Breast density, breast cancer risk, and hormone therapy
10 July, 2017:
The radiographic appearance of the breast on mammography varies among women. Fat appears dark on a mammogram, whereas connective and epithelial tissues are radiologically dense and appear white. This is the basis for determining the mammographic density, and the relative percentage of dense versus lucent breast areas (percent mammographic density, PMD). It has been well established that parameters of breast density correlate with breast cancer risk, with a four- to six-fold-fold gradient in risk between women with 75% or more PMD compared with those with 10% or less [1]. A systematic meta-analysis of data for more than 14,000 cases and 226,000 non-cases from 42 studies found that greater PMD was consistently associated with an increased risk of breast cancer [2]. No differences were observed by age/menopausal status at mammography or by ethnicity. For PMD using pre-diagnostic mammograms, combined relative risks of incident breast cancer in the general population were 1.79 (95% CI 1.48–2.16), 2.11 (95% CI 1.70–2.63), 2.92 (95% CI 2.49–3.42), and 4.64 (95% CI 3.64–5.91) for categories 5–24%, 25–49%, 50–74%, and greater than or equal to 75% relative to less than 5%. While this generalized notion is actually 'old news', there might be, in fact, some deviations from the core principle, which relate to the exact parameter evaluated, the characteristics of the cohort and the technique of density measurement. Some studies picked breast density as the main tested parameter, whereas others used PMD, or changes over time in density or in PMD. As an example, in a study of 6710 women aged 40–49 at intermediate familial risk of breast cancer (average lifetime risk of 23%), the absolute density, but not percent density, was a significant risk factor for breast cancer after adjusting for area of non-dense tissue (OR per 10 cm2 = 1.07, 95% CI 1.00–1.15, p = 0.04) [3]. The effect was stronger in premenopausal women, who made up the majority of the study population. Absolute density remained a significant predictor of breast cancer risk after adjusting for age at menarche, age at first live birth, parity, past or present hormone replacement therapy, and the Tyrer-Cuzick 10-year relative risk estimate of breast cancer. Eng and colleagues drew attention to the potential impact of the mammography technique on the measured density [4]. They pointed out that there is currently no validated estimation method for full-field digital mammography, while differences in the obtained results may occur if various techniques are compared.
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