WASHINGTON — A Genentech drug received federal approval on Friday to treat breast cancer, a surprise decision that could represent a shift in standards for assessing the effectiveness of cancer medicines.

Going against the recommendation of its advisory panel, the Food and Drug Administration judged the effectiveness of Avastin based on measurements of tumor growth, not patient survival.

The ruling prompted a mixed reaction from both doctors and advocates for breast-cancer patients.

Some bemoaned the lowering of medical standards, but others applauded the federal government's endorsement of a drug that is already prescribed "off-label" to an estimated 9,500 patients and for whom insurance coverage is limited.

Avastin, which is already approved in the U.S. for treating lung and colon cancer, was Genentech's best-selling product last year with revenue of $2.3 billion. Through a partnership with Genentech, Swiss drug maker Roche markets the drug in Europe, where it had previously been approved as a breast-cancer treatment.

Shares of Genentech Inc. rose $5.76, or 8 percent, Friday to $77.36 in after hours trading.

FDA approval for drugs targeted at cancer patients who have never been treated before is usually contingent upon data showing a drug extended, or improved the quality of, patients' lives. Avastin showed neither in a study submitted by Genentech, though the drug did slow tumor growth.

Wall Street analysts believe FDA's Avastin decision opens the door for more cancer drugs to be approved for their tumor-shrinking capabilities _ a trend that worries some health experts.

"If FDA sets a precedent of approving a drug based on progression free survival, people are afraid they may stop looking at survival as the most important endpoint," said Dr. Kay Dickersin, director of the Center for Clinical Trials at Johns Hopkins University.

In December, a panel of outside FDA advisers voted 5 to 4 against Genentech's application, indicating the drug's benefits did not outweigh toxic side effects. Nevertheless, U.S. doctors continued prescribing it "off-label," or without a federal endorsement.

Some insurers are reluctant to pay for Avastin's use among breast-cancer patients, which can cost $100,000 for a year's supply.

Dr. Joseph Sparano of Montefiore Medical Center in New York said he prescribes Avastin because it has shown better results at slowing breast cancer growth than any other drug on the market.

The Y-ME National Breast Cancer Organization said the approval gives patients an important new option.

"The benefits we're looking at here matter because they give patients hope," said Margaret C. Kirk, the group's chief executive. "Without disease progression they may survive to see a discovery that can help them." they may survive to see a discovery that can help them."

But the president of the National Breast Cancer Coalition said the decision marked a lowering of standards for FDA. She argued that the drug's effectiveness should not have outweighed safety risks, pointing to six deaths attributed to the drug in the study submitted to FDA.

"All they had was progression-free survival in one trial, no increase in quality of life and patient deaths in the Avastin group," said Fran Visco. "We're very confused why FDA made this decision."

First approved in the U.S. to treat colon cancer in 2004, Avastin was the first drug to fight cancer by blocking nutrients from reaching tumors.