Women who survive five years after being diagnosed with breast cancer have a good chance of remaining cancer-free, a new study shows.

In the most detailed study of its kind, a new report shows 89 percent of these women remain disease-free 10 years after diagnosis, and 81 percent are cancer-free after 15 years.

Authors of the study, published online yesterday in the Journal of the National Cancer Institute, say their findings may reassure breast cancer survivors, many of whom assume their odds are much bleaker.

"Patients often ask me, 'Now that I've survived my breast cancer, what is my future risk of a recurrence?' " says author Abenaa Brewster, an assistant professor at Houston's M.D. Anderson Cancer Center. "This is an answer we've had a hard time giving. They remain really terrified about their risk."

Brewster notes that her study didn't include women who relapsed before five years.

Overall, 89 percent of breast cancer patients live at least five years, according to the American Cancer Society.

About 183,000 women are diagnosed with breast cancer each year, and about 40,000 die of it.

In Brewster's study, all 2,838 patients had surgery to remove the original tumor, and some also had radiation.

All women also took drugs — such as several months of chemotherapy, five years of the pill tamoxifen or both — to prevent cancer from returning.

Thanks to new drugs, women today may fare better than those in the study, who were treated between 1985 and 2001, says the cancer society's Len Lichtenfeld.

Doctors now often prescribe aromatase inhibitors to postmenopausal women when they're diagnosed, Brewster says.

Lichtenfeld says some of the study's findings were surprising.

In the short term, two groups of tumors are usually less likely to return: those that are slow-growing and those that are fueled by estrogen.

But in the long run, Brewster's study shows, these tumors recurred more often. Doctors don't know exactly why, she says.

It's possible that women whose tumors seemed less threatening were "undertreated" and could have benefited from additional therapy, says Joseph Baar, a breast cancer specialist at the Ireland Cancer Center in Cleveland.

Baar notes that doctors have begun a large study to learn which women really need the most intensive therapy and which might safely skip chemotherapy, which can cause serious side effects.