Comparison with Other Risk Factors

The importance of breast density as a risk factor for breast cancer is particularly evident when comparing it with other breast cancer risk factors. For North American women, the strongest risk factor is age. Some other well-known risk factors for breast cancer are summarized in Table 2 (adapted from Kelsey and Gammon [11]). Many of the factors listed carry a risk prediction less than 2.0.

Family history of breast cancer, which is commonly considered to be one of the most important risk factors for breast cancer, only carries a substantial relative risk when multiple first-degree relatives (e.g., a mother and a sister) are involved. This form of family history, however, is present in a very small fraction (estimated 1%) of breast cancer cases. Considerable interest has surrounded the recent discovery of genes involved in inherited breast cancer, namely BRCA-1 and BRCA-2 [12,13]. These genes are seen to play a role particularly in early-onset disease. Women that are heterozygous for either of these genes are at up to a 10-fold increase in risk [14]. Although a genetic mutation is potentially a strong risk factor for disease, such mutations are believed to be associated with only 3-10% of breast cancers [15].

An additional feature of mammographic density, which sets it apart from other risk factors for breast cancer, is that it appears to be modifiable. Mammographic density is known to regress with age, attributed to the hormonal changes associated with menopause. There is evidence that hormone replacement therapy can preserve or increase mammographic density in peri- and postmenopausal women [16]. It has also been shown that a hormonal contraceptive that suppresses ovarian function can result in a significant reduction in mammographic density in just a 1-year interval [17]. It is not yet known whether the reduction in density also reduces breast cancer risk. Nevertheless, the observation that a strong risk factor for breast cancer can be altered gives hope for the development of potential preventive strategies.

21 Quantitative Image Analysis for Estimation of Breast Cancer Risk TABLE 2 Selected risk factors for breast cancer (adapted from Kelsey and Gammon [11])

Group

21 Quantitative Image Analysis for Estimation of Breast Cancer Risk TABLE 2 Selected risk factors for breast cancer (adapted from Kelsey and Gammon [11])

Group

Risk factor

High-risk

Low-risk

Relative risk

Age

Old

Young

>4.0

Country of birth

North America, North Europe

Asia, Africa

>4.0

Socioeconomic status

High

Low

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