Women taking oestrogen only hormone replacement therapy (HRT) for 10 years or more face a 60% greater risk of developing ovarian cancer than if they had never taken it, a new study shows .
The study follows recent research implicating HRT in small increased risks of cardiovascular disease, breast and uterine cancers, blood clots, and stroke. HRT is used by an estimated 13.5 million women in the United States—nearly eight million take oestrogen only pills and around six million take the combined oestrogen and progestogen form.
Dr James Lacey and colleagues from the US National Cancer Institute's Division of Epidemiology tracked the health of 44241 postmenopausal women, who were initially enrolled in the prospective breast cancer detection demonstration project, a breast cancer screening programme conducted in 29 sites across the United States from 1973 to 1980.
The researchers collected monitoring data, including information on type and length of HRT, age at first use, and development of any subsequent cancer, from 1978 to 1998. The data were collected through telephone interviews and questionnaires and covered 31354 women.
Until 1986 there was no attempt to distinguish oestrogen alone from oestrogen plus progestogen treatments, but subsequently this information was collected, including the number of days in the month the treatments were used. After 1993 information on names of drugs and doses was also collected.
Women in whom menopause was surgically induced, such as women who had had a hysterectomy, were included in the study if they had at least one ovary. Women who had had a bilateral oophorectomy or had ovarian or breast cancers before follow up were excluded.
Overall 329 women developed ovarian cancer during the monitoring period. Their average age at the start of the study was 56.6 years.
Compared with no use of HRT, use of oestrogen only HRT was associated with an increased risk of ovarian cancer, and this risk increased over time (rate ratio 1.6 (95% confidence interval 1.2 to 2.0).
The rate ratio was 1.8 (1.1 to 3.0) for 10-19 years of use and 3.2 (1.7 to 5.7) for 20 or more years (P value for trend <0.001). Furthermore, the rate ratio increased by 7% (2% to 13%) for every year of use.
The rate ratio for use of oestrogen plus progestogen HRT was 1.1 (0.64 to 1.7). Switching to this form of HRT after previous oestrogen only use did not completely eliminate the previous risk, conferring a rate ratio of 1.5 (0.91 to 2.4).
The women's international study of long duration oestrogen after menopause (WISDOM) study should continue, says the Medical Research Council, as long as women volunteering for the trial are fully informed of all the known risks and benefits of HRT.