A breast cancer drug could save the lives of thousands of women living with the disease.
Breast tumours are the most common form of cancer in UK women
Research on women with early stage breast cancer found taking letrozole after a course of another drug, tamoxifen, cut death rates by 39%
Until now there have been no treatment options for women who finish their course of tamoxifen after five years.
The international study, of 5,200 women, included five hospitals in the UK.
The women finished their course of tamoxifen, and then took letrozole for an average of two-and-a-half years.
It is recommended that tamoxifen should not be taken for more than five years because its benefits begin to wane, and there is a risk of endometrial cancer, pulmonary embolism and stroke.
However, the risk of a recurrence of breast cancer remains.
The latest study, co-ordinated by the National Cancer Institute of Canada Clinical Trials Group, also found that letrozole cut the risk of the cancer spreading to another part of the body by 40%.
Letrozole - known commercially as Femara - is an aromatase inhibitor. It works by blocking production of the sex hormone oestrogen, which is linked to the development of many breast tumours.
Professor Ian Smith, one of the investigators and head of the breast unit at the
Royal Marsden NHS Trust, said: "This is an exciting development in improving
overall survival of postmenopausal women with breast cancer.
"Femara is the first hormonal therapy to offer a post-tamoxifen option to
reduce the risk of recurrence.
"We now know it may also offer women with node-positive early breast cancer
an improvement in overall survival, nearly halving their residual risk of dying
from this disease."
Samia al Qadhi, joint chief executive of charity Breast Cancer Care, said:
"From our daily contact with women who have breast cancer, we know that they
worry about recurrence of their breast cancer.
"The findings of this trial continue to give hope to the tens of thousands of
women in the UK who are living with breast cancer, and potentially offer women
an option of extended hormone treatment that may maximise their chances of
Earlier this year another study, co-ordinated by Cancer Research UK, found
that patients who switched from tamoxifen to the aromatase inhibitor exemestane
halfway through treatment reduced the risk of the disease returning by a third.
The National Institute of Clinical Excellence (Nice) is currently considering whether to make aromatase inhibitors widely available on the NHS for the treatment of early disease.
The results of the study were presented at the American Society of Clinical
Oncology annual meeting in New Orleans.