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he unsung heroes of medical research are people like Rebecca Lilly and her family.

Last November, when she became the first child ever to receive gene therapy for a recurrent brain tumor, everyone involved knew the odds were long. Indeed, the therapy didn't work a miracle -- her tumor grew back six months later.

But did it work at all?

"She's had six good months," said Roger J. Packer, who directs the brain tumor program at Children's National Medical Center. If he had known last fall that he could get Becca Lilly six good months with any other treatment besides gene therapy, he would have gone for it "in a minute."

The gene therapy approach was twofold. First, doctors injected into Becca's brain a harmless virus designed to smuggle a gene into her tumor cells. If the gene invaded enough tumor cells, it would make them lethally sensitive to a follow-up drug that she received two weeks later.

Whether Becca's tumor responded at all to the gene therapy treatment is "a matter of some debate," said Stephen G. Marcus, vice president of Genetic Therapy Inc., the Gaithersburg biotech company that developed the technology for the treatment tried on her.

Samples of Becca's tumor are being studied by her doctors and at GTI. But measuring the gene therapy's effect or noneffect on her brain tumor is problematic. To cite just one vexing problem, any tumor cells that did take up the gene-carrying virus and were killed by the follow-up drug are gone. They can't be studied because they're not there anymore.

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esides, one case cannot show definitively whether or not a complex experimental treatment works. "A single patient leaves you with a lot of open questions," said neurosurgeon Steven J. Schiff, who operated on Becca. Answers will come only when more patients are treated, and the results can be analyzed and compared statistically, he said.

Medical science advances less often by breakthrough than by a series of small steps. One hunch, one clue, one table of data leads to another, and another. The science of gene therapy is still so new that even dramatic moves like Becca's treatment are "10 or 11 steps" away from finding a cure for brain tumors, said Philip H. Cogen, chairman of pediatric neurosurgery at Children's.

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ven so, Cogen said, doctors learned from Becca's experience a crucial piece of information: that the gene therapy combination is safe and nontoxic in children. That allows doctors to move ahead, fine-tuning the dose and delivery of the gene therapy combination in other patients. A second patient underwent the treatment at Children's last month.

Preliminary studies look for "a glimmer of efficacy," GTI's Marcus said. That her tumor held off for six months is a good sign, but "far short of proof." Under the terms of the experiment, the gene therapy is tried only once per child.

If a roomful of people were to look at a single case such as Becca's and be asked whether the experimental treatment worked, Marcus said, they would tend to split into three groups. Some will say they don't know. Some will say it gave her six good months, so it must have worked. Some will say it probably didn't work, because the tumor came back. No one can say for sure until a group of patients who get the treatment is compared with a group who don't, he said.

What is clear, Marcus said, is that without the contribution of individual patients, there would be no hope of finding a better way to treat a terrible disease.

"Everything starts and ends with individuals like Becca," he said.