A new study has reported that use of hormone replacement therapy (HRT) and ever having given birth differently influence histologic subtype-specific breast cancer risk. Biological differences between ductal and lobular breast cancer suggest differences in factors contributing to their development. The study examined data from the Breast Cancer Surveillance Consortium, which included 3,331,744 mammograms performed on 1,211,238 women. 19,119 of the women were subsequently diagnosed with invasive breast cancer: 14,818 ductal, 1,602 lobular, and 1,601 mixed ductal-lobular.

The associations between family history of breast cancer and risk of breast cancer were found to be similar for each subtype. Similarly for breast density. However, while HRT use was associated with heightened risk of all subtypes, it was most strongly associated with lobular breast cancer. Also, compared to women who had never given birth, women who were mothers had a 20% lower risk of ductal or mixed ductal-lobular breast cancer, but not lobular tumors. On the other hand, late age when first giving birth was associated with increased risk of all subtypes. The authors conclude that similarities in risk factor associations with ductal, lobular, and mixed breast cancer subtypes are more pronounced than differences. In particular, the strongest risk factors for breast cancer overall (family history and breast density) are not histologic subtype-specific. However, HRT use and reproductive history differently influence subtype-specific breast cancer risk.