Early changes in bone mineral content caused by acute irradiation are not observed after daily fractionation: A study by dual-energy X-ray absorptiometry

Abstract

To reveal the effects of irradiation on bone metabolism in terms of time course and the effects of fractionation, early changes in bone mineral content (BMC) in rat tibiae were examined directly after irradiation. The left hind limbs of retired Wistar rats were exposed to a total dose of 60 Gy X-rays either acutely or 2.5 Gy daily fractionately. At 1–45 days (acutely irradiated) or 1–168 days (fractionately irradiated) later, the tibiae were removed and BMC was measured by dual-energy X-ray absorptiometry (DXA). On the computer screen tibiae were divided into four equal parts and the BMC in each was measured. Results were expressed as ratios of BMC in irradiated to nonirradiated tibiae for all parts. At 1–11 days after acute irradiation, changes in BMC ratios were not statistically significant (1.00±0.01 to 1.03±0.01,P>.05, respectively) in the upper part of the tibiae. At 16 days the ratio was 1.09±0.01 (P=.0001) and became 1.12±0.01 (P <.0001) and 1.16±0.02 (P<.0001) 22 and 45 days after irradiation, respectively. There were no statistically significant changes in BMC ratios in the remaining parts of the tibiae. Fractionated irradiation clearly resulted in no significant changes in BMC ratios. The threshold dose to initiate an increase in BMC on the 22nd day after acute irradiation was 20 Gy. These data suggest that early increase in mineral content, which is caused by a single large-dose of irradiation, can be delayed by daily fractionations.