Association between smoking habits and breast density, changes in breast density, and breast cancer in Asian women.

Abstract

To examine the associations among the smoking status, breast density, changes in breast density, and subsequent breast cancer (BC) risk. We conducted a nationwide population-based cohort study of 4.7 million cancer-free women aged ≥40 years who participated in the Korean National Breast Cancer Screening Program from 2009 to 2010. Of them, 4.2 million cancer-free women with the subsequent screening during 2011-2016 were included for changes in breast density. Breast density was assessed using BI-RADS categories, and changes were evaluated among women with repeated screenings from 2011 to 2016. Breast cancer incidence was ascertained using medical records from the NHIS-NHID until December 31, 2023. Former and current smokers had lower odds of dense breasts compared with never smokers (Odds ratio (OR)=0.94, 95% confidence interval (CI): 0.92-0.95; OR = 0.88, 95% CI: 0.87-0.89). Longer smoking durations, greater daily cigarette use, and greater pack-years were associated with progressively lower odds of having dense breasts. Smokers showed a reduced likelihood of increased density (OR = 0.95, 95% CI: 0.94-0.96) and higher likelihood of decreased density (OR = 1.07, 95% CI: 1.06-1.09), particularly among premenopausal women. Smoking was linked to a higher BC risk, especially among postmenopausal women; adjustments for density and density changes did not alter these associations. Smoking was associated with a lower baseline breast density and greater density decline yet increased BC risk in a dose-dependent manner. These findings suggest that the carcinogenic effects of smoking on BC are independent of the breast density or its changes measured using four broad categories.