Alcohol, smoking, and risk of Her2-overexpressing and triple-negative breast cancer relative to estrogen receptor-positive breast cancer.

Epidemiological evidence is limited on how alcohol consumption and smoking are associated with risk of different subtypes of breast cancer, such as triple-negative (TN) and human epidermal growth factor receptor 2-overexpressing (H2E) breast cancers, which may have different etiologies from more common luminal (estrogen receptor [ER+]) breast cancers. In this population-based case-case study, we evaluated the association between alcohol, smoking, and risk of H2E and TN breast cancer, compared with ER+ breast cancers, among women aged 20-69 years. Using polytomous regression, associations between alcohol consumption, smoking, and breast cancer risk were evaluated in 909 ER+, 1,290 TN, and 489 H2E breast cancer patients, with ER+ breast cancer patients as the reference group. Current alcohol consumption at diagnosis was associated with a lower risk of H2E breast cancer (odds ratio = 0.74, 95% confidence interval: 0.58-0.92) relative to ER+ cancers. No difference in association was observed by menopausal status. No association between alcohol consumption and TN breast cancer relative to ER+ breast cancer was observed. Women who smoked did not have an altered risk of TN or H2E breast cancer, relative to ER+ cancer. Our results suggest that alcohol is associated with lower risk of H2E breast cancer relative to ER+ breast cancer. This study adds to the body of epidemiologic evidence that breast cancer etiology differs by breast cancer subtype.