Mean skeletal dose factors for beta-particle emitters in human bone. Part II: surface-seeking radionuclides.

Mean skeletal dose factors for surface-seeking beta-emitting radionuclides have been derived on the basis of the beta-particle dose factors calculated for human bones by Spiers et al. (1978b), or from empirical formulae representing these calculations given by Beddoe and Spiers (1979). The dose factors are derived for the same tissues as in Part I (Spiers et al., 1978a), namely, (1), sDM/Dskel, the mean dose to haemopoietic marrow in trabecular bone, and (2), sDs/Dskel, the mean dose to endosteal tissues lying in a zone 0-10 microns from trabecular surfaces. The dose factors are given as fractions of Dskel, the "dose to bone", calculated from the energy released by the retained radionuclide divided by the total mass of mineral bone. As in Part I, the dose factors are given for three ages, 1.7, 9 and 44 years and extrapolation to lower ages is made on the basis of calculations for a lumbar vertebra of an infant aged 3.5 weeks. The factors are given for six radionuclides covering a range of mean beta-particle energies from 0.0255 MeV (171Tm) to 0.93 MeV (90Y). The dosimetry is also considered for some radionuclides that first deposit on bone surfaces before being translocated to the bone volume. In the case of 45Ca, for example, it is shown that the dose calculation that includes both the surface and the volume depositions gives integrated doses to ten days and 100 days that are respectively 4.5 and 1.9 times the corresponding values calculated conventionally on a volume deposition only. Endosteal dose factors are similarly considered, and other radionuclides included as surface-plus-volume depositions are 47Ca, 89Sr and 90Sr + 90Y.