A relative risk compares the risk of disease among people with a particular risk factor to the risk among people without that risk factor. If the relative risk is above 1.0, then risk is higher among those with the risk factor than those without. Relative risks below 1.0 indicate a protective effect, or lower risk, associated with a particular factor.
Relative risks are useful for comparisons, but they do not provide information about the absolute amount of additional risk experienced by the group with the risk factor in question. For example, current users of combination estrogen and progestin hormone replacement therapy (HRT) have a relative risk of 1.26, or a 26 percent increased risk. Although this increased risk may seem substantial, it proves to be less so in absolute terms because of the very low risk of breast cancer among young women in general.
Among 10,000 women who have been using HRT for 5.2 years, 38 breast cancers would be expected to be diagnosed. Among 10,000 similar women who never used HRT, 30 cases would be expected over the same period. Therefore, the 26 percent increased relative risk results in an absolute risk of only 8 additional breast cancer cases per 10,000 women over a period greater than 5.2 years.
Adapted from the American Cancer Society, Breast Cancer Facts and Figures 2003-2004.1
lifetime risk, or the risk to, say, age 70. Or the risk may be expressed as the probability that a woman of a given age will develop cancer in the next 10 years.
The statistic that one in eight women who survive to age 85 will develop some form of breast cancer in her lifetime is alarming, but this masks the important influence of age on risk (Table 4-1). Fewer than 5 percent of invasive breast cancers occur in women under age 40, whereas over three-quarters are in women over the age 50.
Numerous case-control and cohort studies over the past several decades have identified various factors, some of which have been shown to be consistently associated with risk, such as reproductive hormones, and others that are less consistent, such as dietary factors (Box 4-2, Table 4-2). Risk factors such as body mass index and dietary fat have been associated with specific types of breast cancer whose growth is stimulated by the sex hormones estrogen and progesterone.15 Family history increases risk although not as much as some women believe. Eighty-nine percent of women who develop breast cancer have no family history among their first-degree relatives (mother, daughter, or sister).16 The amount of increased risk depends on how close a relation the affected relative is, the age at which they developed breast cancer, and the number of relatives affected.