San Francisco ? For years, doctors have known exactly what to do with breast cancer patients like Eva Ossorio: Poison them.

Blasting women with toxic chemicals was considered the best way to save lives. The bigger the cancer or the more it had spread, the more vile liquid doctors pumped into veins to try to kill it.

But there’s been a sea change in the last year.

Guidelines recently adopted in Europe and similar ones unveiled this weekend at a conference in Texas will result in far fewer women getting chemotherapy in the future.

The new advice calls for choosing a treatment based on each woman’s particular type of tumor.

“In the past, we made all decisions based on how big the tumor was and whether the lymph nodes were involved. If you had a lot of cancer, you got treated one way, and if you had a little cancer, you got treated another way,” said Dr. Eric Winer of the Dana-Farber Cancer Center in Boston.

Under the new rules, hormone status – whether a tumor’s growth depends on estrogen or progesterone – becomes the single most important factor in picking treatment.

Eva Ossorio, 62, San Antonio, holds a bottle of Femara, a hormone blocker used for breast cancer. Formerly, she would have been given chemotherapy for her relatively large tumor.

That is why Ossorio, a 62-year-old nurse in San Antonio, last week was started on a hormone blocker rather than the chemo she formerly would have been given for her relatively large tumor. She was relieved.

“I don’t care if I die tomorrow. I decided I didn’t want chemotherapy,” she said.

Women have reason to dread it. Chemo is a sledgehammer, killing all rapidly dividing cells whether they are out-of-control cancerous ones or healthy ones that naturally grow quickly, like those lining the mouth and stomach. That’s why chemo causes hair loss, nausea and mouth sores.

But the worst part is, it only helps about 15 percent of those who get it after the usual surgery to remove their tumors. Roughly 25 percent get worse despite chemo. A whopping 60 percent would have been fine with hormones alone.

“For the vast majority of patients, we probably overtreat,” said Dr. William Gradishar of Northwestern University in Chicago.

“It’s not that chemotherapy is not of value, it’s that the value is smaller in women with hormone-driven disease,” said Dr. Robert Carlson, a Stanford University physician who led the U.S. guideline-writing group. “We’re trying to determine if the benefit is so small that we should not be recommending chemotherapy.”

Several developments in recent years help doctors pick who really needs it.

First is the realization that breast cancers have different causes, arise from different types of cells, are driven by different genes, and tend to be different in women before or after menopause.

“Breast cancer must be understood as an umbrella of diseases,” said Dr. Antonio Wolff of Johns Hopkins Medical Institute in Baltimore.

For example, three-fourths of postmenopausal women have tumors fueled by estrogen, called ER-positive disease. Drugs that block this hormone, like tamoxifen and a newer class of medications called aromatase inhibitors, work against those cancers – whether they have spread to lymph nodes or not.

On the other hand, women before menopause often have tumors that are ER-negative and orchestrated by bad genes. Hormones don’t help in that case; these women benefit most from chemotherapy.

If hormone drugs are ball-peen hammers compared to chemotherapy, a medication like Herceptin is an even more refined tool. It targets the one-fourth of breast cancers that have too much of a protein on cell surfaces called HER-2 and leaves healthy cells alone.

The new guidance was developed by the National Comprehensive Cancer Network, a group of leading cancer treatment centers, in cooperation with the American Cancer Society.

They soon will be published and are available now on the network’s web site, www.nccn.org.