Scientists have doubled the five-year survival rate of early stage lung cancer patients by activating their immune systems to attack cancerous cells, a researcher reports.

Patients who received the experimental therapy showed a 63 percent survival rate five years after cancer surgery, compared with 33 percent for patients who received no treatment after surgery, said Ariel Hollinshead of the George Washington University Medical Center."It's come off very well," Hollins-head said Monday after describing results to science writers at an American Cancer Society symposium.

Surgery to remove lung cancer tumors often leaves cancer cells behind. In the new treatment, patients were injected after surgery with proteins drawn from other lung cancer tumors. Those antigens helped the body defend against cancer cells left behind by surgery, Hollinshead said.

Other cancer experts said they were impressed but added that more work was needed.

For early-stage cancer, "it looks like it's going to give people at least a 50 percent chance of being around five years later," while other post-surgical treatment does little good, said John Minton, professor of surgical oncology at the Ohio State University College of Medicine.

"The problem is, it's small numbers," he said of the trial, which included 81 patients. "It looks good, but we need more numbers."

Any time such a jump in survival rates can be demonstrated, "you've got to be impressed," said Dick Rauscher, senior cancer society vice president for research.