Changes in mammographic breast density and concomitant changes in breast cancer risk

Among participants of the biennial Nijmegen breast cancer screening program me, we examined whether diminution of mammographic breast density lowered b reast cancer risk. Post-menopausal breast cancer cases (n = 108), who had t o have participated in all the five screening rounds prior to their diagnos is, were matched to 400 controls on year of birth and screening history. Co ntrols had to be free of breast cancer at the time of the case's diagnosis, Changes in breast density were measured over a 10-year period, by a fully computerized method. Women in whom 5-25% or >25% of the breast was composed of fibro-glandular density showed a threefold increased 10-year risk compa red to women with <5% density. In women with 5-25% density initially, we ob served a trend of decreasing risk with diminishing density: when women with <5% density throughout the whole period formed the reference category, the odds ratio (OR) for those who decreased from 5-25% to <5% density was 1.9 [95% confidence interval (CI) = 0.6-6.1] in contrast to the OR of 5.7 (95% CI = 2.2-15.2) for those with persisting 5-25% density. In women who increa sed from 5-25% density to >25% density the OR was 6.9 (95% CI = 2.1-22.9). In women with >25% density initially, diminishing density was not clearly a ssociated with lowering risk, which may be partly explained by the low numb er of women who decreased to <5% (n = 12), Due to the limited size of the s tudy these results have to be interpreted with caution. Although the result s are not conclusive, they could indicate a trend of decreasing risk with d iminishing breast density, Should this effect be real, it may have great im plications for the primary prevention of breast cancer or for the identific ation of high-risk groups who would benefit by more frequent screening, The refore, large-scale, longterm follow-up studies on the effects of changes i n breast density are needed. (C) 1999 Lippincott Williams & Wilkins.