99
owed by y-ray spectrometry over the course of two or
Jyree weeks from the time of sampling,* and from the
janicilyls of these data, the “zero-time” activites of
2-Pu, 8Ac, “Th, **4Ra, 742Pb, and 7!*Bi, were eal-
njated The activity of the parent isotope, 7*°Th, was
dicermined at a later time from a measurement by
up to 1.64
e-: a¥ spectrometry in a Frisch grid ionization cham-
bir. A more complete description of these analytical
techniques can be found elsewhere.'’ By these means,
k)
e-timates were obtained of the in vivo activities of the
above-mentioned thorium-series nuclides at the mo-
ment that biological activity ccased, i., generally at
the moment of death.
‘The full utilization of these techniques required that
meusurements of the y-ray spectra be commenced
others
nein
within an hour of the time of sampling. For obvious
practical reasons this was not always possible, and
hence the results for the shorter-lived daughter products, particularly ?!2Bi, are not as extensive as for the
other radionuclides.
f this inNormalization of the Activity Measurements
ples from: The activities of the various thorium decay prodreceived ucts in a tissue sample at the time of measurement are
F
reviously, determined by two quite distinct processes. Firstly, in
were ob-¢ tl living animal, there is the process of selective
ration of ptrunslocation of each respective nuclide from one organ
ie Thoro- to unother or to the excreta. This may loosely be
7 difficult termed a “biological process,” though it may depend
umber off on nothing more fundamental than the circulation of
ter death ithe blood, However, the important point is that it
products) ceuses to act at the moment of death, or, for a biopsy
the early sumple, at the time at whichit is withdrawn. Secondly,
of long- there 1s the purely physical process of radioactive
B
growth and decay, which continues regardless of
»Whether the tissue sample exists zn vive or in vitro. It
y through ix the first of these two processes that we wish to underrast. Se- stand, and the second is merely a physical phenometly afterg non that complicates the calculations and interpreta: tion of the measurements.
The method of measurement described above allows
Jt correction te be applied for radioactive growth and
decay occurring between the time of sampling and the
tine of measurement. However, this alone is not sufficient, In order to deduce correctly from the radioac(tivity data some measure of the mobility of each decuy product within the body, it is necessary to take
arcount of radioactive growth and decay in vivo be{ween the time of injection and the time of sampling.
pA hormalization procedure is, therefore, sought by
Wich the steady state activity ratio for any daughter-
-\lso referred to as “zero-time,” meaning in the case of
: 4 “opsv samples, the time of death, and in the case of biopsy
vere fol; Suples, the time of removal of the tissue from the body.
parent pair can be predicted from measurements at
any time after injection, on the assumption that biological processes continue at an unchanged rate. Such
a normalization allows a more meaningful comparison
of biological transport parameters to be made in patients or animals injected with different batches of
Thorotrast, or examined at different times after injection.
For the shorter-lived nuclides, **8Ac, 7?*Ra, 7!°Pb,
and *!°Bi, the normalization procedure presents no
difficulty. The activity ratio for any daughter-parent
pair reaches a steady state value (within <2%) in
about 6 half-lives of the daughter product concerned.
Therefore, the steadystate activity ratios, 77°Ac/**5Ra,
*=4Ra/*28Th, 7°Pb/**4Ra, and 7!°Bi/*!2Pb, are given
directly by the ratios of the observed activities, pro-
vided that the Thorotrast burdenis of at least 3 weeks’
standing (i.e., 6 half-lives of **4Ra).
In the cases of 7°8Ra and 7°Th, the problem is more
complicated. Jn vitro, as may be seen from Figure 84,
their activities relative to *8°Th eontinue to change
significantly until about 50 years have elapsed from
the time of maufacture of the Thorotrast. Moreover,
during approximately the first 10 years, the 7°°Th activity also depends on the initial value of the *°Th/
232Th ratio. In vivo, therefore, the attainment of a
radioactive steady state is likely to take many years,
and until then, the expected steadystate ratios, 7?°Ra/
2322Th and 7*8Th/**8Ra, can only be calculated byreference to their activities in the injection material. Even
when these activities are known, the calculations canido
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Fic. 84.—In vitro activities of "Ra and “*Th in Thorotrast
as a function of time since the preparation date. For *°Th, the
dependence of the activity on the initial **Th/™Th ratio is
shown.
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