Reproductive factors and subtypes of breast cancer defined by hormone receptor and histology

Proc Amer Assoc Cancer Res, Volume 46, 2005 3206 Introduction: Certain reproductive factors are known to protect against breast cancer, but less is known about whether they provide differential protection against subtypes of breast cancer. If reproductive factors act through hormonal mechanisms, they should protect predominantly against cancers expressing estrogen and progesterone receptors. Methods: We examined the effect of selected reproductive factors on subgroups of tumors defined by hormone receptor status as well as histology using data from the National Institute of Child Health and Human Development Women’s Contraceptive and Reproductive Experiences (CARE) study, a multicenter case-control study of breast cancer among White and African-American women aged 35-64. The current analyses are based on data from 3,764 cases with known tumor hormone receptor status and 4,668 controls. The data were analyzed using unconditional logistic regression models with adjustment for potential confounders. Results: Multiparity and early age at first birth were associated with a lower relative risk for ER+PR+ tumors (p for trend=0.0001 and 0.01 respectively) but not for ER-PR- tumors (p for trend=0.27 and 0.85), whereas duration of breastfeeding was associated with lower relative risk for both receptor negative (p for trend=0.0002) and receptor positive tumors (p=0.004). Our results were consistent across subgroups of women defined by age (< 50 versus ≥50) and race. We found few significant differences by histologic subtype, although the strongest protective effect of multiparity was seen for mixed ductolobular tumors. Conclusion: Our results suggest that parity or multiparity and early age at first birth, but possibly not lactation, protect against breast cancer through hormonal mechanisms. These reproductive factors have similar protective effects on breast tumors of lobular and ductal origin.