The medical isotope technetium-99m (Tc-99m) could be made in hospitals rather than nuclear reactors. That is the conclusion of researchers in Canada, who have done theoretical modelling of how the material could be produced and processed in medical cyclotrons. Today Tc-99m is made centrally in a few nuclear reactors and cyclotron-based production could help to alleviate shortages that can occur when a reactor shuts down.
Although Tc-99m is used in a wide range of nuclear medicine procedures, the isotope is produced at only five nuclear reactors worldwide. The fragility of supply was highlighted recently by a global shortage brought about by the unscheduled shutdown of a reactor in Canada. As a result, physicists are keen to develop alternative methods for making the material.
This latest study was done by Anna Celler of the University of British Columbia and colleagues and is part of a C$35 million initiative by the Canadian government to look for alternative manufacturing techniques for Tc-99m.
Unwanted isotopes
In principle, Tc-99m can be made using a hospital’s medical cyclotron to bombard molybdenum with a proton beam, causing the transmutation of some of the molybdenum-100 nuclei into Tc-99m. However, molybdenum targets are expensive and the technique produces other unwanted isotopes that reduce the diagnostic benefit for the patient. The viability of the technique must therefore be carefully scrutinized – however, doing experiments is extremely expensive.
Now Celler and colleagues have developed a theoretical model that predicts the viability of the method and estimates logistical parameters, such as the number of cyclotron runs needed to meet the daily demands of a typical nuclear medicine department. The reaction conditions needed for optimal yields, such as beam energy and target geometry, were also identified.
The researchers used the nuclear-reaction model code EMPIRE-3 to calculate the cross-section, or probability, of each of the possible molybdenum–proton reactions, across an energy range of 6–30 MeV. The simulation confirmed that the numerous molybdenum–proton reactions produced multiple contaminants, including several technetium, molybdenum, niobium and zirconium isotopes. Together with the yield calculations, the EMPIRE-3 simulation also demonstrated that only molybdenum targets enriched with molybdenum-100 were viable for efficient Tc-99m production. Natural molybdenum, with its composition of several isotopes, produced significant amounts of contaminant isotopes.
Ideal proton energy
The researchers also identified 16–19 MeV as the optimal proton energy range for Tc-99m production. In this range, relative Tc-99m yields were greatest when compared with contaminant isotopes. Shorter, multiple molybdenum -100 irradiation cycles per day, each 3–6 hours long, also proved to be the most efficient production schedule.
“We are very happy with these results: not only are our theoretical calculations in agreement with the existing experimental data, but also they provide us with guidance for future experiments and suggest what could be the optimal conditions for technetium production,” said Celler. “The yields are sufficient, so that even cyclotrons designed to produce [positron emission tomography] PET radionuclides can produce sufficient quantities of Tc-99m to meet local needs.”
The researchers are now using their results to calculate radiation doses to patients that will result from the cyclotron-produced technetium. “These dose calculations can then be compared with those related to reactor-produced technetium and will serve as guidance for the selection of target enrichment,” explained Celler.
The research is described in Phys. Med. Biol. 56 5469.
