Image credit: TRIUMF.
The world’s most in-demand isotope for medical-imaging purposes is 99mTc, a daughter of the isotope 99Mo. 99Mo has been produced in plentiful supplies for the entire world chiefly by two research reactors: one in Canada and the other in the Netherlands. Both of these reactors are currently down for difficult repairs related to their age – the younger one is 47 years old.
One mitigating factor in maintaining the supply of 99Mo has been the immense co-operation among medical-isotope suppliers and consumers around the world, primarily brokered through working groups of the International Atomic Energy Agency and several industrial associations. However, in the face of the supply shortages – the pair of reactors produced 65% of the world’s 99Mo – Canada has been examining alternatives.
At the end of March the government of Canada released its policy response to an expert advisory panel that analysed the situation in autumn 2009. The report highlights two main alternatives to manufacturing the 99Mo isotope that is currently in so much demand: cyclotrons (with new target materials) and linear accelerators (using photo-neutron processes on 100Mo or photo-fission of 238U).
Cyclotrons have been used around the world for four decades to produce isotopes useful for medical-imaging purposes ranging from 11C and 18F to 82Sr. The primary method to be explored for the cyclotron approach to the manufacture of 99mTc utilizes the 100Mo(p,2n)99mTc reaction. When bombarding the 100Mo target foil with an energetic proton beam, 99mTc is produced in direct reactions and can then be extracted. High yields of 99mTc from this reaction depend on three things: high-energy cyclotrons, high-intensity beams and high-efficiency 100Mo targets – all of which will be developed and tested in the next year or so.
Along with a team of researchers and clinicians from across Canada, TRIUMF, the University of British Columbia and BC Cancer Agency have received initial Canadian government support to begin benchmarking and then optimizing the 99mTc yield from this process. Other groups are following suit along with several private companies.
If the technology pans out, and the contamination of ground-state 99Tc is controllable in the extracted 99Tc samples, it will be a new “killer app” for medical-isotope cyclotrons. Fine-tuning will be needed to select the optimal beam energy of the protons as well as the target geometries and the extraction and separation procedures. 99mTc produced directly at cyclotrons would be limited to local use because the six-hour half life prevents it from being shipped round the world as 99Mo currently is (with a 66-hour half life). However, this technology could provide an important supplement in major urban centres where cyclotron capacity exists for burgeoning nuclear-medicine departments. Cyclotron-produced 99mTc would reduce the need for 99Mo from reactors.
Independent of this innovation, cyclotrons have a bright future in nuclear medicine. The new isotopes and radiopharmaceuticals being developed using the so-called PET isotopes could eventually overtake the market dominance of 99Mo, so that cyclotrons will be everywhere.
