Background: Breast density is a strong risk factor for breast cancer, but little is known about factors associated with breast density in women over 70.
Methods: Percent breast density, sex hormone levels and breast cancer risk factor data were obtained on 239 women ages 70-92 recruited from 1986 to 1988 in the United States. Multivariable linear regression was used to develop a model to describe factors associated with percent density.
Results: Median (range) percent density among women was 23.7% (0-85%). Body mass index (beta=-0.345, p<0.001 adjusted for age and parity) and parity (beta=-0.277, p<0.001 adjusted for age and BMI) were significantly and inversely associated with percent breast density. After adjusting for parity and BMI, age was not associated with breast density (beta=0.05, p=0.45). Parous women had lower percent density than nulliparous women (23.7 versus 34.7%, p=0.005). Women who had undergone surgical menopause had greater breast density than those who had had a natural menopause (33.4 versus 24.8%, p=0.048), as did women who were not current smokers (26.0 versus 17.3% for smokers, p=0.02). Breast density was not associated with age at menarche, age at menopause, age at first birth, breastfeeding, estrogen levels or androgen levels. In a multivariable model, 24% of the variance in percent breast density was explained by BMI (beta=-0.35), parity (beta=-0.29), surgical menopause (beta=0.13) and current smoking (beta=-0.12).
Conclusion: Factors associated with breast density in older, post-menopausal women differ from traditional breast cancer risk factors and from factors associated with breast density in pre-menopausal and younger post-menopausal women.