The detailed meta-analysis of evidence looked at 52 epidemiological studies, involving a total of 21,488 women with ovarian cancer mainly from North America, Europe and Australia. The results, published in The Lancet, indicate that women who use HRT for just a few years are about 40% more likely to develop ovarian cancer than women who have never taken HRT.
Co-author Professor Sir Richard Peto, who co-directs the Clinical Trial Service Unit at Oxford University said: ‘For women who take HRT for 5 years from around age 50, there will be about one extra ovarian cancer for every 1000 users and one extra ovarian cancer death for every 1700 users.’
Although HRT use fell rapidly about a decade ago, this decrease has now levelled off: around 6 million women are still taking HRT in the UK and United States alone. Existing World Health Organisation, US and European HRT guidelines do not mention ovarian cancer, and UK guidelines, which are currently being revised, state only that ovarian cancer might be increased with long-term use. Previous studies had been too small to assess reliably the risks from just a few years of HRT use.
Co-author Professor Dame Valerie Beral from Oxford’s Cancer Epidemiology Unit, said: ‘The definite risk of ovarian cancer even with less than 5 years of HRT is directly relevant to today’s patterns of use—with most women now taking HRT for only a few years—and has implications for current efforts to revise UK and worldwide guidelines.’
The international study by the international Collaborative Group on Epidemiological Studies of Ovarian Cancer was organised by Oxford University involved over 100 researchers from across the world. They analysed individual participant data from 52 studies, comprising virtually all of the epidemiological evidence ever collected on HRT use and ovarian cancer.
They found a significantly increased risk of developing ovarian cancer in current or recent users (women who had used HRT within the past 5 years), but although the risk of ovarian cancer fell over time after stopping treatment, women who had used HRT for at least 5 years still had a somewhat increased risk of ovarian cancer 10 years later.
The effect of HRT on the risk of developing ovarian cancer was the same for the two main types of HRT (preparations containing oestrogen only, or oestrogen together with a progestagen). The proportional increase in risk was not significantly affected by factors including the age at which HRT began, body size, past use of oral contraceptives, hysterectomy, alcohol use, tobacco use, or family history of breast or ovarian cancer.
There are four main types of ovarian cancer, and the increase in risk was associated only for the two most common types (serous and endometrioid ovarian cancers), and not for the two less common types (mucinous and clear cell ovarian cancers).
Source: The Lancet