One of those patients, Emily Whitehead, now 12 and the first child ever given the altered cells, was at the meeting of the panel with her parents to advocate for approval of the drug that saved her life. In 2012, as a 6-year-old, she was treated in a study at the Children’s Hospital of Philadelphia. Severe side effects — raging fever, crashing blood pressure, lung congestion — nearly killed her. But she emerged cancer free, and has remained so.

“We believe that when this treatment is approved it will save thousands of children’s lives around the world,” Emily’s father, Tom Whitehead, told the panel. “I hope that someday all of you on the advisory committee can tell your families for generations that you were part of the process that ended the use of toxic treatments like chemotherapy and radiation as standard treatment, and turned blood cancers into a treatable disease that even after relapse most people survive.”

The main evidence that Novartis presented to the F.D.A. came from a study of 63 patients who received the treatment from April 2015 to August 2016. Fifty-two of them, or 82.5 percent, went into remission — a high rate for such a severe disease. Eleven others died.

“It’s a new world, an exciting therapy,” said Dr. Gwen Nichols, the chief medical officer of the Leukemia and Lymphoma Society, which paid for some of the research that led to the treatment.

The next step, she said, will be to determine “what we can combine it with and is there a way to use it in the future to treat patients with less disease, so that the immune system is in better shape and really able to fight.” She added, “This is the beginning of something big.”