Cyclotrons

 

Cyclotrons - or particle accelerators - complement nuclear reactors in the production of radioisotopes for medical use.

 

What is a cyclotron?


A cyclotron is a particle accelerator. It is an electrically powered machine which produces a beam of charged particles that can be used for medical, industrial and research processes. As the name suggests, a cyclotron accelerates charged particles in a spiral path, which allows for a much longer acceleration path than a straight line accelerator.

 


 

How does a cyclotron work?


A cyclotron body consists of electrodes, called 'dees' because of their shape, in a vacuum chamber. This vacuum chamber is flat and sits in a narrow gap between poles of a large magnet which creates a perpendicular magnetic field. A stream of charged particles is fed into the centre of the chamber and a high frequency alternating voltage is applied across the electrodes. This voltage alternately attracts and repels the charged particles causing them to accelerate.

 

The magnetic field moves the particles in a circular path and, as they gain more energy from the accelerating voltage, they spiral outwards until they reach the outer edge of the chamber.

 

Modern cyclotrons accelerate negative ions created in a plasma. When these negative ions reach the outer edge of the chamber the excess electrons are stripped off the ions forming positive particles such as a proton or deuteron, which can then be extracted from the cyclotron as a beam. The size of the vacuum chamber determines the length of the spiral path and hence the amount of energy attained by the particle.

 


 

Medical cyclotrons


Medical cyclotrons produce proton beams which are used to manufacture radioisotopes used in medical diagnosis. Radioisotopes produced in a cyclotron decay by either positron emission or electron capture. Positron emission tomography (PET) and single photon emission computed tomography (SPECT), which utilises the gamma rays associated with electron capture, are two imaging techniques that rely on cyclotron-produced radioisotopes.

 


 

How does a PET scan work?


A patient receives an injection of a positron emitting radioactive tracer which is incorporated into a chemical prevalent in the body, such as glucose. The tracer moves through the body and is accumulated by the organ being studied. When positrons collide with other particles they disintegrate and give out two opposing gamma rays. A PET scanner is a specialised body scanner which can detect these two gamma rays and utilises a sophisticated computer to create a 3D image of the organ being studied. This enables diseases and cancers to be quickly and accurately diagnosed.

 

FDG (Fluoro deoxy glucose) is a typical radiopharmaceutical made using a cyclotron radioisotope. It uses Fluorine-18 which only has a short life span - a half-life of 110 minutes - before the activity level diminishes and then cannot be used as needed.

 

FDG is a glucose analogue widely used in PET imaging. When injected into a patient it is taken up by high-glucose-using cells located in areas like the brain, kidney and cancer. After FDG is injected the PET scanner forms images of the distribution of FDG in the body.

 

These images of the distribution of FDG in the body are then assessed to provide a disease diagnosis. This procedure is useful for diagnosing cancers, heart disease and epilepsy; it shows the chemistry in the body, and therefore helps identify problems earlier than otherwise possible.

 


 

Cyclotrons in Australia


A new medical production facility in Australia is the twin PETNET cyclotrons at Lucas Heights. These are small cyclotrons dedicated to making fluorine-18 for FDG synthesis.

 

Two  small cyclotrons are operated commercially in Melbourne by Cyclotek while others are based at the Royal Prince Alfred Hospital (NSW), Peter MacCallum Cancer Institute (VIC), Austin Health and Medical Imaging Australia (VIC), Royal Brisbane Hospital (QLD), Wesley Hospital (QLD) and Sir Charles Gairdner Hospital (WA). Another will be integrated into a new building complex at the Macquarie University Hospital in NSW.

 


 

Why do we need both cyclotrons and reactors?


It depends on the radioactive properties required whether a nuclear reactor or a cyclotron is used to produce a radioisotope.

 

  • Atoms with extra protons in the nucleus are called neutron-deficient and are produced in a particle accelerator such as a cyclotron.
  • Atoms with extra neutrons in the nucleus are called neutron-rich and are produced in a nuclear reactor.

 

Neutron-rich and neutron-deficient radioisotopes decay by different means and hence have different properties and different uses. Radioisotopes made in cyclotrons complement those made in a reactor. Both types of radioisotopes are needed to service all of Australia's nuclear medical needs.

 

More than 80 per cent of the radioisotopes used in medical procedures worldwide come from research reactors. Molybdenum-99 (Mo-99), which decays to form technetium-99m (Tc-99m) - the most commonly used radioisotope - is currently only produced in nuclear research reactors.

 

A recent report (2010) from the OECD Nuclear Energy Agency indicates that non-reactor technologies for Mo-99 production are still decades away from fruition, and expresses strong doubts as to whether they could ever substitute for reactor technologies.  A 2010 article in the European Journal of Nuclear Medicine and Molecular Imaging comes to the same conclusion.

 

In addition, the emerging generation of therapeutic isotopes can only be produced in a reactor such as ANSTO's OPAL reactor.