Doctors at the University of Pennsylvania may have made a breakthrough in the fight against cancer.

An experimental treatment used on patients suffering from the most common type of leukemia proved to be effective beyond physicians' "wildest expectations," said Dr. Carl June a member of the Abramson Cancer Center's research team.

The University of Pennsylvania created the treatment by genetically modifying cells from the patients' own bodies.
    
By removing disease-fighting T cells and injecting a harmless virus into a blood sample, they created what they call "serial killer" cells.
    
When injected back into the bloodstream, those cells hunt down and kill the cancer cells.
    
After one year of the treatment, two of three patients were cancer-free and the third has substantially shown improvement.

"It worked great. We were surprised it worked as well as it did," said Dr. Carl June, a gene therapy expert at the University of Pennsylvania, reported to The Associated Press. "We're just a year out now. We need to find out how long these remissions last."

Researchers say they still have a lot to learn about these re-engineered cells before the treatment can be expanded.

"This is an exciting study," said Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society.

"It is exciting because it takes principles that we have been working on for decades, applies that to patients and produces truly meaningful results," he explained.

The study was published Thursday in the New England Journal of Medicine.