Many post-menopausal women, who have not had a hysterectomy, use combined HRT (containing progestagen and oestrogen) or tibolone (synthetic HRT), because oestrogen-only preparations are known to increase the risk of womb cancer. But little information exists on the incidence of womb cancer in users of these other therapies.
Valerie Beral (Cancer Research UK and University of Oxford) and colleagues recruited 717, 000 postmenopausal women from the UK, aged 50-64 years, who had no previous history of cancer and had not had a hysterectomy, into the Million Women Study, between 1996 and 2001. The women filled in questionnaires about their use of HRT and other personal details and were followed up for an average of 3.4 years. Just under half of the women reported that they had used some form of HRT.1320 womb cancers were diagnosed at follow-up.
The investigators found that, compared with women who had never used HRT, women who last used oestrogen-only HRT or tibolone had a higher risk of womb cancer overall and that the risk increased with longer use of tibolone. By contrast, use of combined HRT did not increase the overall incidence of womb cancer. The researchers also found that among HRT users the risk of womb varied according to bodyweight when compared with women who had never used HRT. In women who were not overweight, a form of HRT, called cyclic-combined HRT, also increased the incidence of womb cancer. In obese women (who normally have a substantially higher incidence of womb cancer than non-obese women), use of both continuous and cyclic combined HRT significantly reduced the incidence of womb cancer, whereas tibolone and oestrogen-only HRT had little additional effect on incidence.