VoL. XXV-.I, No. 316
James S. Robertson and fohn T. Godwin
blood. This figure is strongly dependent upon the
value used for the relative volumeof the trabeculae
in the marrow.
Some ofthe discrepancies which appear in the
response of the hematopoietic tissues to large doses
of 131{ and other agents may be explained by the
absorption of some of the f energy in bone. Bloom
The next factor to be considered is the density of
the bony trabeculae. These are here assumed to have
the sam: composition as that reported for compact
bone (Koch, 1917; Mack, Brown and Trapp, 1949;
Strobino and Farr, 1949). Compact boneis about 70
per cent ash and theashis chiefly Ca,(PO,).. Koch’s
(1917) values for the specific gravity of compact
bone average 1-955, and he quotes other studies
giving results in the same range.
(1948, p. 14) reports that for mice the LD,,/30 days
for 24Na was found to be 30 ye per g, which corresponds to a f-radiation dose of 810 rep., whereas the
LD,,/30 days with X rays on mice was about 530 r.
The calculations for many other agents are compli-
The rate of energy loss per unit of path length for
8 particles depends uponthe velocity of the particle
and upon the electron density of the absorbing
medium. In practice, the range of § particles is
cated by peculiarities of their concentration in bone.
For the very weak emitters, in particular tritium,
the range in the marrow is so short that relatively
little energy is lost by absorption in bone.
Only the § contributions to the dose have been
considered in the above discussion. The y contribu-
expressed in units of areal density, mg/cm?. Decay
of ![ occurs through the emission of one of four
B particles followed by a variety of y rays (Bell and
Graham, 1952). In 87-2 per cent of the disintegra-
tions the 8 has a maximum energy of 0.608 MeV.
The balance of the disintegrations involve maximum
tion may be important too, and may be especially
high if there are iodine-concentrating metastases in
or near the hematopoietic tissues (Rallet a/., 1953).
B energies of 0-812, 0-335 and 0-250 MeV. The
average energy for #1I § particles is about 0-200
MeV (Marinelli et al., 1948; Rall et al., 1953).
In water, the range of the 0-608 MeV particles 1s
about 200 mg/cm? (Glendennin, 1948), or about
2mm, while the range for particles at the average
energy is about 0-4 mm. The corresponding ranges
in bone of density 2.0 are 1 and 0-2 mm. Representative thicknesses of the trabeculae in the fields
studied ranged from 0-04 to 0-25 mm, while
representative distances between trabeculae ranged
from 0-20 to 0-70 mm, so that the paths of most of
the f particles are partly in bone. An exact estimate
of the proportion of energy absorbed in bone would
involve a very complicated analysis of the geometry
involved, but for present purposes it may be assumed
that the division of energy absorption between bone
and soft tissue in the marrow is proportional to the
products of the volumes and densities of the two
phases.
If the bony trabeculae occupy 12 per cent of the
marrow volume as found above, and if the bone
density is taken as 2.0, 21-4 per cent of the mass of
the material in the marrow is bone and about 21 per
cent of the B energy will be absorbed in bone
according to the above assumptions. This means
that the radiation dose which affects the soft tissues
in the marrow is about 79 per cent of the dose
calculated for blood if the concentration of radioactive iodine in the marrow is equal to that in the
ACKNOWLEDGMENT
This research was done under the auspices of the U.S.
Atomic Energy Commission.
SUMMARY
1. The area occupied by bony trabeculae in representative
fields of the bone marrow from human ribs and vertebrae
was found to average 12 per cent ofthe total area.
2. The fraction of the1 8-particle energy absorbed by
bony trabeculae in the bone marrow is estimated as 20 per
cent of the energy released in the marrow.
3. If the concentration of radioactive iodine in bone
marrow equals that in the blood, the §-radiation dose
delivered to the marrow is about 20 per cent less than the
dose delivered to the blood.
REFERENCES
Bett, R. E., and Granam, R. L., Phys. Rev., 1952,
Ixxxvi, 212.
BLoom, W., Histopathology of Radiation, 1948 (McGrawHill, New York).
GLENDENNIN, L. E., Nucleonics, 1948, ii, 1, 12.
Kocn, J. C., Amer. Journ. Anat., 1917, xxi, 177.
Mack, P. B., Brown, W. N., and Trapp, H. D., Amer.
Journ, Roentgenol., 1949, ixi, 808.
MARINELLI, L. D., and Hitt, R. F., Radiology, 1950,
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MarRInELLI, L. D., Quimpy, E. H., and Hine, G. J.,
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Rati, J. E., Foster, C. G., Roseins, J., Lazerson, R.,
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Sripiin, S. M., Yarow, A. A., and Srecer, E., Journ.
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Strosino, L. J., and Farr, L. E., Journ. Biol. Chem.,
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