Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Accelerator-based synthesis of rhenium-186 that enables high spatial
resolution imaging
S. Takeda,∗1 I. O. Umeda,∗1 M. Katsuragawa,∗1 A. Yagishita,∗1 A. Nambu,∗2 X. Yin,∗2 H. Haba,∗2 andT. Takahashi∗1Radiotheranostics is the integration of targetedradionuclide therapy with molecular imaging techniques such as Positron Emission Tomography (PET)and Single Photon Emission Computed Tomography(SPECT). The exponential growth of radiotheranostics in the ﬁeld of oncology is due to its unique mechanism of action, which allows for the targeted elimination of tumor cells with minimal adverse eﬀects. The
radionuclides, used for diagnosis, emit β + particles in
PET and γ rays in SPECT. In contrast, for cancer
therapy, radionuclides that emit energetic α, β − , andauger electron particles are utilized.
Rhenium radioactive isotopes are an attractive option for radiotheranostics, as they emit both particles
suitable for targeted therapy and photons useful for
diagnosis, and can aid in developing radiopharmaceuticals. Rhenium belongs to the same chemical family
as technetium. Technetium-99m (99m Tc) is the mostwidely used radionuclide in nuclear medicine imaging, due to its short half-life and favorable physicalproperties such as low radiation exposure to patients.
Many 99m Tc-labeled probes have been developed andare used clinically to diagnose of various diseases. Mostof the ligands and chelation chemistry developed fortechnetium can be also applied to rhenium. 186 Re istypically produced through neutron irradiation in reactors in other countries, but production in Japan is challenging due to the diﬃculty of reactor production. Additionally, 186 Re produced in reactors includes a large
amount of carrier Re as raw material. At the RIKEN
RI Beam Factory, carrier-free 186 Re can be produced
through proton and heavy ion-induced reactions of186
W using the RIKEN AVF cyclotron. Our goal isto develop new radiotheranostics using 186 Re.1,2)
186
Re emits gamma-rays at 137 keV, allowing visualization of biobehavior of 186 Re-labeled radiopharmaceuticals. However, the emission yield of 137 keV photons is as small as 9.4%. Therefore, high-sensitivity
and high-resolution imaging have been challenging
since the signals (137 keV photons) are prone to becontaminated by the continuum background constituted by scattering of high-energy gamma-rays causedby contamination of rhenium isotopes other than
186Re. In our previous experiment, 186 Re was produced in the 186 W(d, 2n)186 Re reaction using the24-MeV d beam. We found that the byproduct of184
Re signiﬁcantly contributed to the contamination∗1∗2

Kavli Institute for the Physics and Mathematics of the Universe, University of Tokyo
RIKEN Nishina Center- 158 -and caused signiﬁcant degradation in imaging quality.
To improve imaging quality, it is essential to increase
the radionuclidic purity of 186 Re. In this study, we produced 186 Re in the 186 W(p, n)186 Re reaction using the14.8-MeV proton beam on target and successfully improved the radionuclidic purity of 186 Re from 94.43%
to 99.56%.
A 19-MeV proton beam delivered from the RIKEN
AVF cyclotron was degraded to 14.8 MeV through a Taplate (192.6 µm) and irradiated onto a 186 WO3 powder (isotope enrichment of 186 W: 99.79%; thickness:∼200 mg/cm2 ). After irradiation, 186 Re was puriﬁedby chemical separation.3) 3 MBq of 186 Re in 200 µLof 0.01 M HCl was shipped to National Cancer Center
Research Center for the imaging experiment.
We demonstrated the eﬀectiveness of the puriﬁcation for imaging using CdTe-DSD SPECT4) (Fig. 1)
developed by the IPMU team. CdTe-DSD SPECT isa novel small animal imaging system consisting of 8
modules of CdTe Double-sided Strip Detectors5) and amulti-pinhole collimator made of tungsten. The detector has an energy resolution of 1–2 keV (FWHM) in
10–100 keV and 1.6% (FWHM) at 140 keV, which isaround three times better than those of currently available high-grade semiconductor SPECT systems. An
ultra-high spatial resolution of better than 350 µm ispossible. Figure 2 compares phantom imaging resultsfor 186 Re with a purity of 94.43% (left) and 99.56%(right). The image was noisy for lower purity becauseof the continuum background produced by 184 Re. On
the other hand, the preferred image was obtained forhigh-purity 186 Re. This is a promising result, and wewill proceed to in-vivo imaging with mice.

Fig. 1. CdTe-DSD SPECT viewed from the entrance window for small animals.

Ⅲ-3. Radiochemistry & Nuclear Chemistry(A) 186Re purity: 94.43%

RIKEN Accel. Prog. Rep. 56 (2023)(B) 186Re purity: 99.56%
1.0
0.35

0.75
0.5

Fig. 2. The comparison of phantom imaging results for186
Re with a radionuclidic purity of 94.43% (left) and
99.56% (right) taken with CdTe-DSD SPECT.

References
1) I. O. Umeda et al., RIKEN Accel. Prog. Rep. 54, 157(2021).
2) I. O. Umeda et al., RIKEN Accel. Prog. Rep. 55, 139(2022).
3) N. Shigeta et al., J. Radioanal. Nucl. Chem. 205, 85(1996).
4) S. Takeda et al., IEEE Trans. Radiat. Plasma Med. Sci.
(to be submitted).
5) S. Takeda et al., Nucl. Instrum. Methods Phys. Res. A
912, 57 (2018).

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