As a part of a retrospective study of computer-aided detection in breast cancer screening, we recorded the percentage of the breast density on a visual analogue scale on 4,866 routine screening mammograms taken in 1996 as part of the North East of Scotland Breast Screening Service. In these women, 284 breast cancers were diagnosed from the time of the original mammograms up until February 2010.
The main goal was to evaluate breast density as a breast cancer risk factor by detection mode, histology and time since the original mammogram. The association of density with risk of breast cancer was assessed using logistic regression giving odds ratios per 10% increase in density, and by comparison of continuous mean densities between particular groups of cancers and those who did not develop breast cancer.
After adjusting for age, breast density was significantly associated with cancer in the first 6 years after the original mammogram (OR per 10% density = 1.08, 95% CI = 1.01 to 1.15, P = 0.03), but not with cancer more than 6 years after the mammogram (OR = 1.01, 95% CI = 0.93 to 1.10, P = 0.8). There was a statistically significant association between histological type of tumour and breast density (P = 0.02). The mean densities for no cancer, invasive ductal carcinoma, DCIS and invasive lobular carcinoma were 32% (SD 23%), 33% (21%), 35% (26%) and 44% (24%), respectively. The effect of density on risk varied significantly by detection mode (P = 0.02), with highest densities being observed in interval cancers arising more than 1 year after the mammogram (55% compared with 32% in those with no cancer).
Breast density as measured by visual analogue assessment is strongly associated with late interval cancers and with lobular carcinoma. Its predictive value for risk, however, declines with time since its measurement.