There is a new nuclear arms race under way — in hospitals.

Medical centers are rushing to turn nuclear particle accelerators, formerly used only for exotic physics research, into the latest weapons against cancer.

Some experts say the push reflects the best and worst of the nation’s market-based health care system, which tends to pursue the latest, most expensive treatments — without much evidence of improved health — even as soaring costs add to the nation’s economic burden.

The machines accelerate protons to nearly the speed of light and shoot them into tumors. Scientists say proton beams are more precise than the X-rays now typically used for radiation therapy, meaning fewer side effects from stray radiation and, possibly, a higher cure rate.

“I’m fascinated and horrified by the way it’s developing,” said Dr. Anthony L. Zietman, a radiation oncologist at Harvard and Massachusetts General Hospital, which operates a proton center. “This is the dark side of American medicine.”