Copper radionuclides for theranostic applications: towards standardisation of their nuclear data. A mini-review

Copper has several clinically relevant radioisotopes and versatile coordination chemistry, allowing attachment of its radionuclides to biological molecules. This characteristic makes it suitable for applications in molecular imaging or radionuclide targeted therapy. Of particular interest in nuclear medicine today is the theranostic approach. This brief review considers five radionuclides of copper. These are Cu-60, Cu-61, Cu-62, Cu-64, and Cu-67. The first four are positron emitters for imaging, and the last one Cu-67 is a β--emitting radionuclide suitable for targeted therapy. The emphasis here is on theory-aided evaluation of available experimental data with a view to establishing standardised cross-section database for production of the relevant radionuclide in high purity. Evaluated cross section data of the positron emitters have been already extensively reported; so here they are only briefly reviewed. More attention is given to the data of the 68Zn(p,2p)67Cu intermediate energy reaction which is rather commonly used for production of 67Cu.

Cross-section measurements for 68Zn(p,2p)67Cu and 68Zn(p,2n)67Ga reactions using a newly developed separation method for the superposed [Formula: see text]-ray spectra

We have developed a new analytical peak separation analysis for superposed [Formula: see text]-ray peaks on [Formula: see text]Cu and [Formula: see text]Ga to measure the [Formula: see text]Zn(p,2p)[Formula: see text]Cu and [Formula: see text]Zn(p,2n)[Formula: see text]Ga reactions, unlike in most previous works that were employing a radiochemical separation to measure them. Based on the nuclear data such as the [Formula: see text]-ray intensity and the half-life for each nuclide, we may develop a new analytical method that enables us to estimate the respective counts arising from each nuclide, thereby obtaining the nuclear reactions. The newly developed analytical method can universally be applied to separate the superposed [Formula: see text]-ray spectra of any two nuclides, especially superior in separating the nuclides with different half-lives. In comparison with the data in the literature, the two reactions in the present work are in good agreement with those of some previous works. In addition, we compared the present [Formula: see text]Zn(p,2n)[Formula: see text]Ga reaction without the peak separation to the data in the literature without the chemical separation, and find that a good agreement is evident, enhancing the reliability of the [Formula: see text]Zn(p,x)[Formula: see text]Zn and [Formula: see text]Zn(p,3n)[Formula: see text]Ga reactions, which are further measured in the present work.

Optimized production of 67Cu based on cross section measurements of 67Cu and 64Cu using an 18 MeV medical cyclotron

RadioNuclide Therapy (RNT) in nuclear medicine is a cancer treatment based on the administration of radioactive substances that specifically target cancer cells in the patient. These radiopharmaceuticals consist of tumor-targeting vectors labeled with β^-, α, or Auger electron-emitting radionuclides. In this framework, ⁶⁷Cu is receiving increasing interest as it provides β^--particles accompanied by low-energy γ radiation. The latter allows to perform Single Photon Emission Tomography (SPECT) imaging for detecting the radiotracer distribution for an optimized treatment plan and follow-up. Furthermore, ⁶⁷Cu could be used as therapeutic partner of the β⁺-emitters ⁶¹Cu and ⁶⁴Cu, both currently under study for Positron Emission Tomography (PET) imaging, paving the way to the concept of theranostics. The major barrier to a wider use of ⁶⁷Cu-based radiopharmaceutical is its lack of availability in quantities and qualities suitable for clinical applications. A possible but challenging solution is the proton irradiation of enriched ⁷⁰Zn targets, using medical cyclotrons equipped with a solid target station. This route was investigated at the Bern medical cyclotron, where an 18 MeV cyclotron is in operation together with a solid target station and a 6-m-long beam transfer line. The cross section of the involved nuclear reactions were accurately measured to optimize the production yield and the radionuclidic purity. Several production tests were performed to confirm the obtained results.

Upgrade of recommended nuclear cross section data base for production of therapeutic radionuclides

The IAEA Nuclear Data Section has coordinated several actions to setup and improve a database for recommended cross sections and nuclear decay data for various charged-particle reactions that can be used for medical radionuclide production. Some of the earlier evaluations did not provide uncertainties for the recommended cross sections. Updated evaluations with uncertainty quantification for 25 reactions relevant for production of 67Cu, 103Pd, 102mgRh, 114mIn, 125I, 169Yb, 177gLu, 186Re, 192Ir and 210,211At therapeutic radioisotopes are presented. Recommended cross-section data and their uncertainties for production of therapeutic radionuclides are available on the Web page of the IAEA Nuclear Data Section at https://nds.iaea.org/radionuclides and also at the IAEA medical portal https://nds.iaea.org/medportal.

Evaluation of production cross-sections for theranostic 67Cu radionuclide via proton-induced nuclear reaction on 68Zn target

Copper-67 (T_1/2 = 61.83 h, E_β-^mean=141 keV, I_β-^total=100%; E_γ = 184.577 keV, I_γ = 48.7%) is a promising radionuclide for theranostic applications especially in radio immunotherapy. However, one of the main drawbacks for its application is related to its limited availability. Various nuclear reaction routes investigated in the last years can result in ⁶⁷Cu production, although the use of proton beams is the method of choice taken into account in this work. The goal of this work is a revision of the cross-sections aimed at ⁶⁷Cu yield, which were evaluated for the ⁶⁸Zn(p,2p)⁶⁷Cu reaction route up to 80 MeV proton energy. A well-defined statistical procedure, i.e., the Simultaneous Evaluation on KALMAN (SOK), combined with the least-squares concept, was used to obtain the evaluated data together with the covariance matrix. The obtained evaluated data were also compared to predictions provided by the nuclear reaction model codes TALYS and EMPIRE, and a partial agreement among them has been found. These data may be useful for both existing and potential applications in nuclear medicine, to achieve an improvement and validation of the various nuclear reaction models, and may also find applications in other fields (e.g., activation analysis and thin layer activation).

Prominence of Pairing in Inclusive (p,2p) and (p,pn) Cross Sections from Neutron-Rich Nuclei

Fifty-five inclusive single nucleon-removal cross sections from medium mass neutron-rich nuclei impinging on a hydrogen target at ∼250  MeV/nucleon are measured at the RIKEN Radioactive Isotope Beam Factory. Systematically higher cross sections are found for proton removal from nuclei with an even number of protons as compared to odd-proton number projectiles for a given neutron separation energy. Neutron removal cross sections display no even-odd splitting, contrary to nuclear cascade model predictions. Both effects are understood through simple considerations of neutron separation energies and bound state level densities originating in pairing correlations in the daughter nuclei. These conclusions are supported by comparison with semimicroscopic model predictions, highlighting the enhanced role of low-lying level densities in nucleon-removal cross sections from loosely bound nuclei.

Uses of alpha particles, especially in nuclear reaction studies and medical radionuclide production

Alpha particles exhibit three important characteristics: scattering, ionisation and activation. This article briefly discusses those properties and outlines their major applications. Among others, α-particles are used in elemental analysis, investigation and improvement of materials properties, nuclear reaction studies and medical radionuclide production. The latter two topics, dealing with activation of target materials, are treated in some detail in this paper. Measurements of excitation functions of α-particle induced reactions shed some light on their reaction mechanisms, and studies of isomeric cross sections reveal the probability of population of high-spin nuclear levels. Regarding medical radionuclides, an overview is presented of the isotopes commonly produced using α-particle beams. Consideration is also given to some routes which could be potentially useful for production of a few other radionuclides. The significance of α-particle induced reactions to produce a few high-spin isomeric states, decaying by emission of low-energy conversion or Auger electrons, which are of interest in localized internal radiotherapy, is outlined. The α-particle beam, thus broadens the scope of nuclear chemistry research related to development of non-standard positron emitters and therapeutic radionuclides.

Development of a large scale production of 67Cu from 68Zn at the high energy proton accelerator: closing the 68Zn cycle

A number of research irradiations of (68)Zn was carried out at Brookhaven Linac Isotope Producer aiming to develop a practical approach to produce the radioisotope (67)Cu through the high energy (68)Zn(p,2p)(67)Cu reaction. Disks of enriched zinc were prepared by electrodeposition of (68)Zn on aluminum or titanium substrate and isolated in the aluminum capsule for irradition. Irradiations were carried out with 128, 105 and 92 MeV protons for at least 24h. After irradiation the disk was chemically processed to measure production yield and specific activity of (67)Cu and to reclaim the target material. The recovered (68)Zn was irradiated and processed again. The chemical procedure comprised BioRad cation exchange, Chelex-100 and anion exchange columns. Reduction of the oxidation degree of copper allowed for more efficient Cu/Co/Zn separation on the anion exchange column. No radionuclides other than copper isotopes were detected in the final product. The chemical yield of (67)Cu reached 92-95% under remote handling conditions in a hot box. Production yield of (67)Cu averaged 29.2 μCi/[μA-h×g (68)Zn] (1.08MBq/[μA-h×g (68)Zn]) in 24h irradiations. The best specific activity achieved was 18.6 mCi/μg (688.2 MBq/μg).