According to the latest report the drop in the use of hormone replacement therapy (HRT) in mid-2002, following the results of the Women's Health Initiative study (WHI), is linked with a steep decline in new breast cancer diagnoses.
The WHI study showed an increased risk of breast cancer, heart disease, stroke, blood clots, and urinary incontinence among postmenopausal women who were using hormone replacement therapy that included both estrogen and progestin.
The latest research led by scientists at The University of Texas M. D. Anderson Cancer Center used data from the Surveillance, Epidemiology and End Results (SEER) program of the National Cancer Institute (NCI), part of the National Institutes of Health.
Senior investigator Dr. Donald Berry says prescriptions for the two HRT medications Premarin and Prempro dropped from 61 million in 2001 to 21 million in 2004.
The researchers found that the decrease in breast cancer incidence began in mid-2002 and leveled off after 2003.
The decrease occurred only in women age 50-69 and was more evident in women with cancers that were estrogen receptor (ER) positive and need estrogen in order to grow and multiply.
The researchers suggest that extremely small ER-positive breast cancers may have stopped progressing, or even regressed, after HRT was stopped.
Although other possibilities were considered to explain the sudden decrease in new breast cancer cases, the researchers say HRT was the only risk factor that changed substantially from 2002 to 2003 and provides a possible explanation for this trend.
The report is based on population statistics rather than a randomized clinical trial and therefore can not prove a link between HRT and breast cancer incidence.
It does however appear to further strengthen the association between breast cancer incidence and use of HRT.
The women in the WHI study are still being followed and researchers involved are expected to release a follow-up report later this year about the group who received HRT which should provide a much higher level of evidence about the influence of HRT and cessation of HRT on the incidence of breast cancer.
The researchers stress however that their study is not suggesting that all women stop their use of HRT as the risk of developing breast cancer from use of the hormones remains relatively small.
For some women with postmenopausal symptoms, they say the benefits of HRT are well worth that risk.
The report is published in the current issue of the New England Journal of Medicine.