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."