Rex Boyd defends the decision to commission a new nuclear reactor.

It is claimed by opponents of the nuclear industry that Australia's demand for medical radioisotopes can be met by cyclotrons. The truth is that any number of cyclotrons will never replace Australia's need for a reactor.

Australia has two cyclotrons, which use high voltages and electrical fields to accelerate hydrogen atoms through a vacuum chamber. When they collide with a target substance they produce radioactivity.

As a general rule, it is more difficult to make a radioisotope in a cyclotron than in a reactor. Cyclotron reactions are less productive and less predictable than nuclear reactions performed in a reactor.

The cyclotron produces neutron-deficient radioisotopes whereas the reactor produces neutron-rich radioisotopes. Thus the reactor and the cyclotron complement each other in satisfying society's need for a full range of radioisotopes; rarely one acts as a substitute for the other.

A few radioisotopes are exceptions to this rule and can be produced by either facility. One is technetium-99m, currently used in 85% of medical applications. The discovery that technetium-99m can be produced in a cyclotron does not imply that the need for a reactor is disappearing.

The half-life of technetium-99m is 6 hours. This means that this radioisotope must be produced and distributed on a daily basis.

However, when technetium-99m is produced in a reactor it proceeds through a precursor radioisotope, molybdenum-99, which has a half-life of 66 hours. Thus the weekly production of molybdenum-99 generators can meet all the technetium-99m needs of Australian hospitals.

In contrast the cyclotron does not produce molybdenum-99; instead it produces technetium-99m directly. Therefore a network of cyclotrons situated across Australia would be needed to make daily deliveries of technetium-99m to the nation's hospitals. This is one reason why none of the many powerful cyclotrons around the world are used for the manufacture of technetium-99m.

Reliance on cyclotrons for our most frequently used medical isotope would have a serious negative impact on the practice of nuclear medicine. The rapid decay of technetium-99m would limit the number of patients treated in any one day and would preclude the use of nuclear medicine techniques in out-of-hours emergency situations when stocks would be exhausted. Appointments would be subject to technetium-99m availability and patient waiting lists would lengthen.

Economic factors would also militate against cyclotron-produced technetium-99m. The raw materials for reactor production are cheap (a few dollars per kilogram) and readily available, whereas the starting material for the cyclotron-method is a rare form of molybdenum that must be enriched to high levels of isotopic purity (>99%), is not commercially available and would cost millions of dollars per kilogram.

Traces of other molybdenum isotopes in the raw materials can reduce the purity of the technetium-99m. A series of competing nuclear reactions produces undesirable longer-lived technetium radioisotopes, particularly technetium-96, that can accumulate during theday. The level of these impurities may exceed the legal limit and degrade the quality of the scanned image.

Other technetium radioisotopes would expose patients to higher radiation doses. Only 0.1% technetium-96 is necessary before radiation exposure of patients is doubled. Hence before cyclotron-produced technetium-99m could be used, certain regulations governing radiopharmaceutical quality would need changing.

The cyclotron production of technetium-99m is technically feasible but undesirable for all of these reasons.

The frontiers of nuclear medicine now extend beyond the diagnosis of disease with technetium-99m. Other short-lived radioisotopes are being introduced into nuclear medicine with the capability of reducing the pain associated with cancer. Australia must have its own reactor if its community is to have access to these radioisotopes and reap the benefits of the latest advances.

Rex Boyd was formerly the director of the $20 million National Medical Cyclotron Project at Sydney's Royal Prince Alfred Hospital.

Main sources for biefing paper:
ANSTO, and papers at 1999 and 2001 ANA conferences.
NRPB Bulletin #231, Sept 2001.