Inside CERN's cavernous halls near the Swiss-French border, physicist Walter Wuensch and his team are repurposing particle accelerators originally built to unlock the universe's deepest mysteries. Their new target: cancer. The French company Theryq is collaborating with CERN to develop FLASH radiation therapy, a technique that delivers ultrahigh-power radiation in bursts lasting less than one-tenth of a second.

Unlike conventional radiation therapy that requires dozens of sessions and damages healthy tissue, FLASH delivers a single dose of 10 gray or more while causing significantly less injury to normal tissue. This counterintuitive approach emerged from experiments at Institut Curie in the 1990s, where researchers discovered that ultrafast radiation blasts could eradicate tumors in mice without the expected scarring. The findings, published in 2014, initially met skepticism from cancer experts who questioned how healthy tissue could be protected.

At CERN's Linear Electron Accelerator for Research facility, researchers are adapting accelerators to boost electrons to ultrahigh power within short distances. The key advantage is that high-energy electrons can be switched on and off within milliseconds and precisely guided to reach tumors deep within the body. While the mechanism behind FLASH's protective effect remains unclear, the most plausible theory involves how healthy and cancerous cells process reactive oxygen species differently. If ongoing experiments continue to bear out results, FLASH could transform radiotherapy by delivering stronger treatments with fewer side effects and broader access to lifesaving care.