Study upgrades protons' risk to DNA.

Proton radiation--beams of hydrogen nuclei--causes worse breaks in DNA than researchers had expected, a study finds.

This radiation "will not be good for astronauts," says Betsy M. Sutherland of Brookhaven National Laboratory in Upton, N.Y. NASA funded her work to evaluate risks to travelers in space, where they will encounter proton radiation from the sun and exploding stars. The new findings, however, support the use of protons' cell-killing capability in cancer therapy.

Radiation can induce cancer or cause cell death by ripping DNA with a blast of energy. Because radiation sources vary greatly in their energy-release profiles, biologists rank them by their linear energy transfer (LET).

High-LET radiation, such as ionized iron particles, releases copious energy as it passes through cells. Often, it cleaves both strands of a DNA molecule, creating damage that cells find almost impossible to repair. In contrast, low-LET radiation, such as X rays, unloads little DNA-damaging energy as it zips through a cell. If an X-ray photon hits DNA, it typically cuts only a single DNA strand, a break that a cell can often repair.

Protons have a low LET, so scientists expected them to behave as X rays do in their DNA-damaging capability, notes Sutherland. "But they didn't,' the radiation biologist says. "Their damage looked more like that of [heavy] charged particles, even though their LETs were wildly different."

She and Megumi Hada extracted DNA from cells and irradiated it with different types of photon sources and charged particles. In the February Radiation Research, they report that protons create large numbers of predominantly double-strand DNA breaks.--J.R.