They said that Britain's only clinical trial aimed at preventing breast cancer could now be in danger of collapse as doctors fear women at high risk will "inundate" clinics in order to be prescribed tamoxifen, or leave the trial altogether.
In a five-year study by the US National Cancer Institute, tamoxifen was found to almost halve cancer rates among women at high risk of having the disease. The institute notified the 13,000 women in the trial so that those who had been taking the placebo could consider starting tamoxifen therapy.
But the British experts said that the US results had not been released properly and had not as yet been peer-reviewed. They added that the decision to "unblind" the study - that is tell women whether they were on the placebo drug or tamoxifen - meant that results on the long-term effects of taking the drug were "confounded".
As a result, the future of the British trial, part of the International Breast Cancer Intervention Study (IBIS) is now in question, oncologists said yesterday. Professor Tony Howell, of the Christie Hospital, Manchester, co-chairman of IBIS, said: "Our emphasis is to try and get long-term data from [IBIS] so we will know whether it's good for women in the long term. Unfortunately the Americans will not be able to do that now, because they are prematurely stopping their trial."
Professor Michael Baum, professor of surgery at University College London, said: "I don't know what I am going to tell my patients, but I think they should have an informed choice."
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