Risk Factors for Triple-Negative Breast Cancer in Women

Little is known about the etiologic profile of triplenegative breast cancer (negative for estrogen receptor/ progesterone receptor/human epidermal growth factor), a breast cancer subtype associated with high mortality and inadequate therapeutic options. We undertook this study to assess the risk for triple-negative breast cancer among women 45 years of age and younger in relation to demographic/lifestyle factors, reproductive history, and oral contraceptive use. Study participants were ascertained in two previous population-based, case-control studies. Eligible cases included all primary invasive breast cancers among women ages 20 to 45 years in the Seattle–Puget Sound area, diagnosed between January 1983 and December 1992, for whom complete data was obtained for estrogen receptor, progesterone receptor, and human epidermal growth factor status (n = 897; including n = 187 triple-negative breast cancer cases). Controls were age matched and ascertained via random digit dialing. Oral contraceptive use z1 year was associated with a 2.5-fold increased risk for triple-negative breast cancer (95% confidence interval, 1.4-4.3) and no significantly increased risk for non-triple-negative breast cancer (Pheterogeneity = 0.008). Furthermore, the risk among oral contraceptive users conferred by longer oral contraceptive duration and by more recent use was significantly greater for triplenegative breast cancer than non-triple-negative breast cancer (Pheterogeneity = 0.02 and 0.01, respectively). Among women V40 years, the relative risk for triple-negative breast cancer associated with oral contraceptive use z1 year was 4.2 (95% confidence interval, 1.9-9.3), whereas there was no significantly increased risk with oral contraceptive use for non-triple-negative breast cancer among women V40 years, nor for triplenegative breast cancer or non-triple-negative breast cancer among women 41 to 45 years of age. In conclusion, significant heterogeneity exists for the association of oral contraceptive use and breast cancer risk between triple-negative breast cancer and non-triple-negative breast cancer among young women, lending support to a distinct etiology. (Cancer Epidemiol Biomarkers Prev 2009;18(4):1157–66)