Women with early stage breast cancer won a new gene test Tuesday to help predict whether they'll relapse in five or 10 years, information that could influence how aggressively they fight that initial tumor.

The MammaPrint test isn't the first such predictor for breast cancer to hit the U.S. market, but it is the first to win formal approval from the Food and Drug Administration.

The test is far from perfect, warned FDA's Dr. Steven Gutman.

Indeed, it's much better at predicting who isn't likely to relapse than who is, Gutman said. He cited studies suggesting that when the MammaPrint predicts a woman is at high risk of cancer returning in five years, it will be right just a quarter of the time. That compares with 95 percent accuracy if the woman is told she's at low risk of a relapse.

"This information has to be used very carefully by physicians," Gutman said. "This is a complex test. It requires use by people who know their business."

More than 178,000 U.S. women will be diagnosed with breast cancer this year. More than 100,000 of them will have early stage tumors with a remarkably good prognosis: They're small, they haven't yet spread to the lymph nodes, and they're sensitive to hormones.

The vast majority of those women would survive with surgery, radiation and hormone treatment. Yet guidelines today recommend chemotherapy as well for most to catch the few — two or three out of every 100 patients — who need more aggressive help to try to prevent a recurrence years later.

Gene testing can help doctors and patients winnow out who's most likely to need the chemo from who might gamble on skipping it. For about two years, a growing number of breast cancer specialists have begun offering such testing to do just that.

But until now, the FDA hasn't required formal approval of those tests, allowing sales instead as doctor-ordered services that laboratories provide, Gutman said.

Tuesday's approval doesn't mean the MammaPrint works better than what's already used, but that its Dutch manufacturer, Amsterdam-based Agendia, sought formal FDA review and approval. The agency now is debating whether existing gene-predictor tests need similar oversight, Gutman said.

How does MammaPrint work? It analyzes the level of activity of 70 genes in a sample of the woman's surgically removed tumor, and provides a numerical ranking. Women ranked as high-risk have about twice the likelihood of a relapse in the next five to 10 years as women who are ranked low-risk, Gutman said.

The FDA has no information on whether the test results actually improved the survival of high-risk women, Gutman said.

A spokesman for Agendia didn't return a phone call seeking the test's price. Its main U.S. competitor costs $3,000.