A trial comparing anastrozole and tamoxifen for breast cancer (ATAC trial)

The research team found that anastrozole did help stop breast cancer coming back more than tamoxifen, and caused fewer side effects.

This trial recruited 9,366 women who had been through the menopause and had surgery for early stage breast cancer.

• 3,125 had anastrozole
• 3,116 had tamoxifen
• 3,125 had tamoxifen and anastrozole

Tamoxifen and anastrozole are both tablets that you take once a day, every day for 5 years.

The research team discovered quite early on that taking tamoxifen and anastrozole together was no better than taking tamoxifen alone. So this part of the trial was stopped. The women who had been taking both went on to take one or the other.

When the trial started researchers didn’t know for certain that hormone therapies such as tamoxifen and anastrozole worked best for women with hormone receptor positive breast cancer. So not everyone taking part in this trial had hormone receptor positive cancer.

When they first started to look at how well the treatments worked, they discovered that they worked best in the 5,216 women who had hormone receptor positive breast cancer. Of these,

• 2,618 women were taking anastrozole
• 2,598 women were taking tamoxifen

The research team looked at how good the treatments were at stopping the cancer coming back. They found that 10 years after starting treatment, the cancer had come back in

• About 1 in 5 women (19.7%) who had taken anastrozole
• About 1 in 4 women (24%) who had taken tamoxifen

They also looked at the side effects and found that overall anastrozole caused fewer side effects than tamoxifen. The women taking anastrozole were less likely to have problems with vaginal bleeding and discharge, blood clots in the veins (deep vein thrombosis, or DVT) and hot flushes.

The women taking tamoxifen were less likely to have problems with weakening of the bones (osteoporosis) and fractures while they were having treatment. But there was less of a difference between the 2 groups after they had finished treatment.

The risk of developing a second cancer was low for both groups, but for most cancers it was slightly lower for the women taking anastrozole compared to tamoxifen. In particular, the risk of developing cancer in the other breast was lower for women taking anastrozole.

There doesn’t seem to be a significant difference in how long women live after taking these 2 treatments, but it’s really too early to tell yet. There may be differences between the groups in time.

The research team concluded that anastrozole is better than tamoxifen at stopping early stage breast cancer coming back after surgery. And that it caused fewer serious side effects.

We have based this summary on information from the team who ran the trial. The information they sent us has been reviewed by independent specialists (peer reviewed ) and published in a medical journal. The figures we quote above were provided by the trial team. We have not analysed the data ourselves