Tamoxifen use may increase risk of oestrogen-negative
cancers
Women with breast cancer are less likely to develop
oestrogen receptor-positive tumours after receiving preventive tamoxifen
but could be at greater risk from oestrogen receptor (OR)-negative tumours,
researchers from the United States suggest.
Dr Christopher Li and colleagues from the Fred Hutchinson
Cancer Research Centre in Seattle say that women who have been diagnosed
as having breast cancer are more likely than the general population to
develop cancer in the opposite breast.
They conducted a study to find out whether treatment
with tamoxifen for primary breast cancer reduced the likelihood of subsequently
developing OR-positive and OR-negative tumours in the opposite breast.
Overall, use of tamoxifen was associated with a
small reduction in the risk of developing cancer in the opposite breast
(hazard ratio 0.9, 95 per cent confidence interval [CI] 0.7 to 1.2). In
addition, women who had received tamoxifen had a 20 per cent lower risk
of subsequently developing OR-positive cancer in the opposite breast compared
with women who had not received tamoxifen (hazard ratio 0.8, 0.5 to 1.1).
However, compared with non-users, women treated with tamoxifen had nearly
a five-fold increase in risk of developing OR-negative cancer in the opposite
breast (CI 1.4 to 17.4), which was statistically significant (P<0.0001).
“Our results may suggest that tamoxifen selectively
promotes the growth of OR-negative cancer cells in the contralateral breast,
since it inhibits OR-positive cancer cells that are not tamoxifen-resistant,”
the researchers say (Journal of the National Cancer Institute 2001:93;1008).
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