First large study to link estrogen alone to slight increase in breast cancer.
From: Dr. John Provatopoulos B.Sc. M.D.C.M. F.R.S.C. (johnprov@sympatico.ca)
Fri Aug 8 04:28:28 2003
Breast cancer and hormone-replacement therapy in the Million Women Study
Million Women Study Collaborators
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Correspondence to: Prof Valerie Beral, Cancer Research UK Epidemiology
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Unit, Gibson Building, Radcliffe Infirmary, Woodstock Road, Oxford OX2
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6HE, UK
Summary
Background Current use of hormone-replacement therapy (HRT) increases
the incidence of breast cancer. The Million Women Study was set up to
investigate the effects of specific types of HRT on incident and fatal
breast cancer.
Methods 1 084 110 UK women aged 50-64 years were recruited into the
Million Women Study between 1996 and 2001, provided information about
their use of HRT and other personal details, and were followed up for
cancer incidence and death.
Findings Half the women had used HRT; 9364 incident invasive breast
cancers and 637 breast cancer deaths were registered after an average of
2·6 and 4·1 years of follow-up, respectively. Current users of HRT at
recruitment were more likely than never users to develop breast cancer
(adjusted relative risk 1·66 [95% CI 1·58-1·75], p<0·0001) and die from
it (1·22 [1·00-1·48], p=0·05). Past users of HRT were, however, not at
an increased risk of incident or fatal disease (1·01 [0·94-1·09] and
1·05 [0·82-1·34], respectively). Incidence was significantly increased
for current users of preparations containing oestrogen only (1·30
[1·21-1·40], p<0·0001), oestrogen-progestagen (2·00 [1·88-2·12],
p<0·0001), and tibolone (1·45 [1·25-1·68], p<0·0001), but the magnitude
of the associated risk was substantially greater for
oestrogen-progestagen than for other types of HRT (p<0·0001). Results
varied little between specific oestrogens and progestagens or their
doses; or between continuous and sequential regimens. The relative
risks were significantly increased separately for oral, transdermal, and
implanted oestrogen-only formulations (1·32 [1·21-1·45]; 1·24
[1·11-1·39]; and 1·65 [1·26-2·16], respectively; all p<0·0001). In
current users of each type of HRT the risk of breast cancer increased
with increasing total duration of use. 10 years' use of HRT is
estimated to result in five (95% CI 3-7) additional breast cancers per
1000 users of oestrogen-only preparations and 19 (15-23) additional
cancers per 1000 users of oestrogen-progestagen combinations. Use of
HRT by women aged 50-64 years in the UK over the past decade has
resulted in an estimated 20 000 extra breast cancers, 15 000 associated
with oestrogen-progestagen; the extra deaths cannot yet be reliably
estimated.
Interpretation Current use of HRT is associated with an increased risk
of incident and fatal breast cancer; the effect is substantially greater
for oestrogen-progestagen combinations than for other types of HRT.
Lancet 2003; 362: 419-27
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Take care, John
