Risk of Breast Cancer Classified by Joint Estrogen Receptor and Progesterone Receptor Status among Women 20–44 Years of Age

To gain insight into whether breast cancer tumors jointly classified by estrogen receptor (ER) and progesterone receptor (PR) status represent diseases with differing etiologies, data from a population-based case-control study of US women 20–44 years of age were analyzed. Cases included 1,556 women diagnosed between 1990 and 1992. Age- and geographic-frequencymatched controls included 1,397 women identified by random digit dialing. Heterogeneity between ER+PR+ and ER–PR– tumors was most pronounced in relation to age, race, and recreational exercise at 12–13 years of age. Multivariate-adjusted odds ratios for ER+PR+ tumors were 0.64 (95% confidence interval (CI): 0.47, 0.89) for 30–34 versus 40–44 years of age, 0.89 (95% CI: 0.63, 1.25) for Black versus White race, and 0.84 (95% CI: 0.68, 1.03) for exercise at 12–13 years of age above versus at or below the median. Corresponding odds ratios for ER–PR– tumors were 1.24 (95% CI: 0.86, 1.77), 1.51 (95% CI: 1.07, 2.14), and 1.15 (95% CI: 0.90, 1.48). Risk of ER–PR– cancer in relation to menstrual and reproductive (parity and lactation) characteristics, alcohol consumption, and family history of breast cancer was similar to that observed for ER+PR+ tumors. These findings only modestly support the hypothesis that hormonally related risk factors have differing relations with ER+PR+ versus ER–PR– tumors among younger women.