AGE AND MICRODISTRIBUTION OF PB-210 AT CARIBOU BONE SURFACES MEASUREDBY REPEATED ALPHA-SPECTROSCOPY OF PO-210

Purpose: To measure microdistribution and age of (210)pb in Canadian c aribou bone, and assess the accuracy of ICRP biokinetic and dosimetric models for Pb-210-supported Po-210 at bone surfaces. Materials and me thods: The previously described use of alpha spectroscopy to study Pb- 210 microdistribution at bone surfaces was expanded in a study of femo ra from 23 caribou. A new technique for measuring the age in vivo of P b in bone, on the basis of the ratio of Po-210/Pb-210 activity, was ca rried out on eight caribou femora. Results: The measured thickness rho of the layer containing elevated concentration of Pb-210 at femoral e ndosteal surfaces was 0.79 +/- 0.52 mu m (SD). This thickness increase d linearly with age in caribou aged 2-11 years. The surface/volume con centration ratio of Pb-210 (A(s)/A(p)) varied from 1 to 18, geometric mean 3.8. No trend in A(s)/A(t) with caribou age was observed. In the upper 1 mu m/Pb-210 was 17 +/- 22 days old, and at depths of 1 - 22 mu m the age was 146 +/- 17 days. Thus Pb at the bone surface was about nine times younger than in underlying bone. Conclusions: Modifications to the ICRP 67 biokinetic model for Pb are proposed. The alpha dose t o tissue above endosteal surfaces is reduced three times due to the lo w ingrowth ratio of Po-210.