A new Canadian-led study has found that a cancer drug may extend the lives of the thousands of older women diagnosed with breast cancer in Canada every year.

The study found that post-menopausal survivors of early-stage breast cancer who took the drug letrozole, also known as Femara, after five years of tamoxifen treatment reduced their risk of cancer recurrence by 43 per cent, and the risk of death by 26 per cent.

The news is significant because in more than 50 per cent of older breast cancer patients, the cancer returns five or more years later.

The findings will be published in the Nov. 6 issue of the New England Journal of Medicine. The Journal is publishing this material early on its website because of the study's huge implications.

As well, researchers have decided to end the double-blind clinical trial early because it was no longer deemed ethical not to offer the encouraging treatment to the women in the placebo arm of the trial.

"The estimated magnitude of the benefit... was substantially greater than expected,'' John Bryant and Dr. Norman Wolmark wrote in an editorial in the Journal.

Women who are postmenopausal and have hormone-dependent cancer tumours generally receive five years of adjuvant tamoxifen to prevent recurrence. They cannot take the drug longer because studies have shown that longer use actually produces worse outcomes.

So until now, there has been no clinically proven effective drug available to pick up where tamoxifen left off in helping women continue to reduce their risks of cancer recurrence.

This new study, conducted on almost 5,200 women over the age of 50, found that letrozole, a breast cancer drug, can further improve their survival rates if taken immediately following tamoxifen therapy.

After almost two and a half years, there were 207 instances of breast cancer recurrences -- 75 in the letrozole group and 132 in the placebo group.

The study's authors estimate the four-year disease-free survival rates of 93 per cent for those who took letrozole and 87 per cent for those in the placebo group.

The growth of some breast cancers in postmenopausal women is promoted by estrogens that circulate in the blood. Letrozole inhibits aromatase, the enzyme in the adrenal glands that produces the estrogens, estradiol and estrone. 

The study does not apply to pre-menopausal women, who account for about 22 per cent of breast cancer patients, because aromatase inhibitors do not have the same effect in them.

The study's authors were not able to determine how long women who take letrozole should take it, for instance. As well, the long-term effects of letrozole are not known, and there are worries the antiestrogen drugs can lead to osteoporosis.

The study was conducted by researchers from the Canadian Cancer Society, Toronto's Princess Margaret Hospital, the U.S. National Cancer Institute, the Mayo Clinic and the National Cancer Institute of Canada's clinical trial group at Queen's University.

Breast cancer is the most frequently diagnosed cancer in Canadian women, accounting for 30 per cent of all cancer cases. One in nine Canadian women will develop breast cancer during her lifetime. One in 27 will die from it.