|Written by NHS Choices|
|Tuesday, 17 January 2012 10:46|
Several newspapers have today reported that hormone replacement therapy (HRT) does not increase the risk of breast cancer. The reports said that the study that reported the link in 2002 was "fundamentally flawed".
The original research found that women who had taken HRT were twice as likely to develop breast cancer and more likely to die. The Daily Telegraph and other newspapers reported that these 2002 findings "triggered a collapse in confidence in hormone replacement therapy and the number of women taking it halved".
Today's news reports are based on a new analysis of the data from the Million Women Study and two other studies. The researchers wanted to determine whether the association between HRT and breast cancer actually showed that HRT caused an increased risk in breast cancer. They appraised each study against nine criteria that would need to be met before it was possible to say whether HRT caused breast cancer. For example, one criterion assessed was whether the women had taken HRT before they had breast cancer.
This new analysis did not suggest that the original studies were poorly performed but it did highlight limitations to what the studies can tell us.
The analysis found that all three studies failed to meet the majority of the criteria and therefore none of the studies was able to establish whether HRT increases the risk of breast cancer.
Where did the story come from?
The study was carried out by researchers from the University of Cape Town and UK universities including the University of Surrey and Imperial College London. It was published in the peer-reviewed Journal of Family Planning and Reproductive Health Care.
The newspapers reported that this analysis showed that the previous studies had not demonstrated causality.
What kind of research was this?
This was a reanalysis of three studies suggesting that HRT caused an increased risk of breast cancer, to see whether the association was actually causal. The three studies were:
For each of the three studies the researchers looked at whether they met several criteria needed to demonstrate causality, and presented their findings in a series of papers. Besides the causality criteria, the three study designs have the following inherent strengths and limitations:
What did the research involve?
The nine criteria the researchers looked at were:
What were the basic results?
The researchers suggested that the Collaborative Reanalysis study did not adequately satisfy the criteria of time order, bias, confounding, statistical stability and strength of association, dose-duration response, internal consistency, external consistency and biological plausibility.
The Women's Health Initiative looked at two types of HRT: oestrogen-only HRT and combined oestrogen and progesterone HRT. Analysis of the trial of oestrogen plus progesterone HRT found that the findings did not adequately satisfy the criteria or bias, confounding, statistical stability and strength of association, duration response, internal consistency, external consistency or biological plausibility to suggest a causal association. The oestrogen-only HRT trial found that there was no increased risk of breast cancer in women taking this type of HRT relative to the control group.
The researchers said that despite the large size of the Million Women Study (which had followed women receiving oestrogen-only HRT, oestrogen and progesterone HRT, or no HRT) the study failed to satisfy several of the causality criteria. These included time order, information bias, detection bias confounding, statistical stability and strength of association, duration-response, internal consistency, external consistency or biological plausibility.
How did the researchers interpret the results?
The researchers concluded that HRT may or may not increase the risk of breast cancer and none of the three studies is able to establish that it does. The researchers also said that the randomised controlled trial of oestrogen-only HRT had shown no increased risk of breast cancer.
Researchers have reanalysed the design and data from three studies that had suggested that HRT is associated with an increased risk that breast cancer would develop. They wanted to determine whether HRT caused breast cancer to develop (that the link was "causal"). The researchers looked at a pooled analysis study called the Collaborative Reanalysis, the Women's Health Initiative randomised controlled trials and the Million Women Study, which was a large prospective cohort study including 800,000 post-menopausal women.
The researchers found that these studies each failed to meet the majority of nine criteria which would be need to be met in order to say whether the studies could establish causality. These criteria included whether the women in the studies took HRT before their cancer developed. They looked at whether the studies had controlled for any confounding factors (where a factor may be associated with both the likelihood of taking HRT and the likelihood of developing breast cancer). The researchers also looked at the biological plausibility of any causal link.
This research is important because it makes us reconsider the evidence that suggested HRT might increase risk of breast cancer rather than there just being an association between the two. This new research shows that these studies can only tell us that there may or may not be a link between HRT and increased risk of breast cancer. Further, carefully designed studies are needed to determine whether HRT does indeed cause an increased risk of breast cancer.
Women who are taking HRT for menopausal symptoms or who used to take HRT should note the advice from Cancer Help UK, which states that any cancer risk associated with HRT reduces to a normal level five years after you stop taking HRT.