It's not possible to say what exactly causes a woman's breast cancer. However, research has shown that some factors might increase a woman's risk or chance of developing breast cancer.
Having one or more risk factors does not mean you will definitely develop breast cancer, but it might increase your chance of developing breast cancer. Some women with one or more risk factors might never develop breast cancer. Even when a woman has a risk factor, there is no way to determine whether it actually caused her cancer. Most women with breast cancer have no obvious risk factors (aside from getting older).
The most important risk factors for developing breast cancer include:
- Getting older
- Having a strong family history of breast cancer
- Having previously been diagnosed with breast cancer or ductal carcinoma in situ (DCIS).
Other factors that seem to slightly increase a woman's risk of developing breast cancer include:
- Starting menstruation, or "periods", at a relatively early age (before 12 years)
- Starting menopause, or "change of life", at a relatively late age (after 55 years)
- Not having children, or having a first child after 30 years of age
- Not breastfeeding the more months spent breastfeeding, the lower the risk of developing breast cancer
- Taking combined Hormone Replacement Therapy (HRT) after menopause, especially when taken for five years or longer
- Putting on a lot of weight in adulthood, especially after menopause
- Drinking alcohol (more than two standard drinks a day)
- Having previously been diagnosed with lobular carcinoma in situ (LCIS) or atypical hyperplasia (AH)
Most breast cancer is not inherited. Breast cancer is a common disease: about one in eight women in Australia will develop breast cancer by age 85. As a result, many women have a relative with breast cancer simply by chance.
Some women have a strong family history of breast cancer for example, several close blood relatives on the same side of the family who have the disease. These women might have inherited a fault in a gene which might eventually lead to the development of breast cancer. However, this is uncommon and less than five percent of all breast cancers are caused by an inherited gene fault. And although a woman who inherits a gene fault does have an increased chance of developing breast cancer, she may never develop the disease.
Your family history becomes more important in increasing your risk of breast cancer:
- The more blood relatives you have on the same side of the family who have had breast cancer
- The younger these relatives were when they were first diagnosed
- The more closely related these relatives are to you
If you're concerned about your risk based on family history, see your GP.
There are several myths about the risk factors and causes of breast cancer that sound plausible but have little or no scientific theory or data to support them.
There is no body of evidence that an increased risk for breast cancer can be attributed to:
- The use of antiperspirants
- Wearing a bra
- A blow or injury to the breast
- Drinking milk
- Having silicon breast implants
- Having a mammogram
The above information has been reproduced with the kind permission of the National Breast Cancer Centre.