Mean dose factors have been calculated for those tissues in bone that are relevant to the induction of late irradiation effects; namely, red bone marrow and endosteal tissues lining trabecular bone surfaces. The calculations are based on a Monte Carlo computer method for beta-emitting radionuclides which are distributed uniformly throughout the volume of mineralized bone. Results are given for the radionuclides 14C, 45Ca, 22Na, 18F, 32P, 90Y and 90Sr+90Y (all considered for this purpose as volume-seekers) for seven bones from the adult human skeleton and calculations are also presented of average skeletal dose factors. Where possible, results are compared with those derived from other methods based on simple geometrical models.

Based on calculations by Whitwell and Spiers, mean skeletal dose factors for beta-particle emitters have been derived for application to dosimetry in nuclear medicine, radiobiology and radiation protection. The dose factors apply to bone-seeking radionuclides that are depostited more or less uniformly throughout the human mineralized skeleton and are defined as: (1) vDM/DB, the mean dose to the haemopoietic marrow in trabecular bone as a fraction of the "dose to bone", DB; (2) vDs/DB, the mean dose to endosteal (osteogenic) tissue lying in a zone 0--10 micron from trabecular surfaces, also as a fraction of the dose DB. Dose factors are given for three ages, 1.7, 9 and 44 years and for eight radionuclides, ranging in mean beta-particle energy from 0.05 MeV (14C) to 0.93 MeV (90Y). Dose factors, calculated for a single lumbar vertebra of an infant aged 3.5 weeks, enable approximate values of mean skeletal dose factors to be estimated for ages less than 1.7 years. Limited data are also reported on dose factors for the femoral cortex from subjects aged 9 and 50 years.

The method of calculating dose factors for the haemopoitic marrow and endosteal tissues in human trabecular bone, used by Whitwell and Spiers for volume-seeking radionuclides, has been developed for the case of radionuclides which are deposited as very thin layers on bone surfaces. The Monte Carlo method is again used, but modifications to the computer program are made to allow for a surface rather than a volume source of particle emission. The principal change is the introduction of a surface-orientation factor which is shown to have a value of approximately 2, varying slightly with bone structure. Result are given for beta-emitting radionuclides ranging from 171Tm (E (E beta=0.025 MeV) to 90Y(E beta = 0.93MeV), and also for the alpha-emitter 239Pu. It is shown that where the particle ranges are short compared with the dimensions of the bone structures the dose factors for the surface seekers are much greater than those for the volume seekers. For long range particles the dose factors for surface and volume-seeking radionuclides converge. Comparisons are given relating the dose factors calculated in this paper on the basis of measured bone structures to those of other workers based on single plane geometry.

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.