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Effects of Low-Energy X-rays on Trabecular Bone Loss in Mice.  L.C. Smith¹, T.A. Bateman¹. ¹Department of Bioengineering, Clemson Univ., Clemson, SC.

   Radiation therapy for cancer patients involves exposure to photon (gamma/x-ray), electron, and, less commonly, proton radiation. Astronauts on exploratory missions are also exposed to extended periods of lower-dose radiation of different energies from multiple sources and of multiple types. It has been previously demonstrated that a single 2 Gray (Gy) dose of various clinical and spaceflight relevant radiation types causes significant trabecular bone loss in a murine model. However, the current model only accounts for higher-energy radiation sources. This study presents data expanding the model to evaluate bone loss induced by low-energy photon radiation. Ten-week-old, female C57BL/6 mice were exposed to whole body, low-energy x-ray radiation at doses of 0, 2, 4, and 6 Gy to determine the minimum exposure required to achieve bone loss. Two weeks after irradiation, the mice were sacrificed and the hindlimbs collected.  MicroCT analysis was performed on the trabecular bone of the proximal tibia. Significant (p<0.01) loss in trabecular bone fraction (BV/TV) was observed in all dose groups when compared to control: 2 Gy, (-42%), 4 Gy, (-39%), and 6 Gy, (-44%). No differences were observed between dose groups. Connectivity density, trabecular number, and trabecular spacing were also affected. In follow-up experiments, mice were either 1) irradiated with a 2 Gy dose of x-rays and sacrificed 3 days post-exposure or, 2) given an IP injection of Zoledronate (5mg/kg) immediately proceeding exposure to a 2 Gy dose of x-rays and sacrificed after 2 weeks. Preliminary analysis indicated a higher BV/TV, connectivity density, and trabecular number in mice administered zoledronate when compared to those receiving radiation alone. BV/TV was the only factor affected in the mice sacrificed after 3 days.  Further analysis of these data is under way to better quantify the effects of low-energy photon radiation on trabecular bone. Additional studies are planned to evaluate the effects of age, strain, gender and time dependency. (Supported by NSBRI through NASA NCC9-58, Procter and Gamble Pharmaceuticals and South Carolina Space Grant Consortium/NASA EPSCoR)