Trends in Incidence Rates of Invasive Lobular and Ductal Breast Carcinoma

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Abstract
The Collaborative Group on Hormonal Factors in Breast Cancer has reported that current users of combined estrogen and progestin hormone replacement therapy (CHRT) or progestin alone for 5 years or longer have a 53% increase in risk of breast cancer.1 Similarly, the Women's Health Initiative (WHI), a randomized controlled trial, found that CHRT use is associated with a statistically nonsignificant 26% increase in risk of breast cancer after 5.2 years of follow-up.2 However, a growing number of studies report that the risk associated with use of CHRT differs by histological type. Specifically, 5 separate studies have shown that ever use and current use of CHRT are associated with 2.0-fold to 3.9-fold increased risks of invasive lobular carcinoma (ILC), the second most common histological type of breast cancer, but have little impact on risk of the most common histological type, invasive ductal carcinoma (IDC).3-7 These same 5 studies also found that use of unopposed estrogen hormone replacement therapy was not strongly associated with risk of either ILC or IDC. Two of these studies had the power to assess duration of use, and both found that risk of ILC increased as duration of CHRT use increased.5,7 Taken as a whole, these data suggest that ILC is more hormonally responsive than IDC.