To find a treatment that halves the likelihood of disease relapse in the other breast compared to tamoxifen therapy is truly remarkable

Dr Jeffrey Tobias, University College Hospital

Tamoxifen has also recently been shown to reduce the incidence of breast cancer in healthy women who are at high risk of the disease by a third.

Postmenopausal women with early breast cancer were given anastrozole for two-and-a-half years and had a 58% lower risk of developing a new tumour in the other breast than those taking tamoxifen, research shows.

The risk was reduced even further when administered to women with tumours that were fuelled by the female hormone oestrogen.

It was cut by 64% compared to those taking tamoxifen.

About 75% of postmenopausal breast cancer cases are oestrogen sensitive.

A five-year course of tamoxifen cuts the risk of a new cancer in the other breast by 50% - but anastrozole reduces it even further.

Dr Jeffrey Tobias of University College Hospital London, who helped design the study, said: "This is the first time a large-scale clinical trial has proved that another treatment is superior to tamoxifen in postmenopausal women with early breast cancer.

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"It therefore marks an important breakthrough in the management of this devastating disease.

"To find a treatment that halves the likelihood of disease relapse in the other breast compared to tamoxifen therapy is truly remarkable and will be very welcome news for the hundreds of thousands of postmenopausal women diagnosed with early breast cancer every year."

The findings are the latest results of the ATAC (Arimidex, Tamoxifen Alone or in Combination) study, involving more than 9,000 patients in 21 countries.

The women were given either anastrozole, tamoxifen or a combination of both.

The combination therapy was no better than tamoxifen alone.

Anastrozole works by inhibiting production of the female hormone oestrogen in women who have passed the menopause.

Most cases of breast cancer are in postmenopausal women.

Dr Tobias said women given anastrozole had fewer side effects such as blood clots, hot flushes and cancer of the uterus than the tamoxifen group.

At the moment anastrozole is only licensed for use in advanced breast cancer in postmenopausal women.

It is not currently licensed for use in early breast cancer, but regulatory approval for this type of treatment is being sought and is expected to be granted in a number of countries in 2002.