5. Areas of Difference between the Methods ICRP uses 12m2 as the surface area for endosteal tissue. equally divided between trabecular This is and cortical bone and the absorbed fraction is assumed to be the same in each, (ICRP 30, Part 1.) I have not Spiers uses 16m2 for the surface area of endosteal tissue. been able to determine what the distribution cortical bone (ref. 2 and 3). is between trabecular The dose factor for trabecular and endosteal cells is 3.11 while that for cortical endosteal cells is 50% higher at 4.7. could reflect a difference trabecular in the distribution and cortical bone. This of the endosteal mass between Spiers was the absorbed fraction for Pu rather than an average value for alpha emitters as used by ICRP and this would make some difference. Spiers does make the point that cortical endosteal tissue may not be at as high a risk per unit dose as the trabecular endosteal tissue. I think that the dose rates Spiers lists in reference 1 of 129 rad/UCi-y for trabecular endosteal cells and 193 rad/~Ci-y for cortical endosteal cells (compared with 16.9 rad/~Ci-y for bone marrow) are wrong. Based on these numbers, the trabecular endosteal cell ratio to bone marrow is 7.6 while that for cortical endosteal cells is 11.4. This is inconsistent with his published dose factor ratios of 0.26 for marrow, 3.11 for trabecular surface cells and 4.7 for cortical surface cells which give ratios of surface cells to marrow of 12 and 18 respectively. However, as I mentioned before, we use a dose conversion of 338 rem/uCi-y to develop the bone marrow dose and multiply by a factor of 12 to get the surface cell dose.