Radiation therapy and chemotherapy aimed at killing cancer cells may have the undesirable effect of helping to create cancer stem cells, which are thought to be particularly adept at generating new tumors and are especially resistant to treatment, researchers say.

The finding might help explain why late-stage cancers are often resistant to both radiation therapy and chemotherapy, and it could point to new strategies to fight tumors.

Past studies hint that cancer stem cells give rise to new tumors. Researchers suggest they are ultimately responsible for the recurrence of cancers and the dangerous spread of a cancer throughout the body. Scientists also have found that cancer stem cells are more likely than other cancer cells to survive chemotherapies and radiation therapies, probably becausetheir "stemness" allows them to self-replenish by repairing their damaged DNA and removing toxins.

The exact origin of cancer stem cells is debated. One possibility is that normal stem cells — which are valued for their ability to give rise to other cell types in the body — mutate to become cancerous. Another is that regular cancer cells somehow acquire stem cell properties.

The new study suggests regular cancer cells can indeed give rise to cancer stem cells, and that the radiation commonly used to treat cancer can trigger their stemness.

"Radiotherapy has been a standard treatment for cancer for so long, so we were quite surprised that it could induce stemness," said study researcher Dr. Chiang Li, of Harvard Medical School in Boston.

The scientists exposed regular cancer cells to gamma-rays, one form of ionizing radiation. They found that under the conditions that normally foster stem cell growth, regular cancer cells formed balls of cells — a hallmark of cancer stem cells.

Additionally, analysis of these irradiated cancer cells revealed activity of genes linked with stem cell behaviors, according to the findings the scientists detailed online Aug. 21 in the journal PLoS ONE.

Chemotherapy may have similar effects, according to previous findings that Li and his colleagues detailed in July in the journal Cell Cycle.

"So radiation and chemotherapy not only might create cancer stem cells, any pre-existing cancer stem cells in a tumor were veryresistant to radiation and chemotherapy, so they remain as well," Li said. "This could help explain why these therapies are sometimes not as effective as we might hope."

Li cautioned these lab findings might not prove relevant in patients in real life. "This was all carried out in the petri dish," he said. "There is a long way we have to go before we can be sure about its clinical implications for patients, if any."

Still, this research suggests that if scientists find a way to inhibit stemness in cancers, radiation therapy and chemotherapy then might cleanly finish off tumors.

"There are lots of projects both in academia and industry right now to develop cancer stem cell inhibitors, although those are still in early stages," Li said.