Researchers led by Dr. Matthew Ellis of Washington University developed and validated the PEPI score.

"At present about 83 percent of patients are cured of breast cancer," Ellis said in a statement. "The key is to identify that 83 percent so we can treat them with effective available therapies and focus our research efforts on the 17 percent who are resistant to current treatments."

The PEPI score was derived from tumor characteristics present after women with stage 2 and 3 breast cancer, who underwent four months of anti-estrogen therapy before having breast surgery. The PEPI score takes into account: the size of the breast tumor, whether cancer is present in nearby lymph nodes, how fast tumor cells are multiplying and whether tumors lose their estrogen receptors.

The study, published in the Journal of the National Cancer Institute, found that women with a PEPI score of 0 had almost no risk of cancer recurrence during the five-year follow-up and could safely avoid taking chemotherapeutic agents, but women with PEPI scores of 4 or above are at very high risk of having their cancer return and should be given all appropriate post-surgical treatments.