San Antonio ? Researchers in San Francisco say they have found a way to predict which patients with precancerous breast tumors are destined to develop cancer, potentially saving tens of thousands of women a year from unnecessary treatment – and unnecessary anxiety.

In a study presented at the San Antonio Breast Cancer Symposium, which ended Sunday, Thea Tlsty reported that her group had discovered a molecular profile that can distinguish potentially lethal cases of DCIS (ductal carcinoma in situ) from ones that will never become life-threatening.

“This is a big step forward,” said Tlsty, a molecular pathologist at the University of California-San Francisco. “Nobody’s been able to do it before.”

Dr. Peter Ravdin of the University of Texas, who was not involved in the research, agreed. “This is really exciting,” said Ravdin, adding that Tlsty’s work was probably the single most important piece of research to emerge from the four-day annual meeting.

DCIS is generally considered a precursor to breast cancer, but DCIS itself is never life-threatening. In most cases, it does not progress to the kind of breast cancer that has the ability to invade neighboring tissues and spread to other parts of the body.

But an estimated 8 to 15 percent of DCIS patients who have the lesion removed by lumpectomy will develop real breast cancer in the 10 years following their surgery. Because doctors can’t tell which ones will recur, all DCIS patients are treated with lumpectomy, usually followed by radiation, or mastectomy, in which the whole breast is removed. Most also get anti-cancer medication.

DCIS affects more than 50,000 women a year in the U.S. At least another 100,000 women have other precancerous conditions that often are treated in the same way.

Surgery, drugs and radiation have unwanted and sometimes serious side effects. For that reason, the holy grail of cancer research for the past four decades has been to figure out which patients don’t need the harsh treatments and to target cancer therapies to those patients who are most likely to benefit.

Thanks to advances in molecular biology, doctors are getting closer to that goal. In breast cancer, which is ahead of most other areas of research, there are several drugs that are known to help certain patients and not others, and there are several groups of patients who can now be spared the debilitating effects of treatments that won’t help them.