A feasibility study of the production of 131I to be used in radioimmunoassay in Sri Lanka

Abstract

131I is a major radioisotope in nuclear medicine, extensively employed both to diagnose and treat differential thyroid cancers and to manage hyperthyroidism. For therapeutic purposes 131I is required in higher doses, typically exceeding 10 mCi, depending on the stage of the cancer. Beyond its primary roles, 131I is also utilized as a radiolabelled tracer in Radioimmunoassay (RIA) for quantitative detection of hormones, proteins and other antigenic substances. Typical 131I activity used per RIA test ranges from 0.5 to 2 µCi. Since thyroid cancer diagnosis and therapy are critically important, Sri Lanka imports 131I from neighbouring countries at a high cost because it does not have a domestic radioisotope production facility. Consequently, the accessibility of 131I for RIA research in Sri Lanka remains limited, due to its high demand and high importation costs. This study examines the feasibility of producing 131I in Sri Lanka using the neutron activation facility available at the University of Colombo. Considering the current neutron flux of the available source, the focus was placed on generating activity levels appropriate for RIA applications, rather than aiming for the much higher doses required for diagnostic and therapeutic uses. Five grams of 99.9% enriched 130Te was irradiated with a flux of approximately 2.25 x 105 neutrons/cm2/s. During irradiation, 130Te undergoes a nuclear reaction 130Te(n,ɣ)131Te and the resulting 131Te (half-life ̴ 25 minutes) decays to 131I (half-life ̴ 8 days). The production was monitored using a gamma spectroscopy with a sodium iodide scintillation detector. After 40 irradiation cycles, an activity of 6.1±0.3 nCi was measured, demonstrating the feasibility of local 131I production. Utilizing a neutron source with significantly higher flux (approximately 2 x 108 neutrons/cm2/s) and irradiating for more cycles would allow achieving the required RIA dose more readily. The 131I separation process is ongoing and will be reported in future. Preliminary trials using irradiated KIO3 and the wet distillation protocol achieved a separation efficiency of 68%.