Inside the cyclotron bunker that makes western Switzerland’s tracers

In service at Geneva University Hospitals since 2001, the only particle accelerator for medical use in French-speaking Switzerland has been idle for two years while the adjoining laboratories are refurbished. It will be several more weeks before the cyclotron starts producing radiopharmaceuticals again.

The pause makes it possible to enter the unit that houses it, on the second basement level of the hospital. Several airlocks lead to the bunker, a room protected by walls two metres thick. Valentin Bonvin, a physicist specialising in radiation protection who previously worked at CERN, opens the armoured door: twenty-five tonnes of concrete and steel resting on a cushion of air, turning slowly with a loud hiss.

Inside stands a metal cylinder almost two metres high, surrounded by cables, pipes and capillaries. Two people keep the installation running, drawing also on the manufacturer's expertise. The particles are accelerated in a spiral, under the combined action of electric and magnetic fields, in order to bombard water enriched with oxygen-18 with a beam of protons.

Three millilitres and a few hours of life

The installation produces two radioactive isotopes in liquid form, fluorine-18 and nitrogen-13, in a volume of just three millilitres. The liquid is carried to the laboratory through lead-lined capillaries and collected in a shielded cell, where the radiochemical synthesis takes place. Purified and combined with other molecules, it becomes a radiopharmaceutical: joined to glucose, fluorine-18 yields fluorodeoxyglucose, the tracer used in oncology to locate tumours during a PET scan. Nitrogen-13 in the form of ammonia is used instead to image cardiac dysfunction.

With a daily output of forty to fifty doses, the cyclotron supplies the Geneva hospital and the other hospitals in French-speaking Switzerland according to their needs. According to Valentina Garibotto, head of the nuclear medicine and molecular imaging service, this requires very careful coordination upstream. The reason lies in the chemistry: fluorodeoxyglucose has a radioactive half-life of just under two hours, meaning half the initial radioactivity has already gone within that time; for nitrogen-13 ammonia the half-life is ten minutes. The product has to be injected almost at once, or it is of no further use. For the same reason, managing stocks is not a challenge: there are no stocks.

Safety, by contrast, is a constant concern. The installation is subject to strict federal rules, ambient radioactivity is measured continuously, and every member of staff wears a dosimeter. The risk of a chain reaction, Bonvin explains, is nil: with a cyclotron you switch the machine on and carry out the bombardment, and when you switch it off the bombardment stops immediately. There is no nuclear fission at any point. At the end of the visit the physicist triggers the closing of the bunker: a siren and an orange flashing light leave thirty seconds to get out.