Estrogen plus Progestin Effects on Breast Cancer and Mammograms

June 2003

Findings Summary

WHI study findings on the risk of breast cancer in women taking combined estrogen plus progestin (E+P) were published in the July 2002 issue of the Journal of the American Medical Association (JAMA). The E+P study continues to be analyzed. Updated results (June 25 issue of JAMA) include data on additional breast cancer cases, their characteristics, and mammogram results for women in the study.
The 2002 report showed that more women taking E+P developed breast cancer than those taking placebo (inactive) pills. This updated analysis shows that after an average of 5.6 years, 245 of the 8,506 E+P women and 185 of the 8,102 women on placebo developed breast cancer. Of the total cancers, 349 cases were invasive, a type of breast cancer with a greater chance of spreading to other parts of the body. The conclusions below are based on the invasive breast cancer group.
  • The increased risk of breast cancer due to E+P was eight additional cases of breast cancer for every 10,000 women over one year
  • Overall, there was a 24% increase in the risk for breast cancer due to E+P
The breast cancers in the E+P group had similar characteristics (looked the same under a microscope) to those in the placebo group. However, the tumors in the E+P group tended to be larger and more advanced (had spread to the lymph nodes or elsewhere in the body). A more advanced stage is usually associated with poorer outcome. At this time, no direct statements can be made about the prognosis of the breast cancer found in women taking E+P until more follow-up information is collected.
After even one year, quite a few more women had abnormal mammograms in the E+P group (9.4%) compared to the placebo group (5.4%); this pattern continued until the study ended. An abnormal mammogram is a breast X-ray that results in a recommendation for additional medical evaluation (most often, a shorter time between mammograms, but sometimes, a breast biopsy or other tests). Although we have known from other studies that E+P use increases the density of breast tissue on mammograms, the increase in abnormal mammograms with E+P use seen in this study is a new finding.
Further WHI studies are being done to learn what happens to breast cancer rates and mammograms after E+P use is stopped.