Purpose: As the influence of estrogen alone on breast cancer detection is not established, we examined this issue in the Women's Health Initiative trial, which randomly assigned 10,739 postmenopausal women with prior hysterectomy to conjugated equine estrogen (CEE; 0.625 mg/d) or placebo.
Methods: Screening mammography and breast exams were performed at baseline and annually. Breast biopsies were based on clinical findings. Effects of CEE alone on breast cancer detection were determined by using receiver operating characteristic (ROC) analyses of mammogram performance.
Results: After a 7.1-year mean follow-up, fewer invasive breast cancers were diagnosed in the CEE than in the placebo group, but the difference was not statistically significant. Use of CEE alone increased mammograms with short-interval follow-up recommendations (cumulative, 39.2% v 29.6.3%; P < .001) but not abnormal mammograms (ie, those suggestive of or highly suggestive of malignancy; cumulative, 7.3% v 7.0%; P = .41). Breast biopsies were more frequent in the CEE group (cumulative, 12.5% v 10.7%; P = .004) and less commonly diagnosed as cancer (8.9% v 15.8%, respectively, with positive biopsies; P = .04). Mammographic breast cancer detection in the CEE group was significantly compromised only in the early years of use.
Conclusion: CEE alone use for 5 years results in approximately one in 11 and one in 50 women having otherwise avoidable mammograms with short-interval follow-up recommendations or breast biopsies, respectively. Although the breast biopsies on CEE were less commonly diagnosed as cancer, breast cancer detection was not substantially compromised. These findings differ from estrogen-plus-progestin use, for which significantly increased abnormal mammograms and a compromise in breast cancer detection are seen.