Microdosimetry for leukaemogenic target cells for bone-incorporated alpha-emitting radionuclides

Neutron-induced autoradiographs of femoral bone shafts taken from mice injected with 40 kBq.kg^{-1 239}Pu, ²⁴¹Am or ²³³U, 1, 28 or 224 days previously, were analysed morphometrically in order to present a comparison of the changing radionuclide distribution patterns. Microdosimetric methods were used to calculate localised dose rates throughout the bone marrow, and hence to those cells thought to be target cells for leukaemia induction. Initially all three radionuclides were found to be primarily deposited on the endosteal and periosteal surfaces of the bone. ²⁴¹Am was additionally deposited on the surfaces of the vascular canals. ²³⁹Pu subsequently became either buried in the bone matrix or resorbed into macrophages, which by 224 days were seen to have migrated towards the central venous sinus. ²⁴¹Am deposits were also present in macrophages, but to a lesser extent. In contrast uranium was not taken up into macrophages. Mean dose rates to the axial region of marrow, believed to contain the leukaemogenic target cells, were calculated for plutonium as 0.11, 0.07 and 16.78 mGy.d^-1 and for americium as 0.23, 5.19 and 2.28 mGy.d^-1 at 1, 28 and 224 days respectively. For uranium a dose rate of zero was calculated at all time points.