Estrogen Replacement Therapy and Risk of Breast Cancer: Results of Two Meta-analyses

To the Editor.—  I have read the interesting meta-analysis concerning breast cancer and estrogen replacement therapy conducted by Dupont and Page¹ who have found an overall relative risk of 1.08 (95% confidence interval [CI], 0.96 to 1.2) for breast cancer in women who took 0.625 mg/d or less of conjugated estrogens. The authors also observed that "literature does not permit a definitive conclusion concerning whether breast cancer risk increases with the duration of estrogen replacement therapy." These findings are at variance with the results of another, subsequently published, meta-analysis of the effects of estrogen replacement therapy on breast cancer risk by Steinberg et al.² Using a summary dose response slope, Steinberg et al have calculated the proportional increase in risk of breast cancer for each year of estrogen use across 16 studies. They have found a 30% increase in risk of breast cancer (relative risk, 1.3; 95% CI,