What do orbiting astronauts and cancer patients have in common? They're both bombarded with low levels of ionizing radiation. Now, a new study finds that such radiation can destroy a large amount of the "spongy" part of bones in mice, potentially increasing the risk of fracture.
Bioengineer Ted Bateman of Clemson University in South Carolina, has long been intrigued by bone weakening in space. For a while now, scientists have known that high doses of solar radiation and limited gravity can contribute to bone loss, but no one had looked at the effects of lower doses of radiation, says Bateman. In addition, low doses of therapeutic radiation are known to weaken the bones of cancer patients--they tend to have more fractures than the average person, for example--but just how this radiation leads to bone loss was unclear.
To get a better idea of what was going on, Bateman and colleagues zapped four groups of mice with different types of radiation. One group of 10 mice got a 2 Gray (Gy) dose of gamma radiation (equivalent to a daily therapeutic dose for a cancer patient); another group received 2 Gy of proton radiation (what an astronaut on the moon might experience during a solar flare); and two more groups got 2 Gy of either ion or carbon radiation (about twice the dose an astronaut would get during a 2-and-a-half year Mars mission).
The team then waited 110 days and sacrificed the mice. Using micro-computed tomography--a 3-D imaging technique that reveals inner bone structure--the researchers found that, compared to nonirradiated controls, the gamma group had 29% less spongy bone tissue, an essential component for structure and stability. The proton, ion, and carbon groups had about 34% to 39% less spongy tissue than did controls. Furthermore, each radiated group lost anywhere from 46% to 64% of the spongy connections that help bones support weight.
This dramatic weakening of bone architecture is surprising, says Bateman, especially because it results from a single dose of low-level radiation. Given the magnitude of this effect, he says, "it may be appropriate to shield bone or slightly modify [cancer] treatment regimens to reduce the increased fracture risk." As for astronauts, Bateman says scientists should further investigate the risk of bone damage from low-dose radiation. The team reports its results online today in the Journal of Applied Physiology.
Bioengineer Stefan Judex of Stony Brook University in New York notes that because Bateman's group used young mice "that had not yet reached peak bone mass," their bones might be more sensitive to radiation than the bones of mature animals. Still, he says, "the study is important in the way we think about the danger of radiation in space and on Earth."