Exposure-to-absorbed-dose conversion for human adult cortical bone.

The conversion of measured exposure to absorbed dose at a point in bone, under conditions of electron equilibrium, involves a factor (the f-factor) which is proportional to the ratio of the spectrum-averaged photon energy-absorption coefficient for bone to that for air. This paper gives mass energy-absorption coefficients and f-factors for three compositions of human adult compact or cortical bone recommended in publications by the ICRU and the ICRP, for photon energies from 1 keV to 1.5 MeV. Spectrum-averaged f-factors for a number of calibration x-ray beams ranging from 10 to 250 kVp have been calculated and compared to corresponding results obtained with the use of an equivalent photon energy derived from the measured thickness of the half-value layer. At low photon energies (approximately less than 200 keV), the new f-factor results reflect: (a) the rather large differences due to the differing calcium contents among the recommended compositions for bone: and (b) the generally poor predictions obtained when replacing a broad energy spectrum by an equivalent photon energy.