Cadmium is a widespread
heavy metal pollutant that may act as an exogenous estrogenic
hormone. Environmental
cadmium exposure has been associated with risk of
breast cancer in retrospective studies. We prospectively assessed the relationship between
cadmium exposure, evaluated by
creatinine-normalized urinary
cadmium concentration, and invasive
breast cancer among 12,701 postmenopausal women aged ≥50 years in a Women's Health Initiative study of bone mineral density. After a median of 13.2 years of follow-up (1993-2010), 508 cases of invasive
breast cancer and 1,050 comparison women were identified for a case-cohort analysis. Multivariable Cox regression was used to calculate hazard ratios and 95% confidence intervals. Risk of
breast cancer was not associated with urinary
cadmium parameterized either in quartiles (comparing highest quartile with lowest, hazard ratio = 0.80, 95% confidence interval: 0.56, 1.14; P for trend = 0.20) or as a log-transformed continuous variable (per 2-fold higher urinary
cadmium concentration, hazard ratio = 0.94, 95% confidence interval: 0.86, 1.03). We did not observe an association between urinary
cadmium and
breast cancer risk in any subgroup examined, including never smokers and women with body mass index (weight (kg)/height (m)(2)) less than 25. Results were consistent in both
estrogen receptor-positive and
estrogen receptor-negative
tumors. Our results do not support the hypothesis that environmental
cadmium exposure is associated with risk of postmenopausal
breast cancer.