Chemotherapy helps improve breast cancer survival in post-menopausal women, adding to a long-standing debate about how best to treat these women, U.S. researchers said on Thursday.

A gene-based test called Oncotype DX made by Genomic Health Inc may help identify a small group of women who are not likely to benefit from chemotherapy, a second study found.

The main study proves that adding chemotherapy to treatment with the estrogen-blocking drug tamoxifen can help prevent cancer from coming back in women with estrogen-receptor positive breast cancers, the most common kind in which a hormone is driving the cancer.

We have a survival benefit that lasts for a very long time ... for women who got both modalities of treatment versus women who just got tamoxifen, said Dr. Kathy Albain of Loyola University Health System in Maywood, Illinois.

She presented findings from both studies at the American Association for Cancer Research San Antonio Breast Cancer Symposium.

It is considered a landmark study in the clinical trials literature because it is the only one really demonstrating the survival advantage of chemotherapy added to tamoxifen, Albain said in a telephone interview.

Up until this trial, studies adding common chemotherapy drugs to tamoxifen or tamoxifen alone were essentially negative.

For the study, the team followed nearly 1,500 post-menopausal women with estrogen-receptor positive breast cancers that had spread to at least one lymph node.

Some of the women got both tamoxifen and a chemotherapy drug known as anthracycline, and some got tamoxifen alone.

The team found that the women who got the chemotherapy were 24 percent less likely to have their cancer come back.

They were also 17 percent less likely to die during the 10-year study period, but this finding was just shy of meeting statistical significance.

The team also found that giving tamoxifen after chemotherapy ended instead of during chemotherapy improved a woman's survival chances.

In a second study led by Albain, published in the journal Lancet Oncology, the team evaluated whether the Oncotype DX test can predict which women would benefit from chemotherapy.

The test examines 21 genes from a tumor sample to see how active they are, and produces a score that predicts chemotherapy benefit. It is most commonly used in women with estrogen-fed tumors whose cancer has not spread to a lymph node.

But Albain's study suggests it may also be useful in identifying women whose tumors had spread that would not benefit from chemotherapy.

Albain said a large clinical trial is getting started that will confirm whether the test is effective, but that will be expensive and take many years.

Meanwhile, she said, some doctors like herself plan to use the test on certain patients to give them more choices about their breast cancer treatment options.

Medicine Breast cancer Cancer