Breast cancer risk factors and mammographic breast density in women over age 70
Breast density is a strong risk factor for breast cancer, but little is known about factors associated with breast density in women over 70. 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. Median (range) percent density among women was 23.7% (0–85%). Body mass index (β= −0.345, p<0.001 adjusted for age and parity) and parity (β= −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 (β=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 (β= −0.35), parity (β=−0.29), surgical menopause (β=0.13) and current smoking (β= −0.12). 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.