WOMEN taking certain forms of hormone replacement therapy are at greater risk of developing breast cancer, a huge study of Swedish women has confirmed.
After monitoring 29,508 women on HRT for 10 years, investigators have found that the greatest risks are posed by so-called combination therapies. But there is little extra risk if therapy is based on oestrogen alone, although this is only suitable for some women.
Combination therapies include progesterone to counteract the effects of oestrogen alone, which is linked to cancer of the uterus. Progesterone prevents the uterine cells from multiplying, reducing the chances of uterine cancer. The downside is that the hormone makes breast tissue denser and more prone to cancer.
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鈥淲e found that all therapies containing progesterones strongly increased the risk of breast cancer,鈥� says H氓kan Olsson of University Hospital in Lund, the leader of the research team. Overall, 7 out of 100 women taking the most risky form of HRT from the age of 50 for at least four years would develop breast cancer, compared with just 2 out of 100 among those who never had this type of HRT, the team reports in Cancer (vol 97, p 1442).
However, women should consult their doctor before stopping treatment. The results suggest combined therapies are safe over less than two years and the higher risk of cancer disappears five years after the last dose.
The results resemble those from the Women鈥檚 Health Initiative trial in the US, abandoned last year after higher rates of breast cancer, heart attacks and strokes were observed. Unlike the US trial, though, the Swedish compared different forms of HRT. The women at greatest risk were those on 鈥渃ontinuous鈥� combined therapy, taking oestrogen and progesterone every day. Women on this therapy for four years were 4.6 times as likely to be diagnosed with breast cancer as women on other therapies or not taking HRT.
Next came 鈥渟equential鈥� combined therapy, where the corresponding risk was 2.2. Women on sequential treatment take the same two hormones in a way that mimics the normal menstrual cycle, and so take less progesterone than those on continuous therapy.
Women on oestrogen only were just 0.35 times as likely to develop breast cancer. 鈥淲omen using this don鈥檛 have higher risk,鈥� says Olsson. But this kind of HRT is only given to women who have had hysterectomies and thus cannot get cancer of the uterus.
鈥淭he good news is that the findings allow us to construct safer therapies for women,鈥� Olsson says. One option is for women to have an intrauterine device fitted that releases progesterone only in the womb. This avoids exposing the breasts to the hormone. Or women can be given tibolone, a more 鈥渕ale鈥� version of progesterone that does not increase the density of breast tissue.