Assessing risk for breast cancer

Abstract

Risk factors for breast cancer can be allocated to one of four major groups: family history/genetic, reproductive/hormonal, proliferative benign breast disease and mammographic density. These four factors have now been thoroughly studied, and accurate quantitative estimates for the risk are now available for many of them. The most useful summary comes from the Oxford collaboration, which has now produced a series of papers estimating the risk associated with individual factors [1-3]. In terms of family history, compared with a woman with no affected relatives, a single affected firstdegree relative roughly doubles the risk; two such individuals triple the risk, and three or more quadruple the risk. The age at which cancer occurs in a mother or sister also affects risk, with young age at onset leading to higher risk; risk is approximately threefold for onset under age 40 years, twofold for age 40 to 50 years and 1.5-fold for age 50 to 65 years, and there is little increase for older ages unless there are multiple affected cases. This is further complicated by the fact that the relative risk is greatest when the woman herself is young, especially if the family member had early onset cancer, and diminishes as the women ages and does not develop cancer. A relative with bilateral breast cancer can be treated as having two affected relatives for the purposes of these calculations. Reproductive factors are also well established risk factors, with age at first childbirth being the most well known. Nulliparous women have similar risk to that in women whose first child was born when they were aged 30 years, with a later first birth giving rise to a higher risk (especially within 5 years after delivery) and women giving birth when they were young at lower risk. The relative risk decreases by about 3% for every year younger (maternal age) that childbirth occurs, so that a woman whose first child was born when she was aged 20 years has about a 30% lower relative risk than a woman whose first child was born when she was aged 30 years. Because the absolute lifetime risk for breast cancer is about 10%, this translates into a 3% lower absolute risk. Subsequent births reduce relative risk by about 7% per birth, but these also have a similar but weaker link to age at first childbirth.