Breast cancer risk after estrogen replacement therapy: results from the Toronto Breast Cancer Study.

The authors examined noncontraceptive estrogen use in relation to breast cancer risk in women under age 70 in a case-control study conducted in Toronto, Canada. Cases were 607 women with incident primary breast cancer, identified at the time of hospitalization for treatment. They were compared to 1,214 controls matched to the cases on neighborhood and decade of age. Information was obtained through personal interviews conducted in the subjects' homes. Most estrogen users had taken conjugated estrogens, and only 7% had also taken progestogens. Compared with never use, the estimated relative risk for ever use of unopposed conjugated estrogens was 0.9 (95% confidence interval (Cl) 0.6-1.2) after allowance for multiple confounding factors. Relative risk estimates for most duration categories were close to 1.0; the estimate for the longest duration category, greater than or equal to 15 years of use, was elevated (1.5, 95% Cl 0.6-3.8), but there was not a significant trend with increasing duration. The estimate for current use or use that ended less than 12 months before interview and had lasted for at least 5 years was 0.9 (95% Cl 0.4-1.9). The results provide evidence against an increase in risk among women who used unopposed conjugated estrogens for less than 15 years and for recent users; for women with durations of at least 15 years, an increase could not be ruled out.