Nulliparous women are a growing population in the U.S. that is known to be at elevated risk for breast cancer. A term pregnancy is believed to exert a protective effect on the ductal structures in the breast through terminal differentiation. It is biologically plausible that limited ductal maturation in nulliparous women renders the breast more susceptible to the effects of hormonal factors.
We assessed the association between hormonal and other factors and breast cancer risk by parity among postmenopausal women. Data were combined from four prospective cohort studies [The Breast Cancer Detection Demonstration Project Follow-up Study, the NIH-AARP Diet and Health Study, The Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial, and The United States Radiologic Technologist Study]. Each dataset was analyzed using multivariable Cox Proportional Hazards models to estimate hazard ratios (HR) and 95% confidence intervals (CI) separately for nulliparous and parous women for known risk factors. Study-specific results were combined using a meta-analytic random effects model.
A total of 1,611 breast cancers were reported among 32,636 nulliparous women and 8,284 breast cancers among 207,763 parous women. The meta-analytic HRs for the one of the strongest risk factors, current use of menopausal hormone therapy for ≥5 years, were 1.46 (95% CI: 1.10-1.82) and 1.50 (95% CI: 1.42-1.59) for nulliparous and parous women, respectively. Patterns of risk were similar for nulliparous and parous women for other factors including ages at menarche and last menstrual period, body mass index, family history of breast cancer, and benign breast disease. Some study-specific variability was detected.
Similar associations between hormonal risk factors and breast cancer were observed for nulliparous and parous women. However, given their elevated baseline risk, these risk factors are likely to contribute to a greater absolute risk of breast cancer among nulliparous women.
Citation Information: Cancer Prev Res 2008;1(7 Suppl):A79.
Seventh AACR International Conference on Frontiers in Cancer Prevention Research-- Nov 16-19, 2008; Washington, DC