June 10, 2008 -- What doesn't kill cancer cells makes them stronger, Duke researchers have observed.

Doctors use radiation and chemotherapy to destroy cancer cells. About half of patients are cured -- that is, all of their tumor cells die.

The other half of the time, some tumor cells survive treatment. These cancer cells are more aggressive than they were before treatment, says Mark W. Dewhirst, DVM, PhD, professor of radiation oncology at Duke University.

"When you give a tumor treatment, whatever cells survive are going to be more resistant to that treatment," Dewhirst tells WebMD. "Those not killed are healthier cancer cells."

This does not mean radiation and chemotherapy don't work. It does mean that additional new treatments will be needed. And to know what treatments will work best, Dewhirst says doctors need to know how cancer cells survive radiation and chemotherapy.

The key may be a protein called HIF -- hypoxia-inducing factor. Government, university, and drug-company researchers are racing to develop new drugs that inhibit HIF. But patients may not have to wait that long: Existing drugs, already approved by the FDA for cancer treatment, turn out to be potent anti-HIF agents.

Why is HIF suddenly a big deal? It's the key to a different way of looking at cancers.