Cross-section measurements of lanthanum radioisotopes towards an optimized production of 133La and 135La for theranostic applications.

The theranostic pair 133La/135La has recently attracted growing interest as a promising candidate for combined diagnostic and therapeutic applications, owing to the favorable nuclear and chemical properties of lanthanum. 135La (t1/2 = 18.91 h) decays by electron capture (EC) to the ground state of 135Ba, emitting Auger cascade electrons that are potentially useful for targeted internal radiotherapy. In contrast, 133La (t1/2 = 3.912 h) is a β+-emitter suitable for PET imaging. Moreover, 133La has been proposed as a diagnostic partner for 225Ac (t1/2 = 9.9 d), due to their close chemical similarity and the longer half-life of 133La compared with other PET radiometals such as 68Ga (t1/2 = 1.13 h). A feasible approach for producing 133La and 135La is the irradiation of enriched 134Ba and 135Ba targets, respectively, with low- to medium-energy proton beams (up to 25 MeV maximum energy) readily available from medical cyclotrons. Although these production routes have been previously explored, the available nuclear data on lanthanum radioisotopes remain limited and often inconsistent. In this work, we measured the cross sections of reactions producing lanthanum isotopes up to 18 MeV, using natural BaCO3 as well as enriched 134BaCO3 and 135BaCO3 targets, with some cross sections reported here for the first time. The half-lives of La radioisotopes were also determined and compared with the literature values. The resulting data were used to optimize the production parameters for 133La and 135La, and several irradiation tests were carried out to validate our findings.