To determine the relationship between breast density, presenting features and molecular subtype of cancer, and surgical treatment received.

Retrospective review of a prospectively maintained database. Eligible patients had stage 1-3 cancer, were treated between 1/2005 and 6/2007, and had estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) measurements and films available for review. Density was classified at presentation as 1-4 using the Breast Imaging Reporting and Data System (BI-RADS) classification.

1,323 patients were included. Significant differences across the four density groups were present in age, race, multicentricity/focality, and presence of an extensive intraductal component (EIC). When density was combined into two groups, after adjustment for age, only an EIC and mammographically occult cancer were significantly more common in the dense groups. Extremely dense breasts (BI-RADS density 4) more commonly had luminal A tumors (p = 0.05), lobular cancers (p = 0.03), multicentricity (p = 0.02), and occult tumors (p < 0.0001). Greater density was associated with increased mastectomy use, with 61% of the extremely dense group having mastectomy versus 43% of those of lesser density (p = 0.01).

Cancers in extremely dense breasts occur in younger women, are more often mammographically occult, and appear to be phenotypically different from those arising in other density groups. The more common use of mastectomy may be related to these features, although density itself is not a selection criterion for mastectomy.