Background: Mammographic density (MD) is a strong risk factor for breast cancer. We examined how breast cancer risk factors are associated with MD area (cm2) change across age.
Methods: We conducted a cohort study of 31 782 Swedish women ages 40-70 years at time of baseline mammogram. Lifestyle and reproductive risk factors were assessed by a web-based questionnaire. MD was measured as dense area using the STRATUS method (mean over the left and right breast). Linear regression analyses with adjustments for age, body mass index (BMI), and menopausal status at baseline were performed to assess the association between breast cancer risk factors and mean baseline MD. To investigate mean MD change across age, linear regression analyses with adjustments for age, BMI, menopausal status, and age at last mammogram were performed. All tests of statistical significance were two-sided.
Results: Except for oral contraceptive use, established lifestyle and reproductive risk factors for breast cancer were associated with baseline mean MD. The overall average annual MD change was -1.0 cm2. BMI and physical activity were statistically significantly associated with MD change. Lean women (BMI <20 kg/m2) had a mean MD change of -1.13 cm2 per year (95% confidence interval = -1.25 to -1.02) compared with -0.46 cm2 per year (95% confidence interval = -0.57 to -0.35) for women with BMI 30 or higher. The annual MD change was -0.4 cm2 larger in women who were very physically active compared with less physically active women.
Conclusions: Our results indicate that all risk factors for breast cancer, except oral contraceptive use, are associated with baseline MD but that only age, BMI, and physical activity are determinants of MD change.