Doctors have used it for several years to guide treatment for certain women with early breast cancers, especially those that not spread.

Some experts say the findings could change the way surgeons treat early breast cancers that have spread to the lymph nodes. But other doctors say they want more proof before they make a change.

Another study found that a gene test can help predict whether some women need chemo at all - even among those whose cancer has spread to their lymph nodes, which typically brings full treatment now.