco and cancer, and the disease burden from tobacco in Australia.

Non-modifiable risk factors

Some breast cancer risk factors cannot be modified through lifestyle or behavioural change. These include sex and age, family history of breast cancer, genetics, personal history of breast disease, and hormonal factors. Having an understanding of these risk factors is to key to ensuring there are appropriate targeted efforts for early detection in place, such as through risk-based breast cancer screening.

Sex and age

Breast cancer is much more common in women, compared to men (125.6 and 1.1 cases per 100,000, respectively).⁽²⁴⁾ Breast cancer risk increases with age. In Australia, the risk of being diagnosed with breast cancer for a woman before age 75 is approximately one in 10, versus one in 47 before the age of 50.⁽²⁴⁾ More than 80% of breast cancers in Australia are diagnosed in women over the age of 50.⁽²⁵⁾ It is important to note that the association between breast cancer and age is not due to overdiagnosis through screening. The median age of a breast cancer diagnosis in 1982 was 60.2 years, and in 1992, after the introduction of BreastScreen Australia, the median age was 60.4 years.⁽²⁴⁾

Family history & genetics

Family history is an important risk factor for breast cancer. Evidence shows that there is a two-fold increase in risk of developing the disease for women with breast cancer in their first-degree family (parents, siblings, or children), and a larger increase in risk among women with a first-degree relative diagnosed ≤50 years.^{(26, 27)} A smaller increase in breast cancer risk is associated with affected second-degree relatives (aunts, nieces, cousins, or grandparents).⁽²⁸⁾ However, 8 out of 9 women who develop breast cancer do not have a first-degree relative with breast cancer.⁽²⁹⁾

The most common cause of a hereditary breast cancer is from a mutation in either the BRCA1 or BRCA2 genes. ⁽³⁰⁾ BRCA1 and BRCA2 mutations are estimated to increase the cumulative risk of breast cancer by 72% and 60% respectively, to the age of 80 years.⁽³¹⁾ Breast cancer incidences increased rapidly in early adulthood until ages 30-40 years for BRCA1 and until ages 40-50 for BRCA2 carriers, then remained at a similar, constant incidence until 80 years.⁽³¹⁾

Breast conditions

Personal history of breast cancer and other pre-invasive breast conditions is associated with increased breast cancer risk.^{(30, 32)} 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.⁽³²⁾

Breast density

Mammographic breast density (MD) reflects the proportion of the breast composed of fibro-glandular tissue and is a strong risk factor for breast cancer.^{(33, 34)} MD cannot be felt, but it can be seen on radiology scans. Radiologists can classify MD along a scale from mostly fatty (very low density) to mostly dense (extremely dense), using guidelines such as the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS). Breast density can also be measured using imaging processing.⁽³⁵⁾

Dense breasts are also associated with reduced sensitivity for detection of cancer, due to potential masking (hiding or camouflaging) of tumours.⁽³³⁾

Extremely dense breasts on mammography are associated with a ≥2-fold increase in breast cancer risk in women aged 40-49, compared with breasts that were almost entirely fat.⁽¹⁸⁾ A meta-analysis of observational studies demonstrated that the risk of breast cancer increased nearly 2-fold for each 25% increase in percent density in post-menopausal Asian women.⁽³⁶⁾ Breast density is partially modifiable and can be impacted by age, parity, weight, alcohol, diet, and hormone replacement therapy (HRT) use. On average, breast density is reduced with menopause, and by medications such as tamoxifen.^{(37, 38)}

Hormonal factors

High endogenous oestrogen levels are associated with increased breast cancer risk for post-menopausal women,⁽³⁹⁾ with an estimated two-fold higher risk among women in the top 20-25% compared to bottom 20-25% of oestrogen levels.⁽⁴⁰⁾ However, no association has been confirmed in pre-menopausal women.

Early menarche (onset of menstrual periods) 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.⁽⁴¹⁾ Women with shorter menstrual cycles and later menopause are also at increased risk of breast cancer.⁽⁴¹⁾

Although to a lesser extent than other risk factors, reproductive history and breast-feeding also appear to influence breast cancer risk. The risk of breast cancer is lower in women who have children compared to those who have no children or have their first child at 30 years or older. ^{(18, 42, 43)} Breast cancer risk decreases by around 7% with each birth following the first.⁽⁴⁴⁾

The World Cancer Research Fund (WCRF) has also found strong evidence to suggest that breastfeeding reduces the risk of breast cancer,⁽³⁾ especially for women who breastfeed for more than 12-months, compared to shorter durations.^(45-48) There is evidence to suggest that ever having breastfed is significantly associated with a reduced risk of developing luminal and triple-negative subtypes of breast cancer by 23% and 21%, respectively.⁽⁴⁸⁾