Reproductive factors and histologic subtype in relation to mortality after a breast cancer diagnosis

Evidence suggests that certain reproductive factors are more strongly associated with the incidence of lobular than of ductal breast cancer. The mechanisms influencing breast cancer incidence histology may also affect survival. Women with invasive breast cancer (N = 22,302) diagnosed during 1986-2005 were enrolled in a series of population-based studies in three US states. Participants completed telephone interviews regarding reproductive exposures and other breast cancer risk factors. Histologic subtype was obtained from state cancer registries. Vital status and cause of death were determined through December 2006 using the National Death Index. Women were followed for 9.8 years on average with 3,050 breast cancer deaths documented. Adjusted hazard rate ratios (HR) and 95% confidence intervals (95% CI) were calculated using Cox proportional hazards regression models for breast cancer-specific and all-cause mortality. Parity was inversely associated with breast cancer-specific mortality (P (Trend) = 0.002). Associations were similar though attenuated for all-cause mortality. In women diagnosed with ductal breast cancer, a 15% reduction in breast cancer-specific mortality was observed in women with five or more children when compared to those with no children (HR = 0.85, 95% CI: 0.73-1.00). A similar inverse though non-significant association was observed in women with lobular subtype (HR = 0.70, 95% CI: 0.43-1.14). The trend did not extend to mixed ductal-lobular breast cancer. Age at first birth had no consistent relationship with breast cancer-specific or all-cause mortality. We found increasing parity reduced mortality in ductal and lobular breast cancer. The number of full-term births, rather than age at first birth, has an effect on both breast cancer-specific and overall mortality.