Women with high mammographic breast density have at least four times the risk of breast cancer compared to those with less dense breasts (which have a higher percentage of fat). In fact, high breast density is one of the most powerful risk factors for breast cancer. Women with a family history of breast cancer tend to have dense breasts, suggesting that breast density is a heritable risk factor. Circulating sex hormone levels are also important risk factors for breast cancer that are partially heritable. Now a new study has reported that breast density and levels of sex hormones are traits that influence breast cancer susceptibility independently.

High breast density increases breast cancer risk but does not reduce survival

Breast density is not fixed it can be increased or reduced by diet, environmental exposures and other factors. For example, women can potentially reduce their breast density by lowering their consumption of red meat and saturated fat, as well as limiting their exposure to cadmium. Both chemotherapy and treatment with aromatase inhibitors reduce breast density. Women with a reduction in breast density over time have a lower risk of breast cancer than women whose breast density remains stable.

High breast density appears to acts as a general stimulator of tumor development, interacting with other risk factors to determine hormone receptor profile. Breast cancer survivors with dense breasts have higher rates of local recurrence (in the same or opposite breast) and new primary breast tumors (i.e., arising after treatment). Radiotherapy after breast conserving surgery reduces these risks. However, high breast density does not appear to be associated with increased risk of distant metastasis or increased risk of death from breast cancer after accounting for other patient and tumor characteristics. In other words, high breast density is a risk factor for breast cancer development but does not appear to influence the risk of death after breast cancer is established.

Latest research finds no association between breast density and sex hormones

The study referenced at the beginning of this news article was designed to investigate whether there is a shared genetic basis between breast density and sex hormone levels. To conduct the study, the authors used data concerning 1,286 women from 617 families to estimate the heritability of circulating estradiol, testosterone and sex hormone binding globulin (SHBG) levels, as well as three measures of breast density (dense area, non-dense area and percent mammographic density). Results were adjusted for body mass index (BMI).

The estimated degree of heritability was 63% for percent mammographic density, 40% for circulating estradiol, 25% for testosterone, and 58% for SHBG. No significant associations were found between estradiol, testosterone, or SHBG levels and any of the three breast density measures studied. In fact, no evidence of a shared genetic basis between any of the hormone levels studied and any measure of breast density was observed. The authors conclude that breast density and circulating sex hormones influence breast cancer susceptibility separately.

Please see our article on breast density for more information.