(Yo)
228Th PRODUCED BY GROWTH SINCE To
TOTAL 228 Th
100
T
{
80}—
60/— LOWACTIVITY THOROTRAST
/ [228 th/232th-0.25 AT To}
40h20 ;—
HIGH ACTIVITY THOROTRAST
[228Th/232Th= 1.00 AT To]
|
0.2
1.0
tot
J
yi 4 t
2.0
LO
20
TIME IN YEARS SINCE PREPARATION DATE To
yg. 86.—Proportion of ?8Th in Thorotrast produced in viéro since the date of preparation, relative to the total **%Th
nelivily.
TABLE 49.
Nuclide
2327
228P 9
228Th
24Ra
207m
22Ph
Others
ApproximaTe Excretion Rates or THorruM-Serres Nucuwpes tn Long-Term Tuorotrast Patients
70 cnereted ne een
<1073
0.08
<0.01
1-2
—fa)
1
Negligible
Priority route of excretion on ritesfeubaleboty of Reference for excretion rate
Feces
Feces
Feces (?)
Feces
Breath
Urine and feces
_
—
8Ra/?’Th =
228TH/2%8Ra =
24Ra8TH =
20Km/24Ra =
22Pb /20Em =
—
0.50
0.91-1.00
0.90-0.95
0.90-0.92
0.99
9
6, 9
6
6,9
10, 13, 14
6
Qn account of the short half-life of ?°Em it is not meaningful to express the excretion of this nuclide in the units of °% body
burden per day.
the RES probably reflect the proportion of **°Th in the
F cciginal injeetion solution which existed in an ionic or
FP soltble form. In the Thorotrast ampoule prior to inf jection this proportion would be expected to grow with
tie in the manner of Figure 86. Since *°Th grows
| from 228Ra and there are no intermediate high energy
, recoil events, the maximum value of this proportion
; would be expeeted to approach the analogous figure of
P?Ra (Le., ~75%). Some support for this theory can
he found in the fact that the reported values of the
"Th. ??8Ra activity ratios in Tables 42 and 43 are
}lorser for the “young” batches of Thorotrast, and
F stucllcr for the “old” batch of Thorotrast.
f
lor Thorotrast burdens of many years standing, the
Values of (1 — fg) ealeulated for the RES probably
% tetlect the proportion of 28Th in a soluble form which
af his grown in vivo from the 228Ra trapped within the
| Particles. That is, unlike the original injection mateF Vint. it does not include the 2°8°Th which grows from
F~'uble 2°8Ra, because the latter is largely excreted
| tomthe body (all but ~1% within its mean lifetime).
BE “ee there are no high energy recoil events to give
/™-" to the direct escape of °°*Th from the particles,
the steady state activity ratios *°8Th/228Ra would be
expected to be close to unity. In the two long-term
eases presented in Tables 42 and 43, this expectation
is borne out. Nevertheless, other workers have reported
a significant washout of **°Th from the RES in longterm Thorotrast patients (~10%, see Table 49), and
there have even been observations of 228Th in the excreta.‘?) Thus, although there is clear evidence for the
translocation of *7°Th at early times after administration, its later behaviour is uncertain.
The proposed recoil mechanism would lead to the
prediction that, on account of the higher recoil energy
(Table 47), the retention of 224Ra atoms within the
Thorotrast particles should be lower than that of **8Ra.
In fact the 7*4Ra/**8Th steady state activity ratios
turn out to be higher than the 2*°Ra/***Thratios in the
RES (e.g., in Tables 42 and 48, ~0.5 vs. ~0.3), but
this is readily explained on the assumption that ?74Ra
atoms which are not bound within the particles have a
biological half-life in the RES of several days. ICRP
quotes a figure of 10 days.4®
The occurrence of several high-energy recoil events
between **4Ra and 7?2Pb presumably implies that most