Researchers were so encouraged by early results from a study on preventing breast cancer recurrence that they halted their work so more women can benefit from the findings.

The study, published online Thursday by the New England Journal of Medicine, showed breast cancer patients who follow up five years of tamoxifen (search) treatment with letrozole (search), an estrogen suppressor, cut the risk of recurrence by nearly half.

Doctors involved in the study said the findings could benefit hundreds of thousands of women uncertain about what to do after taking tamoxifen, which loses much of its effectiveness after five years. The drug is the top hormonal treatment for estrogen-fueled tumors.

"The result has provided women with hope," said Kathy Anderson, a breast cancer survivor who took part in the study.

The study involved more than 5,000 women in North America and Europe with the most common form of breast cancer who had completed the recommended five years of tamoxifen treatment. They were given either letrozole or a dummy pill, and results showed that within an average of 2.4 years, 207 had a cancer recurrence — 75 of those on letrozole and 132 of those taking the placebo.

Because of those results, the research was halted so those participants getting the placebo could begin taking letrozole. Publication of the results, which will appear in the journal's Nov. 6 issue, was moved up because of the importance of the findings.

Letrozole is made by Novartis Pharmaceuticals and sold under the brand name Femara (search). It paid more than half the cost of conducting the $15 million study, and also supplied all the letrozole and placebo pills used, officials said.

Doctors who ran the study told a news conference Thursday that the opportunity to help so many women prevailed over the desire for more substantive long-term findings.

"This is available and can provide potentially meaningful reduction in risk of recurrence," said Dr. James Ingle, of the Mayo Clinic in Rochester, Minn.

A journal editorial published alongside the study supported the decision.

"At a minimum, suitable patients must be apprised of these important observations and must be given the opportunity to receive letrozole, with an understanding of the limitations of the data," said the editorial by Dr. Norman Wolmark of Allegheny General Hospital in Pittsburgh.

Estrogen fuels the growth of about half of all breast cancers, especially those in older women. Tamoxifen is given to almost all such U.S. patients after surgery to help prevent breast tumors from returning.

Tamoxifen, the top treatment to stall tumor growth, prevents estrogen from linking up to a receptor on the surface of cancer cells.

However, tamoxifen's effectiveness ends after five years, apparently because the body develops a resistance to it, said Dr. Paul Goss of Princess Margaret Hospital in Toronto. He headed the study by 18 doctors from Canadian, U.S. and European hospitals, universities and cancer centers.

Estrogen pushes dormant tumors to grow, he said, so the study looked at what happened if patients took an estrogen inhibitor such as letrozole. Goss and Ingle said further study was required on the effects of prolonged letrozole use.

Side effects include increased risk of osteoporosis, hot flashes, night sweats, and pain in the bones, joints or back. Letrozole costs about $6 per pill and is taken daily, Ingle said.

Goss said the findings help him go to work "with a lighter step" because he can tell patients that "yes, something is happening."

Anderson, a 50-year-old elementary school administrator diagnosed with breast cancer more than eight years ago, said she had no idea during the study if she was taking letrozole or the placebo. She said she was relieved to hear earlier this week it was letrozole.

"There is anxiety about recurrence. It fluctuates day-to-day," she said. "My recurrence rate has just been cut in half."