Marked Decline in Breast Cancer Risk After Stopping Contraceptive pills

Following the release of the 2002 report of the Women's Health Initiative (WHI) trial of estrogen plus progestin (EPT), the use of menopausal hormone therapy in the United States decreased substantially. Subsequently, the incidence of breast cancer also dropped, suggesting a cause-and-effect relation between hormone treatment and breast cancer.

In a study published in N Engl J Med 2009, Leon Speroff, MD and Wendy Y. Chen, MD, MPH analyzed the results of the WHI randomized clinical trial—in which one study group received 0.625 mg of conjugated equine estrogens plus 2.5 mg of medroxyprogesterone acetate daily and another group received placebo—and examined temporal trends in breast-cancer diagnoses in the WHI observational-study cohort. Risk factors for breast cancer, frequency of mammography, and time-specific incidence of breast cancer were assessed in relation to combined hormone use.

RESULTS: In the clinical trial, there were fewer breast-cancer diagnoses in the group receiving estrogen plus progestin than in the placebo group in the initial 2 years of the study, but the number of diagnoses increased over the course of the 5.6-year intervention period. The elevated risk decreased rapidly after both groups stopped taking the study pills, despite a similar frequency of mammography. In the observational study, the incidence of breast cancer was initially about two times as high in the group receiving menopausal hormones as in the placebo group, but this difference in incidence decreased rapidly in about 2 years, coinciding with year-to-year reductions in combined hormone use. During this period, differences in the frequency of mammography between the two groups were unchanged.

The authors concluded that increased risk of breast cancer associated with the use of estrogen plus progestin declined markedly soon after discontinuation of combined hormone therapy and was unrelated to changes in frequency of mammography.

This report from the WHI is not the first about the change in breast cancer prevalence that occurred after the initial WHI publications in 2002, but it is of value because it documents that the change cannot be attributed solely to a decrease in the use of mammography.

The WHI recently updated results on breast cancer risk after discontinuation of the combination EPT arm. Overall results were published for the post-intervention phase in 2008. With mean follow-up of 2.4 years for this phase, a nonsignificant increased risk of breast cancer was still observed (HR, 1.27; 95% CI, 0.91-1.78), with a possible downward trend in cancer risk with follow-up.

This newest report is a more detailed analysis on breast cancer outcomes and also drew data from the observational cohort of the WHI. HRs for 6-month time intervals were calculated for both the intervention and nonintervention phase of the clinical trial. In an intention-to-treat analysis, there was no difference in the slopes of the HRs over time comparing the intervention and post-intervention phases (P = 0.28). However, after adjusting for adherence by censoring nonadherent subjects, there was a statistically significant difference in the slopes with HRs increasing over time for women using EPT, but declining after discontinuation (P = 0.005). Although breast cancer cases appeared to decline more in the intervention arm compared to placebo, mammography rates were similar in both arms. During the post-intervention phase, both breast biopsies and mammograms with abnormalities were more common in the EPT arm compared to placebo. Similar results for breast cancer incidence rates were seen using the observational data, although mammography rates were higher among women who used HT compared to those who did not.

After the WHI results were released in 2002, there was a large decline in HT use in the United States. Several investigators then reported a decline in US breast cancer rates correlating with reporting of the WHI trial results. Given the temporal association, it was speculated that this decrease in breast cancer incidence may have been due to the widespread discontinuation of HT, although other investigators noted that there was also a concomitant decline in mammography utilization, which may also have explained the changes. However, mammography rates were similar in the intervention arms of the WHI trial and were stable over time for women in the observational study, so it would seem unlikely that changes in mammography utilization alone can explain the recent decline in US breast cancer rates.