What're the risk factors for breast cancer?
Age - The incidence of breast cancer increases with age and doubles every 10 years until the menopause when the rate of increase slows. Approximately a quarter of breast cancers affect women under the age of 50, a half occur between the ages of 50 and 69 and the remaining quarter develop in women who are 70 years or older.
Geographical variation - There is quite a difference in incidence and death rate of breast cancer between different countries. The biggest difference is between Eastern and Western countries. Recent, age-adjusted figures show that the rate of breast cancer per 100,000 women is 24.3 in Japan and 26.5 in China compared to 68.8 in England and Wales and 72.7 in Scotland and 90.7 in North America in white females. However, studies of women from Japan who emigrate to the US show that their rates of breast cancer rise to become similar to US rates within just one or two generations, indicating that factors relating to everyday activities are more important than inherited factors in breast cancer.
Reproductive factors - Women who start menstruating early in life or who have a late menopause have an increased risk of breast cancer. Women who have natural menopause after the age of 55 are twice as likely to develop breast cancer as women who experience the menopause before the age of 45.
Age at first pregnancy - Having no children and being older at the time of the first birth both increase the lifetime incidence of breast cancer. The risk of breast cancer in women who have their first child after the age of 30 is about twice that of women having their first child before the age of 20. The highest risk group are those who have their first child after the age of 35 and these women have an even higher risk than women who have no children. These observations indicate a ‘menstrual cycle effect’. During the monthly cycle a woman’s fluctuating hormone levels cause several changes within breast tissue, which are repeated every month. These changes possibly encourage or amplify abnormalities in the cell repair processes within breast tissue, which can in some cases lead to breast cancer later in life. Women who have fewer menstrual cycles before their first pregnancy, either through being older when they start menstruating or younger when they first get pregnant, run less chance of such an abnormality occurring.
Inherited risk - Up to 10 per cent of breast cancer in Western countries is due to an inherited factor. This factor can be passed on from either parent and some family members pass on the abnormal gene without developing cancer themselves. It is not yet known how many breast cancer genes there are, but to date, two specific breast cancer genes have been identified (BRCA1 and BRCA2).
Previous breast disease - Women with certain benign changes in their breasts are at increased risk of breast cancer. These women present with a breast lump, have an operation and the breast tissue removed shows severe overgrowth of the cells lining the breast lobule. . The technical name for this type of breast condition is ‘severe atypical epithelial hyperplasia’. Although benign in itself, its occurrence in a particular woman multiplies her risk of developing breast cancer during her life by a factor of four.
Radiation - Doubling of the risk of breast cancer was observed among teenage girls exposed to radiation during the second world war. Another study of women who received radiation to the chest as a result of repeated X-rays for tuberculosis, found that there was a risk among women who were first X-rayed between the ages of 10 and 14 years. Fortunately, as TB itself has been prevented, this risk is less relevant today. Other studies have shown that women with Hodgkin's disease who received radiation therapy to the chest have an excess risk of breast cancer. As they are surviving to older age they are now developing not only unilateral but bilateral breast cancer. The increase in risk depends on the dose and the age at which they received radiation. Data has also suggested that there is increased risk of breast cancer in the other breast in patients having radiation to one breast.
Lifestyle - Although there is a close correlation between the incidence of breast cancer in a country and the dietary fat intake of that country, more detailed studies have shown that there does not appear to be a particularly strong or consistent relationship between fat intake in any individual and their risk of developing breast cancer.
Weight - Being overweight is associated with a doubling of the risk of breast cancer in postmenopausal women whereas amongst premenopausal women obesity is associated with reduced breast cancer incidence.
Alcohol intake - Some studies have shown a link between the amount of alcohol people drink and the incidence of breast cancer, but this relationship is not consistent and may be influenced by dietary factors other than alcohol.
Hormones - Women who take the oral contraceptive pill are at a slight increased risk while they take the Pill and they remain at risk for 10 years after coming of the Pill. The increased risk is, however, very small and cancers diagnosed in women taking the oral contraceptive Pill are less likely to have spread than those cancers diagnosed in women who have never used the oral contraceptive. The duration of use, age at first use, dose and type of hormone within the contraceptive appears to have no significant effect on breast cancer risk. Women who begin taking the Pill before the age of 20 appear to have a higher risk than women who begin taking oral contraceptives at an older age.
Hormone replacement therapy - Among current users of hormone replacement therapy (HRT) and those who have stopped using it one to four years previously, there is an increased risk of breast cancer. The risk increases 1.023 times for each year of HRT use. This increased risk is very similar to the effect of a delay in the menopause by one year. The risk of breast cancer in a woman who has not used HRT increases 1.028 times for each year she is older at the menopause. HRT using a combination of oestrogen and progestogen has been shown to be associated with a slightly higher risk of breast cancer than oestrogen-only HRT. Cancers diagnosed in women taking HRT tend to be less advanced clinically than those diagnosed in women who have not used HRT. Current evidence suggests that HRT does not increase breast cancer mortality.