A trial looking at letrozole after hormone therapy for early breast cancer in postmenopausal women (SOLE)

Cancer type:

Breast cancer




Phase 3

This trial compared taking letrozole all the time with stopping and starting treatment. It was for women who had been through the menopause and had early breast cancer that has tested positive for hormone receptors.

More about this trial

The first treatment for early stage breast cancer is usually surgery. After surgery, women who have hormone receptor positive breast cancer will have hormone therapy as part of their treatment. This helps to reduce the risk of breast cancer coming back.

Letrozole (Femara) is type of hormone therapy called an aromatase inhibitor. When this trial was done, it was common to have hormone therapy such as tamoxifen for 5 years after surgery. And research had shown that having letrozole for another 5 years, after the first 5 years of hormone therapy, may further reduce the risk of the cancer coming back.

The results of a small study had shown that having intermittent letrozole may be as good as, or even better than, having it continuously. The researchers wanted to look at this in a larger trial.

The aim of this trial was to compare intermittent and continuous letrozole for postmenopausal women after their first 5 years of hormone therapy.

Summary of results

This trial recruited nearly 5,000 women who’d already had surgery and then 4 to 6 years of hormone therapy for early stage breast cancer.

The women taking part were put into 1 of 2 groups at random, and:

  • half took letrozole every day for 5 years
  • half took letrozole on and off (intermittently) for 5 years

The women in the intermittent group took letrozole once a day for 9 months followed by 3 months of no treatment, for 4 years. They then took letrozole every day for the 5th year.

The research team looked at the number of women who had no signs that their cancer had come back. They found it was nearly 9 out of 10 women in both groups:

  • 87.5% of women who had continuous letrozole
  • 85.8% of women who had intermittent letrozole

When they looked at how many women were living at the end of the 5 years, they found it was more than 9 out of 10 in each group:

  • 93.7% of women who had continuous letrozole
  • 94.3% of women who had intermittent letrozole

Side effects
They also looked at the side effects and quality of life of the two groups, and there were just a few differences. A small number of women who had continuous treatment had more problems with their mood, hot flushes, sleeplessness and bone and joint pain.

The research team concluded that having intermittent letrozole doesn’t reduce the risk of early stage breast cancer coming back. But that the side effects were similar, and quality of life was slightly better. They suggest that some women who have been through the menopause and have early stage breast cancer, may benefit from short breaks in letrozole treatment.

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

Recruitment start:

Recruitment end:

How to join a clinical trial

Please note: In order to join a trial you will need to discuss it with your doctor, unless otherwise specified.

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Chief Investigator

Professor A M Thompson

Supported by

Breast International Group (BIG)
Experimental Cancer Medicine Centre (ECMC)
International Breast Cancer Study Group (IBCSG)
NIHR Clinical Research Network: Cancer

If you have questions about the trial please contact our cancer information nurses

Freephone 0808 800 4040

Last review date

CRUK internal database number:


Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Last reviewed:

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