Risk Factors for Breast Cancer
The direct causes of breast cancer are not known, however a number of well established risk factors have been identified.
Gender and age
All women are at risk of developing breast cancer, however the risk increases with age. Seventy-five per cent of breast cancers occur in women over 50 years. Men can also develop breast cancer however it is rare and accounts for only one per cent of all breast cancers. For further information, visit the National Breast Cancer Centre website.
Family history
Your family history becomes more important in increasing your risk of breast cancer with:
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the more blood relatives you have on the same side of the family who have had breast cancer
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the younger these relatives were when they were first diagnosed
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the more closely related these relatives are to you.
Risk factors for women at or slightly above average risk
Risk factors for women at or slightly above average risk of developing breast cancer (covers more than 95 per cent of the female population) are:
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no confirmed family history of breast cancer
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one first degree relative (first degree relative = parents, siblings, children) diagnosed with breast cancer at aged 50 or older
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one second degree relative (second degree relative = aunts, uncles, nieces, nephews, grandparents on both sides of the family) diagnosed with breast cancer at any age
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two first degree or second degree relatives diagnosed with breast cancer at age 50 or older, but on different sides of the family.
Risk factors for women with moderately increased risk
Risk factors for women with moderately increased risk of developing breast cancer (covers less than 4 per cent of the female population) are:
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one or two first degree relatives diagnosed with breast cancer before the age of 50
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two first degree or second degree relative on the same side of the family diagnosed with breast or ovarian cancer.
Risk factors for women with potentially high risk
Risk factors for women with potentially high risk of developing breast cancer (covers much less than one per cent of the female population) are:
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two first degree or second degree relatives on one side of the family diagnosed with breast cancer or ovarian cancer plus one or more of the following features on the same side of the family:
- additional relative(s) with breast or ovarian cancer
- breast cancer diagnosed before the age of 40
- ovarian cancer diagnosed before the age of 50
- bilateral breast cancer
- breast and ovarian cancer in the same woman
- Jewish ancestry
- Breast cancer in a male relative
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one first degree or second degree relative diagnosed with breast cancer at age 45 or younger plus another first or second degree relative on the same side of the family with sarcoma (bone/soft tissue) at age 45 or younger
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member of a family in which the presence of a high risk breast cancer gene mutation has been established.
Susceptibility to breast cancer occurs through both paternal and maternal lines of inheritance and the risk increases according to the number of relatives affected. The genes that are known to be related to breast cancer include the BRCA-1 and BRCA-2 'breast cancer' genes. Less than five per cent of all breast cancers are caused by an inherited gene fault. The risk of developing breast cancer only occurs when the gene is found to be abnormal. The detection of such an abnormality is very difficult, as there is no simple test for the presence of an abnormal BRCA gene.
If women are concerned about family history as a risk factor for breast cancer, they should discuss this with their doctor. For further information, visit the National Breast Cancer Centre website.
Reference: National Breast Cancer Centre (2000). Advice About Familial Aspects of Breast Cancer and Ovarian Cancer - A Guide for Health Professionals.
Reproductive and menstrual factors
On average, the older a woman is when she gives birth to her first child the greater her risk of developing breast cancer. The reasons for this is unclear but are believed to be due to either changes in breast tissue, after childbirth, making them less susceptible to cancer or changes in hormonal levels. Women who have never borne a child are also at increased risk (Kelsey and Gammon, 1991).
Women who have their first period before 15 years and late menopause (>50 years) increases the risk of developing breast cancer. The reasons for this are as yet undetermined, however the number of years of menstrual activity appears to be a significant factor (Kelsey and Gammon, 1991).
Hormonal factors
Oral Contraceptives
An analysis of worldwide epidemiological data on the relation between risk of breast cancer and use of oral contraceptives concluded that there was a small increase in risk while women were using combined oral contraceptives and in the 10 years after stopping (Collaborative Group on Hormonal Factors in Breast Cancer, 1996).
In summary, the Collaborative Group on Hormonal Factors in Breast Cancer found:
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a 24 per cent average increased risk (higher in younger women and lower in older women) for current users
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no excess risk once 10 years had passed without using oral contraceptives
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less clinically advanced breast cancer.
In absolute terms, because the incidence of breast cancer is low at younger ages when women use oral contraceptives, the excess number of breast cancer cases due to oral contraceptive use is small.
Hormone Replacement Therapy (HRT)
Recent research has shown that combined hormone replacement therapy increases both incidence and mortality from breast cancer. Breast cancers in combined hormone replacement therapy users tend to be more advanced than in non-users. The increased risk of breast cancer can occur after three years of continuous use and it may occur earlier. More information about hormone replacement therapy and breast cancer screening can be found in the Hormone Replacement Therapy - Advice for Women and Hormone Replacement Therapy - Information for General Practitioners information sheets, or by visiting the National Breast Cancer Centre website.
Previous history of other cancers
Women with a history of primary breast cancer have three to four times the risk of developing breast cancer in the future. Women with a history of cancer of the ovaries or cancer of the lining of the womb (endometrium) have between 1.3 to 1.4 times the relative risk of developing breast cancer compared to women who have never had this type of cancer. Women who have a previous history of primary cancer of the ovaries or womb (uterus) have a two-fold risk of developing breast cancer. The relationship between breast cancer and cancers of the colon, thyroid and salivary glands have also been reported but are not as strongly associated.
Diet
The World Cancer Research Fund and the American Cancer Association have reviewed much of the literature on breast cancer and diet and conclude that:
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rapid early growth and greater adult height increases the risk of developing breast cancer (mainly due to the fact that rapid growth leads to earlier periods, which is a risk factor in itself)
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diets high in fruits and vegetables have been shown to reduce the risk of breast cancer whereas diets high in red meat, fats and alcohol possibly increase the risk of developing cancer. There does appear to be a dose-response relationship between breast cancer and alcohol consumption, with women drinking three or more standard drinks a day being at greater risk for developing breast cancer. This evidence is not yet conclusive.
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isoflavones and lignans, which occur mainly in soybean and wholegrain products, contain phyto-oestrogens. These substances are able to bind to oestrogen receptors and therefore have been speculated to have anti-cancer properties. More research is this area is required.
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there is no evidence that regular consumption of coffee increases the risk of breast cancer.
The most effective dietary means of reducing the risk of developing breast cancer is to increase the consumption of fruits and vegetables, avoid alcohol, maintain body weight within the recommended levels and engage in regular physical activity. For further information, visit the National Breast Cancer Centre website.
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