Chicago ? Cancer researchers meeting in Chicago had both good and bad news Monday for breast cancer patients who are eligible for treatment with the new-generation drug Herceptin – at least 40,000 a year in the U.S. alone.

The good news is that Herceptin continues to prevent relapses years after patients stop taking it. The bad news is that the laboratory tests used to determine eligibility appear to be highly inaccurate. That means some women who could benefit from Herceptin don’t get it, while others are subjected to the side effects and high cost of the drug with no hope of a benefit.

“It is a very big deal,” said Dr. Larry Norton, head of breast cancer programs at Memorial Sloan-Kettering Cancer Center in New York.

About one-quarter of breast cancer patients carry extra copies of a gene called HER2, which makes their tumors more aggressive but also makes them respond to Herceptin, a synthetic antibody that targets the HER2 protein. Herceptin – one of the first in a new generation of molecularly targeted cancer drugs – has been shown to reduce the risk of recurrence by about 50 percent. That is a huge benefit because breast cancer doesn’t become life-threatening unless it recurs and spreads to other parts of the body.

On Monday, Dr. Edith Perez of the Mayo Clinic in Jacksonville, Fla., presented trial results showing that, four years after treatment, the women who got Herceptin in addition to chemotherapy were 35 percent more likely to be alive and 52 percent more likely to be relapse-free, with no increase in negative side effects.

“This is good news for patients,” she said.

But one of the slides Perez screened at the annual meeting of the American Society of Clinical Oncology showed that some patients who were classified as HER2-negative got the same benefit as those who tested positive. Perez’s data contained too few patients to be statistically significant, but another researcher was scheduled to present similar findings today in a bigger set of patients.

That caused a buzz among the cancer doctors gathered at McCormick Place, many of whom said the finding raised serious questions about their ability to offer patients the best possible treatment.

Dr. Kathy Albain, director of breast research at Loyola University Health System in Maywood, Ill., predicted the data would cause “mass confusion,” adding, “It will give me pause to re-measure someone who is initially HER2-negative.”

Perez also suggested it might make sense to retest anyone whose first test was negative, just in case it’s a false result. “I’m worried about excluding patients who might benefit,” she said.

But Dr. Dennis Slamon of UCLA, who is largely credited with developing Herceptin, noted that false-positive results also are problematic because the drug costs about $3,000 a month and has sometime-serious side effects, including a heightened risk of heart failure. Dr. Michael Press, a pathologist at the University of Southern California, said as many as one-third of positive antibody tests “could be false-positives.”

Slamon called for a high-level task force to figure out how to reduce errors in testing for HER2. As a first step, he said, any negative tumor samples of women who got Herceptin in the clinical trials should be retested by independent, “blinded” pathologists to make sure they’re true negatives.