WASHINGTON (AP) -- For post-menopausal women with breast cancer that is affected by estrogen, chemotherapy may offer no benefit and a five-year course after surgery of tamoxifen, which blocks the hormone, may be the only treatment needed, a study shows.

The finding, appearing this week in the Journal of the National Cancer Institute, could mean that some of the thousands of American women diagnosed each year with breast cancer could avoid the debilitating side effects of taking drugs that chemically kill cancer cells and damage other cells in the body.

The study also found that for post-menopausal women whose breast cancer was not affected by estrogen, chemotherapy followed by tamoxifen offered the best hope of disease-free survival. Tamoxifen works by blocking the action of estrogen, a hormone that can promote the growth of cancer cells.

"We're hoping this study will question the routine use of chemotherapy in the ER-positive (estrogen sensitive) group," said Richard D. Gelber, a biostatistician at the Dana-Farber Cancer Institute in Boston and one of dozens of co-authors. "This is the largest study ever concentrating on this population."

The information should help physicians tailor cancer treatment for specific patients, Gelber said.

Dr. Andrew D. Seidman, a breast cancer specialist at Memorial Sloan-Kettering Cancer Center in New York City, said the study "will reaffirm the way that many oncologists are now practicing" and should help doctors base cancer therapies on the specific characteristics of their patients.

It shows chemotherapy is appropriate for some women, but not others, and that patients who need the therapy can be specifically identified based on the estrogen sensitivity of their tumors and on other factors, Seidman said.

The study, compiled by researchers from nine countries, was limited to breast cancer surgery patients who had completed menopause and whose disease had not spread to the lymph nodes. About 43 percent of the approximately 184,000 women in the United States diagnosed with breast cancer each year fit this category, the study said.

Researchers further divided the 1,669 patients in the study into groups based on whether their cancers were affected or not affected by estrogen.

There were 382 in the study whose tumors were not affected by estrogen, described in medical terms as being estrogen receptor negative, or ER-negative. There were 1,217 who were ER-positive. For the remaining 70 patients, the ER status was unknown.

About half in each of these groups were treated with chemotherapy followed by five years of tamoxifen. The other half of each group received tamoxifen only.

Researchers followed the patients for an average of almost six years and found the value of chemotherapy depended directly on the estrogen sensitivity of the patients' disease.

Among the ER-positive patients, chemotherapy provided no benefit. The five-year, disease-free survival rate for those who only took tamoxifen was 85 percent, while it was 84 percent for those who had both chemo and tamoxifen.

However, for the ER-negative patients, chemotherapy could be a life saver. The five-year, disease-free survival rate for those who had both chemo and tamoxifen was 84 percent. For ER-negative patients who took only tamoxifen, the survival rate was only 69 percent.

Gelber said the study may improve the lives of patients who are ER-positive because it will ensure they don't have to endure chemotherapy's side effects, such as hair loss, nausea and persistent tiredness.

"There is a quality of life burden associated with chemotherapy," said Gelber. "It is not a free ride. It should not be given frivolously."

On the other hand, he said, the study showed that chemo's side effects were not severe enough that the therapy should be avoided by the ER-negative patients who would benefit.

Gelber said a quality-of-life segment of the study showed the side effects from chemo are usually temporary and go away after the therapy ends.

Copyright 2002 The Associated Press. All rights reserved.