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.