Background: Mammographic breast density may be greater in the luteal phase (days 15-30) than the follicular phase (days 1-14) of the menstrual cycle; this may have implications for when mammography screening should occur. Objective: Examine whether percent breast density, breast area, or dense area differ by menstrual phase. Methods: We identified 204 premenopausal women with regular periods who were <55 years (mean = 45.0 years) and had two screening mammograms within 9 to 18 months, with one screening between days 9 and 14, and one screening between days 22 and 35 of the menstrual cycle. We measured percent breast density, breast area, and dense area using the Cumulus software. We used linear regression to test for differences in breast density, breast area, and dense area from follicular to luteal phase, adjusting for change in weight and time between exams. Results: The mean (SD) percent breast density was 35.8% (21.3) in the follicular phase and 36.7% (21.3) in the luteal phase. Multivariable analyses showed small but not statistically significant increases in percent density [1.1%; 95% confidence interval (95% CI), -0.2% to 2.3%] and breast area (16.7 cm2; 95% CI, -2.8 to 36.2) and a statistically significant increase in dense area (13.1 cm2; 95% CI, 0.1-26.1) in the luteal compared with the follicular phase. Conclusions: Breast density, breast area, and dense area have small, but probably not clinically meaningful, increases in the luteal phase of the menstrual cycle. However, there are other factors that may differ by menstrual cycle phase that we were unable to assess (e.g., breast compression), which may ultimately influence mammographic sensitivity by menstrual cycle phase.
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