A longitudinal study of the effects of menopause on mammographic features.

Menopause has an important influence on risk of breast cancer. We have examined longitudinally the effect of menopause on mammographic densities, a strong risk factor for the disease, in women in the Canadian National Breast Screening Study. Baseline mammograms from women in the National Breast Screening Study, who were premenopausal at entry and had undergone menopause after entry, were compared with the mammogram that most closely followed menopause, using a computer-assisted method of measurement. The changes seen in the mammograms of these subjects were compared with those in an age-matched group of women who were also premenopausal at entry, had been followed for the same length of time, and had not experienced menopause. The results of this longitudinal study show that menopause has effects on characteristics of the mammogram that, over the same period of time, are greater than the effects of age. These effects are a reduction in the area of radiologically dense tissue, an increase in the area of nondense tissue, and a decrease in the percentage of density. However, these changes do not fully account for the effects of age on mammographic densities seen in cross-sectional data. Menopause is associated with distinct changes in the mammogram that account for some, but not all, of the observed association of increasing age with decreasing percentage of mammographic density. The observed changes in the morphology of the breast may explain some of the effect that menopause has on breast cancer risk.