EPR DOSIMETRY OF CORTICAL BONE AND TOOTH ENAMEL IRRADIATED WITH X-RAYS AND GAMMA-RAYS - STUDY OF ENERGY-DEPENDENCE

Previous investigators have reported that the radiation-induced EPR si gnal intensity in compact or cortical bone increases up to a factor of two with decreasing photon energy for a given absorbed dose. If the E PR signal intensity was dependent on energy, it could limit the applic ation of EPR spectrometry and the additive reirradiation method to obt ain dose estimates. We have recently shown that errors in the assumpti ons governing conversion of measured exposure to absorbed dose can lea d to similar ''apparent'' energy-dependence results. We hypothesized t hat these previous results were due to errors in the estimated dose in bone, rather than the effects of energy dependence per se. To test th is hypothesis we studied human adult cortical bone from male and femal e donors ranging in age from 23 to 95 years, and bovine tooth enamel, using 34 and 138 keV average energy X-ray beams and Cs-137 (662 keV) a nd Co-60 (1250 keV) gamma rays. In a femur from a 47-year-old male (su bject I), there was a difference of borderline significance at the alp ha = 0.05 level in the mean radiation-induced hydroxyapatite signal in tensities as a function of photon energy. No other statistically signi ficant differences in EPR signal intensity as a function of photon ene rgy 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 intens ity at the lowest energy compared with the highest energy in subjects 1 and 3. Further analysis of the data from subject 1 revealed that thi s trend, which is in the opposite direction of previous reports but is consistent with theory, is statistically significant. There were no e ffects of energy dependence in the tooth samples.