There is no clear evidence to confirm the link between the use of hormone replacement therapy and the fall in the number of new breast cancer cases, a new study suggests.
The stats and time trends neither back up nor refute the claim, so no firm conclusions can be drawn, the authors in the last of a series of five critiques of the published data in three major studies on HRT said.
A trio of studies - the Collaborative Reanalysis (CR); the Women's Health Initiative (WHI); and the Million Women Study (MWS) - prompted the use of HRT to tumble, starting in 2002, after they concluded that it causes breast cancer.
Two subsequent studies published in 2006 and 2007 suggested that the drop-off in HRT use was directly linked to a decline in breast cancer rates, a claim that has gained considerable currency.
One of these studies tracked new cases of breast cancer between 1975 and 2004, from nine US National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) registries.
This showed that between 2002 and 2003, the numbers of new cases fell by 6.7 percent.
Between 2001 and 2004, the incidence among 50 to 69 year old women fell by 11.8 percent and by 11.1 percent among those aged 70 and above.
The second study used pharmacy data from the Kaiser-Permanente-Northern California (KPNC) health plan to look at HRT use between 1994 and 2004 and rates of breast cancer among women between the ages of 50 and 74.
This showed that the rates of single and combined HRT fell by 58 percent and 38 percent respectively, between 2001 and 2003, while new breast cancer cases fell by around 11 percent in 2003.
The authors of the current analysis point out that in both studies the fall in breast cancer incidence started in 1999, three years before the sharp fall in HRT use began.
Between 1999 and 2002, the use of HRT fell by 1 percent every quarter, which would not have had any noticeable impact on breast cancer rates, they said.
Between 2002 and 2004 it fell by 18 percent, after which it levelled off.
In the SEER study, the declines in breast cancer were similar for both early stage and advanced cancers.
This suggests that advanced cancers would have rapidly shrunk very soon after HRT discontinuation, which is "unlikely," the authors said.
Similarly, the KPNC study claim that HRT discontinuation might have prompted the 11 percent fall in breast cancer in 2003 "is not credible," the authors said.
The study is published in the Journal of Family Planning and Reproductive Health Care.