Nuclear Medicine - Source of Radionuclides, With Notes On A Few Radiopharmaceuticals

Source of Radionuclides, With Notes On A Few Radiopharmaceuticals

About a third of the world's supply, and most of North America's supply, of medical isotopes are produced at the Chalk River Laboratories in Chalk River, Ontario, Canada. (Another third of the world's supply, and most of Europe's supply, are produced at the Petten nuclear reactor in the Netherlands.) The Canadian Nuclear Safety Commission ordered the NRU reactor to be shut down on November 18, 2007 for regularly scheduled maintenance and an upgrade of the safety systems to modern standards. The upgrade took longer than expected and in December 2007 a critical shortage of medical isotopes occurred. The Canadian government unanimously passed emergency legislation, allowing the reactor to re-start on 16 December 2007, and production of medical isotopes to continue.

The Chalk River reactor is used to irradiate materials with neutrons which are produced in great quantity during the fission of U-235. These neutrons change the nucleus of the irradiated material by adding a neutron, or by splitting it in the process of nuclear fission. In a reactor, one of the fission products of uranium is molybdenum-99 which is extracted and shipped to radiopharmaceutical houses all over North America. The Mo-99 radioactively beta decays with a half-life of 2.7 days, turning initially into Tc-99m, which is then extracted (milked) from a "moly cow" (see technetium-99m generator). The Tc-99m then further decays, while inside a patient, releasing a gamma photon which is detected by the gamma camera. It decays to its ground state of Tc-99, which is relatively non-radioactive compared to Tc-99m.

The most commonly used radioisotope in PET F-18, is not produced in any nuclear reactor, but rather in a circular accelerator called a cyclotron. The cyclotron is used to accelerate protons to bombard the stable heavy isotope of oxygen O-18. The O-18 constitutes about 0.20% of ordinary oxygen (mostly O-16), from which it is extracted. The F-18 is then typically used to make FDG (see this link for more information on this process).

A typical nuclear medicine study involves administration of a radionuclide into the body by intravenous injection in liquid or aggregate form, ingestion while combined with food, inhalation as a gas or aerosol, or rarely, injection of a radionuclide that has undergone micro-encapsulation. Some studies require the labeling of a patient's own blood cells with a radionuclide (leukocyte scintigraphy and red blood cell scintigraphy). Most diagnostic radionuclides emit gamma rays, while the cell-damaging properties of beta particles are used in therapeutic applications. Refined radionuclides for use in nuclear medicine are derived from fission or fusion processes in nuclear reactors, which produce radionuclides with longer half-lives, or cyclotrons, which produce radionuclides with shorter half-lives, or take advantage of natural decay processes in dedicated generators, i.e. molybdenum/technetium or strontium/rubidium.

The most commonly used intravenous radionuclides are:

• Technetium-99m (technetium-99m)
• Iodine-123 and 131
• Thallium-201
• Gallium-67
• Fluorine-18 Fluorodeoxyglucose
• Indium-111 Labeled Leukocytes

The most commonly used gaseous/aerosol radionuclides are:

• Xenon-133
• Krypton-81m
• Technetium-99m Technegas a radioaerosol invented in Australia by Dr Bill Burch and Dr Richard Fawdry
• Technetium-99m DTPA