t
Ur
113
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natirlichen Thoriumreihe. Ph.D. Thesis, Homburg/
Saar, 1964.
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Mass Spectrometry, Ed. M. L. Smith. Butterworths,
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7
38. Marinelli, L. D. IAEA-WHO Panel Meeting on Dosimetry
and Toxicity of Thorotrast, Vienna, October 1965. Int.
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tope. In general, it is necessary to distinguish three
types of atom; those which enter the circulation in the
injection materials (tvpe I), those which grow77vivo as
daughters of injected atoms (type II), and those which
grow in vivo as granddaughters of the injected atoms
{type III). (In the present context, since we are dealing
with growth and decay on a time scale long compared
to the half-life of Ac, the decay chain ?°Th:*%Ra:°%Th
is visualized as a_ parent:daughter: granddaughter
series). At the time of injection, therefore, all the
2Th, “Ra, and “*Th atoms are of type I. At anylater
time, ?%Th atoms are of types I, IT, and III, *8Ra atoms
are of types I and II, and all the °®Th atoms remain
of type I.
For a Thorotrast body burden of very long standing
(sufficiently long for essentially all the 8Ra atoms of
type I to have decayed away), we can write the follow-
ing for the activities in any tissue sample and in the
whole body. (See Table 53.)
TABLE 53.
ActTiviTtes FoR 4 LONG-STANDING THOROTRAST
Bopy BurpEN
22Th
(type I)
Tissue sample activity
Whole body activity
Tissue/whole body
ad»
Ag
a
!
28Ra
(type IT)
228Th
(type ITI)
Filado]
fidg
afs/fi
frlfiaAo]
felfidol
afifo/fife
Aa is the activity of ™?Th in the whole body (equally the
activity of “2Th in the total injection material) and a is the
fraction of this administered material contained within the
tissue sample.
These expressions define the quantities f; and f: for
the tissue sample, and f, and f2 for the whole body, as
F 41. Rotblat, J. and Ward, G. B. Nature 172, 769 (1953).
the steady state activity ratios which it is desired to
calculate. It follows that, of all the ?®Ra atoms born
within the entire body which have not undergone decay
F APPENDIX
within the body; and amongst the different tissues these
Tech. Report 106, p. 79.
Calculation of the Steady State Activity Ratios
( SRA/27H) and (Th/*™Ra) in vivo
‘. rigorous mathematical treatment of the #*Ra and
““Th activities 2 vivo is complicated by the fact that
Thorotrast is a nonhomogeneous material in which the
+ radioactivity is probably distributed throughoutseveral
different. physical phases in proportions whichare differcnt for each nuclide. For this reason alone, and without
tesird to differences in physical half-life, it cannot be
‘issumed that different isotopes of the same element will
‘ow the same pattern of tissue distribution in vivo
F (. ™8Th and 2®Th, or 2*Ra and 2%Ra). Moreover, a
thorough analysis of the problem even demands that
f ‘“ctinetions be drawn between atoms of the same iso-
by the time of sampling, a fraction f; remains then
retained atoms are distributed in the proportions afi//t .
Similarly for “®Th, f, measures the fraction retained in
the whole body, and afif2/fif2 measures the distribution
of the retained atoms amongthedifferent tissues.
This discussion refers to a Thorotrast burden of such
long standing that none of the type I **Ra atoms and
none of the types I and II ?8Th atoms survive. At
earlier times it is obviously necessary to consider the
fates of theRa andTh atoms contained within the
administered Thorotrast. For #*Ra it will be assumed
that all type I atoms distribute themselves in the same
proportions af,/f, among the different tissues as do the
type II 28Ra atomsreferred to in Table 53 and that the
whole-body retention is f,. Similarly, the 28Th atoms
of type IT will be assumed to distribute themselves in the
pb ay