Flaws found in HRT study that caused breast cancer panic
Further doubt has been cast on a study suggesting hormone replacement therapy (HRT) increases the risk of breast cancer.
A link between HRT and the disease has proved controversial, with several studies suggesting the finding has been blown out of proportion.
However, other research has reinforced the link or shown a drop in breast cancer rates alongside declining HRT use.
Writing in the Journal of Family Planning and Reproductive Health Care, experts said the Million Women Study in the UK -- which has reiterated the link several times -- does not in fact establish HRT as a cause of breast cancer. However, they did not rule out that HRT may increase risk of the disease. First published in 2003, the study found that using combined HRT doubles a woman's risk of developing breast cancer compared with women not taking HRT.
An update last August found similar results after a longer follow-up, including a "rapid fall in risk after HRT is stopped".
But the experts, from South Africa, Germany and the UK, analysed the study and said: "HRT may or may not increase the risk of breast cancer, but the Million Women Study did not establish that it does."
The authors examined criteria applied to research to show a causal link, such as biases and biological implausibility, to review the findings of the study.
Their analysis highlighted several design flaws that they say would have skewed the findings. Inviting women to join the study would in itself have increased the number already aware of breast lumps or pre-cancerous changes, leading to higher numbers of cancers being detected (detection bias), they added.
This is borne out by a higher rate of cancers among the study participants than in the general population, irrespective of whether or not they used HRT. The authors also argued that crucial data was often also missing.
In the third report from the Million Women Study, follow-up data on HRT use was not available for 57-62pc of the study participants.
The researchers added: "Many commentators, and the investigators, have repeatedly stressed that it was the largest study of HRT and breast cancer ever conducted.
"Yet the validity of any study is dependent on the quality of its design, execution, analysis and interpretation. Size alone does not guarantee that the findings are reliable."