EPR dosimetry of cortical bone and tooth enamel irradiated with X and gamma rays: study of energy dependence.

Previous investigators have reported that the radiation-induced EPR signal intensity in compact or cortical bone increases up to a factor of two with decreasing photon energy for a given absorbed dose. If the EPR signal intensity was dependent on energy, it could limit the application of EPR spectrometry and the additive reirradiation method to obtain dose estimates. We have recently shown that errors in the assumptions governing conversion of measured exposure to absorbed dose can lead to similar "apparent" energy-dependence results. We hypothesized that these previous results were due to errors in the estimated dose in bone, rather than the effects of energy dependence per se. To test this hypothesis we studied human adult cortical bone from male and female donors ranging in age from 23 to 95 years, and bovine tooth enamel, using 34 and 138 keV average energy X-ray beams and 137Cs (662 keV) and 60Co (1250 keV) gamma rays. In a femur from a 47-year-old male (subject 1), there was a difference of borderline significance at the alpha = 0.05 level in the mean radiation-induced hydroxyapatite signal intensities as a function of photon energy. No other statistically significant differences in EPR signal intensity as a function of photon energy were observed in this subject, or in the tibia from a 23-year-old male (subject 2) and the femur from a 75-year-old female (subject 3). However, there was a trend toward a decrease (12-15%) in signal intensity at the lowest energy compared with the highest energy in subjects 1 and 3. Further analysis of the data from subject 1 revealed that this trend, which is in the opposite direction of previous reports but is consistent with theory, is statistically significant. There were no effects of energy dependence in the tooth samples.

Estimation of the absorbed dose in radiation-processed food--3: The effect of time of evaluation on the accuracy of the estimate.

Electron paramagnetic resonance (EPR) spectrometry is evaluated as a method to retrospectively assess the absorbed dose to radiation-processed chicken (containing bone). Decay of the hydroxyapatite paramagnetic center EPR signal intensity was monitored at three different dose levels (0.5, 3.0, 7.0 kGy) up to 20 days, and the dose was assessed for each level at 1, 8, and 20 days after irradiation. It was determined that the time of evaluation (up to 20 days post-irradiation) did not adversely affect the estimate for 0.5 and 3.0 kGy bone, and only moderately affected the 7.0 kGy estimates.