CONTINUOUS, LOW DOSE IRRADIATION RESULTS IN NO DETRIMENT TO BONE AFTER 4, 12, AND 28 DAYS

As the focus shifts to potential Lunar and Mars missions, there are many aspects of the space environment that are still unknown, one of those being radiation exposure. There are many radiation studies examining the effects of acute, high dose radiation on bone; however, it has been discovered that the total exposure astronauts will experience will be much lower than the doses these studies have utilized. The purpose of this study was to determine the effect of a continuous, low-dose irradiation exposure on bone at various time points. Sixty-six male C57Bl/6 mice, aged 47-49 weeks, were placed in the continuous radiation field for either four, twelve, or twenty-eight days. The radiation field was created by locating four activated ^60Co wires emitting gamma radiation around a mouse rack in a lead-shielded room. At a dose rate of 6.49 mGy/day, the total cumulative dose was approximately 25.6 mGy for four days, 77.88 mGy for 12 days, and 181.72 mGy for 28 days. Fluorochrome calcein labels were injected to obtain bone formation rate post termination. DXA scans were performed prior to radiation exposure and again just before termination to determine bone mineral density and body composition. At termination, tibiae and femurs were collected from all animals for histomorphometry assessments. Also, osteocyte apoptosis was quantified in cancellous bone via staining for Annexin V. Continuous radiation exposure at this low dose rate did not have any impact on total body bone mineral density, lean mass, fat mass, and percent fat at any time point. There were also no differences in cortical or cancellous bone formation rate at any time point in radiation-exposed mice vs. control animals. Also, bone volume and trabecular microarchitecture were not affected by radiation at any time point. Osteoid and osteoclast surfaces were not different after radiation at any time point, nor were there any differences in osteocyte apoptosis at any time point examined. These results are quite different from those seen after acute, high-dose radiation exposures, after which decreased bone volume, depressed bone formation rate, and increased osteoclast surface are often observed. This data begins to elucidate a time course of continuous low dose radiation impacts on bone, similar to radiation that might be experienced during transit to Mars and alludes to the potential that it may not be deleterious to bone health, as previously assumed