The latest data from an extensive ongoing project involving 145,000 women with early breast cancer found that chemotherapy and hormone treatment continue to protect many women from dying from the disease for at least 15 years. The protection often gets stronger over time, increasing the likelihood that the therapy is truly eradicating cancer from their bodies.

The findings provide the most convincing support yet for using aggressive strategies against the most common malignancy to strike women, and they help explain why the death rate from breast cancer has been dropping in many countries, including the United States and Britain, experts said.

"This is really good news," said Sarah Darby of the University of Oxford in England, who led the analysis being reported in tomorrow's issue of the journal the Lancet. "It means that the standard therapies we're giving women really are working. It's really quite exciting."

The analysis showed, for example, that a middle-aged woman with a diagnosis of early-stage breast cancer cuts her risk of dying by about half by undergoing six months of chemotherapy and taking hormone treatment for five years, if she is medically eligible for both.

Every year, breast cancer is diagnosed in about 211,240 women in the United States and about 40,410 die from the disease, making it the most common cancer among women and second-leading cancer killer, after lung cancer.

For most women, it is now standard practice to treat early breast cancer with surgery and radiation, followed by chemotherapy to reduce the risk of a recurrence by attempting to wipe out any cancer cells lurking elsewhere in the body. If their tumors are sensitive to the hormone estrogen, many women also take the estrogen-blocking drug tamoxifen for about five years to further reduce the risk of recurrence. (A new generation of hormone therapy has begun to replace tamoxifen.) Although earlier studies have shown that the approach reduced the chances of a relapse and increased the odds of survival, there have been haunting concerns about how long those benefits last inasmuch as breast cancer can hide in the body for years or even decades before reemerging.

The new findings should alleviate lingering doubts, reassuring women who went through the sometimes grueling regimens that they were worthwhile and encouraging women who are not receiving such treatment to do so, Darby and others said.

"There are a lot of women out there who are not getting these treatments and could benefit from them," Darby said. "They are not a magic bullet, but as these data show, they clearly do offer substantial benefit."

The study also offers long-sought confirmation that follow-up therapies really are enabling more women to survive the disease and not just holding the cancer at bay, Darby and other experts said.

"One of the big concerns people have had was whether these treatments really were curing you or just putting off the disease for a little while," said Susan Love, a breast cancer expert at the University of California at Los Angeles. "Now with these data you can say, 'No, it really is curing some people.' "

Because breast cancer therapies have evolved since the studies in the analysis were conducted, survival rates may be even better now, the researchers said. But they noted that much more work is needed to prevent breast cancer, to develop targeted treatments tailored to women's cancers and to find better ways to predict an individual's prognosis.

"While these numbers clearly translate into more women living longer, which is good news, it doesn't translate into an individual prediction," said Barbara Brenner of Breast Cancer Action, a patient advocacy group. "People always want to know, 'What is going to happen to me?' This study can't answer that."

The Early Breast Cancer Trialists' Collaborative Group, based at Oxford, has been conducting comprehensive assessments of existing data on the effectiveness of this approach every five years. Previous analyses showed benefits after five and 10 years. In the latest analysis, the researchers extended the analysis to 15 years -- the longest follow-up ever -- using data from 194 studies begun around the world from 1985 to 1995.

Overall, the women receiving treatment were significantly more likely to survive than those who did not, with the benefits varying based on a variety of factors, including age and whether their tumors were sensitive to estrogen.

The analysis found, for example, that if a 50-year-old woman had a 1-in-5 risk of dying from her breast cancer, that risk could be reduced by about half -- to about 1 in 10 -- by six months of chemotherapy and five years of tamoxifen.

The payoff increased with time. In women younger than 50, for example, the benefit of chemotherapy was twice as great at 15 years as at five years -- 10 percent survival benefit vs. 4.7 percent. The benefit of five years of tamoxifen was almost three times as great at 15 years as at five years -- 9.2 percent vs. 3.6 percent.

The benefit probably increases over time because the proportion of surviving women who are cured by the treatment grows, experts said.

"The most impressive finding is the divergence of the survival curves for breast cancer over time," wrote Karen Gelmon of the BC Cancer Agency in Vancouver, British Columbia, and colleagues in an editorial accompanying the findings. "The survival curves suggest that adjuvant systemic therapies do cure a proportion of women with early-stage breast cancer."