Timing of hormone therapy influences breast cancer risk

Women starting hormone therapy at the time of menopause have a greater risk of breast cancer than those starting therapy later, an Oxford University study has found.

The results, published in the Journal of the National Cancer Institute, add to our picture of how the balance of risks and benefits of hormone therapy vary for individual women. But there is no need for any immediate changes in the way hormone therapy is currently prescribed, the researchers say.

Dr Jane Green, one of the co-authors of the study, said: ‘The main new finding is that the risk of breast cancer in women taking hormone therapy for the menopause depends on how soon after menopause therapy is started.

‘Women starting hormone therapy within five years of the menopause – including those starting before or at menopause – have a higher risk of breast cancer than women starting therapy five or more years after menopause.’

This new finding adds to the now well-established effects of hormone therapy on women’s risk of breast cancer, Dr Green explains.

These include a greater risk of cancer with combined oestrogen-progestagen therapy than with oestrogen alone; an increased risk among women currently on hormone therapy that falls off rapidly once they stop; greater risks the longer hormone therapy is used; and a greater relative risk in lean women than in those that are obese.

But few studies have looked at the timing of hormone therapy as a risk factor, although two previous studies suggested the interval between menopause and initiating hormone therapy may influence breast cancer risk.

Professor Dame Valerie Beral and colleagues at the Cancer Epidemiology Unit, Oxford University, set out to investigate this question by updating their 2003 analysis of data from the Million Women Study. The researchers estimated the adjusted relative risks of breast cancer in hormone therapy users and past users compared to non-users among the 1.13 million UK women in the study.

Dr Green says there is no need for any immediate changes in current practice, which is to prescribe hormone therapy to women for a short period (~2–5 years), usually for menopausal symptoms, ie around the time of the menopause or soon after.

‘Obviously if you are taking HRT for symptoms then it will have to be close to time of menopause and these findings do not change the overall risks on which current guidelines are based,’ she says.

‘However, it is clearer than ever that the balance of risks and benefits will vary for individual women and we still lack information on details of timing for other effects such as risk of blood clots and strokes.

‘The details of the pattern of risk with hormone therapy are becoming clearer, and we hope that ultimately this will help women and their health advisors to make better individual decisions.’

The study was funded by Cancer Research UK and the Medical Research Council.