Production of thulium-167 for medical use by irradiation of lutetium, hafnium, tantalum and tungsten with 590-MeV protons

Study of thulium-167 cyclotron production: a potential medically-relevant radionuclide

Introduction: Targeted Radionuclide Therapy is used for the treatment of tumors in nuclear medicine, while sparing healthy tissues. Its application to cancer treatment is expanding. In particular, Auger-electron emitters potentially exhibit high efficacy in treating either small metastases or single tumor cells due to their short range in tissue. The aim of this paper is to study the feasibility of a large-scale production of thulium-167, an Auger-electron emitter radionuclide, in view of eventual systematic preclinical studies. Methods: Proton-irradiated enriched erbium-167 and erbium-168 oxides were used to measure the production cross sections of thulium-165, thulium-166, thulium-167, and thulium-168 utilizing an 18-MeV medical cyclotron equipped with a Beam Transport Line (BTL) at the Bern medical cyclotron laboratory. The comparison between the experimental and the TENDL 2021 theoretical cross-section results were in good agreement. Additional experiments were performed to assess the production yields of thulium radioisotopes in the BTL. Thulium-167 production yield was also measured irradiating five different target materials (167 Er 2 O 3, 168 Er 2 O 3, nat Tm 2 O 3, nat Yb 2 O 3, 171 Yb 2 O 3) with proton beams up to 63 MeV at the Injector II cyclotron of Paul Scherrer Institute. Results and Discussion: Our experiments showed that an 8-h irradiation of enriched ytterbium-171 oxide produced about 420 MBq of thulium-167 with a radionuclidic purity of 99.95% after 5 days of cooling time with a proton beam of about 53 MeV. Larger activities of thulium-167 can be achieved using enriched erbium-168 oxide with a 23-MeV proton beam, obtaining about 1 GBq after 8-h irradiation with a radionuclidic purity of < 99.5% 5 days post end of bombardment.

Radiochemical studies relevant to cyclotron production of the therapeutic radionuclide 167Tm

Thulium-167, having a half-life of 9.25 d, is an important therapeutic radionuclide. It was produced via the natEr(p,xn)167Tm nuclear reaction by irradiation of a sedimented target with 15 MeV protons at a beam current of 20 μA. Radiochemical separation of 167Tm from a solution of the irradiated erbium target in hydrochloric acid was performed using di-(2-ethylhexyl) phosphoric acid (HDEHP) in n-hexane and in chloroform separately. The effect of different molarities of HDEHP on extraction efficiency was investigated. The radiochemical yield of 167Tm amounted to about 80 ± 5% while using n-hexane/HDEHP (1 M). The experimentally obtained production yield was about 3.2 MBq/μA h. It was comparable with the theoretical value of 4.1 MBq/μA h obtained from nuclear model calculations using the TALYS and EMPIRE codes.

Experimental study of the excitation functions of proton induced nuclear reactions on (167)Er for production of medically relevant (167)Tm

(167)Tm (T(1/2)=9.25d) is a candidate radioisotope for medical therapy and diagnostics due to its Auger-electron and low-energy X- and gamma-ray emission. Excitation functions of the (167)Er(p,n)(167)Tm reaction and (168)Er(p,n)(168)Tm, (167)Er(p,2n)(166)Tm, (166)Er(p,2n)(165)Tm disturbing reactions were measured up to 15MeV by using the stacked foil irradiation technique and gamma-ray spectroscopy. The measured excitation functions agree well with the results of ALICE-IPPE, EMPIRE-II and TALYS nuclear reaction model codes. The thick target yield of (167)Tm in the 15-8MeV energy range is 6.9MBq/microAh. A short comparison of charged particle production routes of (167)Tm is given.

Separation of carrier-free ytterbium and thulium produced in 80 MeV 12C6+ irradiated gadolinium foil target by liquid-liquid extraction with HDEHP

Heavy ion activation of natural gadolinium foil with 80 MeV 12C6+ results in the formation of carrier-free ytterbium isotopes, 165,166,167Yb and their corresponding decay products, 165,166,167Tm, in the matrix. Carrier-free ytterbium and thulium isotopes have been separated quantitatively from the bulk target matrix gadolinium by employing di-(2-ethylhexyl)phosphoric acid (HDEHP) as a liquid cation exchanger.