CHICAGO — Researchers say extending hormone therapy to 10 years reduces the risk of a recurrence of breast cancer in many older women and decreases the chance of a new cancer emerging in the other, healthy breast.

The study, released Sunday at the annual meeting of the American Society of Clinical Oncology, involved post-menopausal women who had a type of early-stage breast cancer known as hormone receptor-positive. That means the cancer was driven by estrogen, progesterone or both.

The data showed that women benefited from taking the drug letrozole, part of a class of estrogen-reducers called aromatase inhibitors, for 10 years. Typically, that kind of drug is taken for five years, sometimes after a course of another hormone therapy, tamoxifen.

While many women with early-stage breast cancer live for a long time, they “face an indefinite risk of relapse,” said the study’s lead author, Paul Goss, director of breast cancer research at Massachusetts General Hospital. He said the new research “provides direction for many patients and their doctors, confirming that prolonging aromatase inhibitor therapy can further reduce the risk of breast cancer recurrences.”

However, women who were treated with the drug for a total 10 years didn’t live longer than those who were given a placebo in the study. Goss said at a news briefing Sunday that he’s confident a survival benefit will emerge in the data in coming years.

The results were published online in the New England Journal of Medicine, which also featured an editorial calling the study “reassuring” and saying that “the findings have direct application for clinical practice.”