Nuclear medicine uses technetium-99m among other things for tumor diagnostics. With over 30 million applications worldwide each year, it is the most widely used radioisotope. The precursor material, molybdenum-99, is mainly produced in research reactors. A study at the Heinz Maier-Leibnitz Research Neutron Source (FRM II) at the Technical University of Munich (TUM) now shows options to significantly reduce the radioactive waste produced during processing to a medical product.

Over 85 percent of all nuclear medicine diagnostic examinations use technetium-99m (Tc-99m). In Germany alone, more than 3 million doses are deployed every year. Coupled to suitable organic molecules, technetium is distributed throughout the body via the blood and accumulates in tumors, for example. When it decays there, the released radiation reveals the precise location of the tumor.

Technetium-99m is produced by irradiating uranium plates, so-called targets, with a high neutron flux that is practically only available at research reactors. Initially, starting from uranium-235 this produces molybdenum-99 (Mo-99), which decays to Tc-99m with a half-life of 66 hours. With a half-life of six hours the latter converts to Tc-99, emitting gamma radiation that can be measured.
More waste from low-enriched uranium
The political push to replace highly enriched uranium with low-enriched uranium also applies to targets being used in the medical field. This is why the Mo-99 irradiation facility currently under construction at FRM II is designed for targets with low-enriched uranium.

"However, this gives rise to a severe problem: the less the uranium plates are enriched with uranium-235, the lower the specific yield of Mo-99 during irradiation," says Dr. Tobias Chemnitz, instrument scientist at the MEDAPP medical irradiation facility at FRM II.

To meet world-wide demand of Tc-99m, at least twice as many uranium plates must be irradiated and processed, depending on the technology used. This produces correspondingly higher volumes of waste. Chemnitz addressed this problem in his doctoral thesis at the Technical University of Munich.

New process avoids up to 15,000 liters of liquid radioactive waste
The final irradiated plates comprise only about 0.1 percent Mo-99. To ensure a purity sufficient for medical applications, the Mo-99 must be carefully separated from the remaining material.

Currently, there are two standard processes in use, based on an acidic and an alkaline process, respectively. In the alkaline variant, the entire target is initially treated with caustic soda. In the process, Mo-99 is preferentially dissolved, while the uranium is insoluble in this solution and remains as a solid. The residual fission products are then separated from the aqueous solution in an elaborate chemical separation process.

Molecular Imaging Homepage