WASHINGTON - A drug made by Genentech Inc. received federal approval yesterday to treat breast cancer, a decision that could represent a major shift in standards for assessing the effectiveness of cancer medicines.

Going against the recommendation of its advisory panel, the Food and Drug Administration cleared Genentech's Avastin, which already has been approved for treating lung and colon cancer, based on findings that the drug slowed tumor growth.

Advocates for breast-cancer patients applauded the approval.

Avastin was approved in advanced breast cancer under the FDA's accelerated-approval program, which allows the agency to approve products for cancer and other life-threatening diseases based on initial positive clinical data.

Genentech said that the FDA had granted the drug preliminary approval, and that the company would have to submit additional data.

FDA approval for late-stage cancer treatments is usually contingent upon data showing a drug extended, or improved the quality of, patients' lives. Avastin showed neither in a study, according to Genentech's application.

Instead, Genentech showed that the drug slowed tumor growth without actually increasing life expectancy. Cancer experts have long debated whether that measure is as significant as survival rates for measuring a drug's effectiveness.

Wall Street analysts believe the FDA's Avastin decision opens the door for other 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 end point," said Kay Dickersin, director of the Center for Clinical Trials at Johns Hopkins University. The questions surrounding Avastin's effectiveness have caused debate among cancer experts, doctors and patients.

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.

Despite the panel's narrow recommendation against the drug, many U.S. doctors continued prescribing it "off-label," or without a federal endorsement, for breast cancer. Genentech previously has estimated that more than 9,000 breast-cancer patients receive the drug this way.

Some insurers have been reluctant to pay for the injectable drug, which can cost $100,000 for a year's supply.

Joseph Sparano said he prescribed Avastin for his patients because it had shown better results at slowing breast-cancer growth than any other drug on the market.

The Y-Me National Breast Cancer Organization called Avastin an important new option for patients.

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

Genentech shares closed down 15 cents to $71.60. After the FDA decision, the stock rose $6.39 to $77.99 in after-hours trading.