"Hugely important is whether this has to do with the fact that it just makes the bone hostile, somehow, to metastasis or if there is a more global anti-metastasis effect," said the oncology group's president, Dr. Nancy Davidson of Johns Hopkins University.

"Either of those would be good and would teach us a lot about what to do next."

Breast cancer is the most common cancer in women. About 184,450 cases and 40,930 deaths from the disease are expected in the United States this year.

Standard treatments are surgery, chemotherapy, radiation and hormone-blocking drugs if the tumors are like those in the study — helped to grow by estrogen or progesterone.

The hormone-blockers often weaken bones, so bisphosphonates like the osteoporosis pill Fosamax have become increasingly popular to treat this side effect. However, using them to treat the cancer itself is a very different approach.

Lab studies hinted it would work, and Gnant's is the first to test it in a large group of breast cancer patients.

All had surgery to remove their tumors and were taking hormone-blocking drugs — goserelin plus either tamoxifen or anastrozole — treatments that made them menopausal. Half also were given infusions of Zometa once every six months.

The women were treated for three years and studied for two more. By then, only 6 percent of those given Zometa had suffered a relapse or died, compared to 9 percent of the others. That translated to a 36 percent decline in risk.