Thirty years ago, Professor Dame Valerie Beral starting recruiting for a new study investigating the links between hormone replacement therapy (HRT) and breast cancer risk.
At the time, more and more women in the UK and elsewhere were taking HRT to relieve menopausal symptoms amid claims it could improve their general wellbeing and life expectancy. But emerging evidence from small studies suggested that HRT might increase women’s risk of breast cancer.
In search of some definitive data, Beral and her team at the Oxford Cancer Epidemiology Unit (now part of Oxford Population Health) set out to recruit a million women in their 50s and early 60s who they could track for a number of years through regular questionnaires and NHS records. Between 1 May 1996 and 2001, some 1.3 million women – a quarter of all UK women between 50 and 64 – filled in the recruitment questionnaire and brought it to their breast screening appointment.
As well as looking at the associations between HRT use and breast cancer risk, the Million Women Study would provide the first opportunity to investigate how changes in behaviour and lifestyle had affected the long-term health of this generation of women.
As most diseases become more common after middle age, the researchers also understood the value of targeting this age group of women, recalls Professor Gill Reeves, the current Director of the Cancer Epidemiology Unit, who was working as the study’s statistician at the time.
Landmark findings on HRT
In 2003, Beral and her team published their first major findings from the study in The Lancet. The results showed that women taking combined oestrogen and progestogen HRT were at twice the risk of developing breast cancer compared to non-users, while those taking oestrogen only HRT had around a one-third higher risk.
The authors estimated that the use of HRT by women aged 50 to 64 in the UK over the previous decade had resulted in 20,000 additional breast cancers.
The findings made headlines in the UK and around the world and, along with similar results from smaller studies, dramatically changed attitudes towards HRT. Clinical guidelines also changed in response to the findings and women were advised to take HRT for shorter periods, or not at all if they were at high risk of breast cancer. As HRT use fell in many countries, the incidence of breast cancer in women over the age of 50 also decreased.
Further results from the study revealed that HRT slightly increased women’s risk of ovarian cancer and that oestrogen-only HRT increased the risk of endometrial (womb) cancer in post-menopausal women who had not undergone a hysterectomy.
In recent years, HRT use has once again been on the rise among UK women and there is increasing public discussion about the risks and benefits of the next generation of hormonal therapies.
A resource for years to come
The sheer size of the Million Women Study means it continues to generate data on many aspects of women’s health from risk factors for rare medical conditions to more common diseases of old age. None of this would be possible without the willingness of participants to share so many details of their lives, says Co-Principal Investigator Associate Professor Sarah Floud.
‘We would like to thank every woman who has taken the time to fill out our questionnaires and provide blood samples when asked,’ adds Reeves. ‘Their data are invaluable and will continue to be invaluable to researchers for many years to come.’
Read the full story on the Oxford Population Health website.