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Meeting abstract

The Collaborative Group on Hormonal Factors in Breast Cancer has brought together and reanalysed about 90% of the worldwide epidemiological evidence on the relation between risk of breast cancer and use of hormone replacement therapy (HRT). Data on 52,705 women with breast cancer and 108,411 women without breast cancer from 51 studies in 21 countries were collected, checked and analysed centrally. Among current users of HRT or those who ceased use 1–4 years previously, the relative risk of having breast cancer diagnosed increased by a factor of 1.023 (95% confidence interval [CI] 1.011–1.036; 2P = 0.002) for each year of use; the relative risk was 1.35 (95% CI 1.21–1.49; 2P = 0.00001) for women who had used HRT for 5 years or longer (average duration of use in this group 11 years). Five or more years after cessation of HRT use, there was no significant excess of breast cancer overall or in relation to duration of use. There was no marked variation in the results according to hormonal type or dose but little information was available about long durations of use of any specific preparation. Cancers diagnosed in women who had ever used HRT tended to be less advanced clinically than those diagnosed in never-users. In North America and Europe the cumulative incidence of breast cancer between the ages of 50 and 70 in never-users of HRT is about 45 per 1,000 women. The cumulative excess numbers of breast cancers diagnosed between these ages per 1,000 women who began use of HRT at age 50 and used it for 5, 10 and 15 years respectively, are estimated to be 2 (95% CI 1–3), 6 (95% CI 3–9) and 12 (95% CI 5–20). Whether HRT affects mortality from breast cancer is not known.

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