PRODUCING RADIO-METALS IN LIQUID TARGETS: PROOF OF FEASIBILITY
WITH 94mTc
Hoehr, C. 1¶, Morley, T.1, Trinczek, M.1, Hanemaayer, V.1, Ruth, T.1, Schaffer, P.1, Benard, F.2
1
TRIUMF, Vancouver, BC, Canada, 2BC Cancer Agency, Vancouver, BC, Canada.
Corresponding author email address: choehr@triumf.ca
¶
Material and methods: The isotope 94mTc has a halflife of 52.0 minutes, a large positron branching ratio of
72%, and a medium positron end-point energy of 2.47
MeV. This makes it a suitable candidate for PET. Its
main appeal is in its chemical exchangeability with
99m
Tc, the most common radioisotope for imaging in
nuclear medicine, widely used in SPECT. A range of
radiopharmaceuticals are currently available for technetium and 94mTc can therefore be used as a possible
PET alternative to the established SPECT tracer. This
provides a possibly unique solution to future shortages
of 99mTc. For our preliminary studies, we used Mo of
natural abundance, either as MoO3 or as (NH4)6Mo7O24
dissolved in water, H2O2, and NH4OH. The metal-salt
solution was loaded into a modified liquid-target system on the TR13, a 13MeV negative hydrogen-ion
cyclotron at TRIUMF. All runs were performed with a
beam current of 5 µA for a maximum of 60 minutes.
The irradiated solution containing the desired 94mTc
was then transferred to a purification system utilizing
solid phase extraction as a means to separate the 94mTc
from the target solution [2].
Table 1: The different solutions that were used.
(NH4)6Mo7O24·
H2O2
H2O
NH4OH
4H2O (g)
(ml)
(ml)
(ml)
A
6.5
1
16
B
13.0
1
14
C
19.9
1.2
12
MoO3 (g)
D
6
4
10
4
E
12
4
10
6
F
6
1
10
4
Irradiation results varied widely when using different
concentrations of Mo (Table 1). After an initial rise due
to the proton-beam strike within the target solution, the
pressure continues to rise. With the maximum safety
threshold of target pressure being 450 psi, not all solutions were able to run for a full 60-minute irradiation
(see Figure 1).
450
400
350
300
pressure (psi)
Introduction: Widespread acceptance of new and
promising radioisotopes can be challenging due to their
limited availability. This is typified by many radiometals that have demonstrated promise as molecular
imaging probes, or are of interest based on their known
emission properties; yet their production is difficult to
execute in a hospital-based medical cyclotron facility.
To produce these isotopes most facilities would require
the purchase of an isotope generator (if available) or
invest substantially in installing a solid-target infrastructure – two expensive options, especially if preliminary biological studies are required in order to justify
further investment.
The approach described here provides a facile method
for most medical cyclotron facilities to produce clinically useful quantities of radio-metals by adapting
liquid targets already in place for the production of
other PET isotopes, such as 18F. This allows for the
production of 94mTc without a significant investment in
a solid-target station and transfer system. This liquidtarget production method can be easily extended toward other metal radioisotopes and could facilitate a
quick turnaround for the investigation of these isotopes
as alternatives for a specific study. Liquid targets have
already been successfully used for the production of
89
Zr, 68Ga, 86Y, 123I, and 77Br [1].
250
200
solution F
solution E
solution D
solution A
solution C
solution B
150
100
50
0
0
10
20
30
40
50
60
time (min)
Figure 1: Pressure rise during irradiation of different solutions.
Results: Quantities large enough for pre-clinical
studies were achieved. The maximum yield after a 60minute run at EOB was 120±30MBq (solution C, Table
1). Future studies will examine the production metrics
for other isotopes (89Zr, 68Ga, 86Y [1]).
References:
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Cuninghame et al., Int J. Appl. Radiat. Isot.27 597
(1976). J.L. Hutter at al., Appl. Radiat. Isot. 43
1393 (1992). E. Galiano et al., Appl. Radiat. Isot. 49
105 (1996).
2. T. Morley et al., Nuc. Med. Biol. in print (2012).