PHILADELPHIA When doctors saw the report on Bill Ludwigs bone-marrow biopsy, they thought it was a mistake and ordered the test repeated.But the results came back the same: His lethal leukemia had been wiped out by an experimental treatment never used in humans.We were hoping for a little improvement, remembers the 72-year-old retired New Jersey corrections officer, who had battled the disease for a decade. He and his oncologist both broke down when she delivered the good news in 2010. Nobody was hoping for zero cancer.The pioneering therapy with Ludwig and a few other adults at the University of Pennsylvania hospital paved the way for clinical trials with children. Six-year-old Emily Whitehead, who was near death, became the first pediatric recipient in 2012. Like Ludwig, she remains cancer-free.Such results are why the treatment is on track to become the first gene therapy approved by the Food and Drug Administration. An FDA advisory committee will decide Wednesday whether to recommend approval of the approach, which uses patients own genetically altered immune cells to fight blood cancers.If the panel gives the nod, the agency probably will follow suit by the end of September. That would open the latest chapter in immunotherapy – a true living drug, says Penn scientist Carl June, who led its development.