Many women are concerned about risks of breast cancer, especially if they have a close relative who has had the disease. Here's some information, compiled from factsheets from the UK Cancer Research Campaign(date of publication: 1998)
There are large variations in the risk of breast cancer, both between countries where recorded incidence rates vary 30-fold, and also within countries over time. In addition, migration studies show an increase in risk when people migrate from low to high risk countries. Japanese migrants to the USA acquire incidence rates similar to the host population within two generations: the younger the migrant the sooner their risk approximates that of the host population. These observations suggest that genetic determinants of the disease are less important than environmental factors, and that prevention is possible. Although a number of risk factors have been linked to breast cancer, many relating to hormonal status, no major risk factor has been identified which can be used as a basis for primary prevention.
The strongest risk factor is age: the older the woman the higher the risk. Reproductive factors are linked to hormonal exposure: in general the greater the exposure to oestrogen, the higher the risk. Early age at menarche increases the risk and correlates with the international pattern of the disease. Low risk countries, such as China, have a later average age at menarche (17 years) compared to 12.8 in the USA. Late menopause after the age of 55 years approximately doubles the risk compared to menopause before the age of 45. An artificial menopause caused by bilateral oophorectomy before the age of 35 reduces the risk by two-thirds compared to women with a natural menopause (risk calculated before widespread administration of hormone replacement therapy (HRT)). Nulliparity and having a child after the age of 30 years approximately doubles the risk compared to a woman who has her first child before the age of 20. Age at menarche, age at menopause, parity and early pregnancy are well-established risk factors but it is not yet clear how these events affect a woman’s risk of developing breast cancer. Prenatal exposure to oestrogen is also under investigation.
A relatively small proportion of women, 5-10% of the population, are at increased risk due to an inherited susceptibility associated with a family history of breast and ovarian cancer, often occurring at young ages. The relative risk (RR) for a woman with one first degree relative (mother or sister) with breast cancer is raised slightly to between 1.5 and 2, and if two first degree relatives are affected the RR rises to between 4 and 6. The risk increases if the relative has bilateral disease and if the disease occurs at a young age: when both these conditions are fulfilled the cumulative probability of developing breast cancer may be as high as 40%.
Recently two breast cancer genes have been identified accounting for approximately 85% of families with four or more cases of breast cancer diagnosed under the age of 60 years. Both are large genes with many mutations. BRCA1 is a gene on the long arm of chromosome 17 and BRCA2 is on the long arm of chromosome 13. It is estimated that 2% of all breast cancers are due to BRCA1, 8% of breast cancers in women under 30 years and up to 20% of cases in Ashkenazi Jews who have breast cancer before the age of 40 years Other rare inherited conditions such as the Li-Fraumeni syndrome (associated with mutations in the p53 tumour suppressor gene on the short arm of chromosome 17), also increase the risk of breast cancer. In total around 5% of all breast cancer cases (around 1725 cases annually) may be caused by cancer predisposition genes. How best to identify, treat and counsel women at increased risk is currently being intensively investigated.
The heterogenous category known as benign breast disease carries only a slightly raised risk (RR 1.5) for malignant disease. However, for women within this category who have atypical hyperplasia, their risk is quadrupled (RR 4.0). Ninety percent of benign biopsy specimens do not show atypical hyperplasia. Cancer in the other breast also raises the risk four-fold, and both this and atypical hyper-plasia are associated with familial breast cancer.
The administration of artificial hormones in the form of oral contraceptives and HRT has also been studied. A recent meta-analysis of oral contraceptive studies concludes that there is a small increase in the risk of having breast cancer diagnosed during combined oral contraceptive use and in the following ten years, but the excess disappears ten years or more after cessation of use. The health benefits of HRT at present seem to outweigh the disadvantages, but some studies do show an increase in breast cancer risk after more than 10 years of use of unopposed oestrogen. Evaluation of the effects of oral contraceptives and HRT is continuing.
The effect of dietary factors on the aetiology of breast cancer is still unclear. Animal studies linking high fat diets with increased risk and international correlations between fat intake and breast cancer have not been confirmed by large prospective studies. There are many difficulties in measuring fat intake and it has been suggested that dietary fat intake during early life may be more important than during mid-life. Obesity in young women is associated with reduced risk.
Many of the risk factors detailed above will be difficult or impossible to change. However, there is evidence that certain forms of behaviour may protect against breast cancer, such as high levels of physical activity, breast feeding and diets high in fibre, fruit and vegetables.
For more information in the UK: leaflets are available from your GP, or local Well Woman clinic. Ask your GP for more specific personal risk assessment. Also see: tamoxifen.