Age and microdistribution of 210Pb at caribou bone surfaces measured by repeated alpha spectroscopy of 210Po.

Purpose: To measure microdistibution and age of 210Pb in Canadian caribou bone, and assess the accuracy of ICRP biokinetic and dosimetric models for 210Pb-supported 210Po at bone surfaces.

Materials And Methods: The previously described use of alpha spectroscopy to study 210Pb microdistribution at bone surfaces was expanded in a study of femora from 23 caribou. A new technique for measuring the age in vivo of Pb in bone, on the basis of the ratio of 210Po/210Pb activity, was carried out on eight caribou femora.

Results: The measured thickness p of the layer containing elevated concentration of 210Pb at femoral endosteal surfaces was 0.79 +/- 0.52 microm (SD). This thickness increased linearly with age in caribou aged 2-11 years. The surface/volume concentration ratio of 210Pb (As/Av) varied from 1 to 18, geometric mean 3.8. No trend in As/Av with caribou age was observed. In the upper 1 microm/210Pb was 17 +/- 22 days old, and at depths of 1-22 microm 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 to tissue above endosteal surfaces is reduced three times due to the low ingrowth ratio of 210Po.