Medical Isotopes

Radioisotopes in Medicine - by Dale Dewar, MD.  Past President, Physicians for Global Survival.

Here are some of my thoughts, in summary:

The nuclear industry promotes nuclear power and medical uses of radionuclides as inextricably linked.  In 1991, when Bill C-204 (placing a moratorium on further nuclear power plants in Canada) was introduced in parliament, the industry went into full propaganda mode, even claiming that x-ray machines would be affected!   (Well, perhaps not  “claiming” but not denying the rumour either.)

Radioisotopes entered medical use before nuclear reactors. At first, naturally occurring radium-226 and radium-224 were used. In the 1940's, use of a cyclotron for bombarding various target materials created new radioisotopes. From their discovery, physicians were fascinated by these substances that could destroy cells with or without direct contact. Strontium-90 was used as a salt to destroy superficial skin cancers. The early brachytherapy* beads contained radium-226, iridium-192 or palladium-103. Their use has always required a cautious approach, finding a dose that cured with a minimum of side effects. There is no doubt that current radioisotopes are useful for both diagnostic and treatment purposes.

Some of the radioisotopes in current use:

1.  Cobalt-60 is produced in some nuclear power reactors.  It is a gamma emitter and used to sterilize medical instruments and as an external treatment of some solid cancers.

2.  Technetium-99m is produced through the decay of Molybdenum-99 which, in turn, is produced in nuclear reactors designed for isotope production (not nuclear power).  Tc-99m is used in imaging to make diagnoses. Tc-99m is a gamma emitter but has such a short half life that it poses next to no problem in terms of waste; its short-lived activity means that the patient is exposed to a low dose of gamma rays. When its availability was limited last year, physicians discovered that judicious use of PET scans and ultrasound could serve much the same purpose.

3.  Gallium-67 is cyclotron-produced by the proton irradiation of enriched zinc oxide. It is a gamma-emitter with a half-life of 78.3 hours. For cancer staging, gallium scans have largely been replaced by PET scans, or positron emission tomography.  Gallium-67 continues to be useful in the imaging of inflammation and infection.

4. Cesium-131 is probably the “new” radionuclide for brachytherapy, the insertion of radioactive beads into the cancerous growth. It is produced by radioactive decay from a neutron-irradiated naturally occurring barium-130 or from enriched barium containing barium-131. The source of the neutrons can be a nuclear reactor or another neutron generating device such as a linear accelerator or neutron generator.

5. Iodine-131 is a fission product of uranium in nuclear reactors or of plutonium in nuclear bombs. Its principle use is for ablation of thyroid cancer cells but it is also used in brachytherapy.

Radionuclides and nuclear power?

1. New reactors are not required to keep medical sources supplied.  We will have enough for many years with our current reactors.

2. Research indicates that molybdenum-99, the radioisotope whose shortage caused havoc last winter, can be produced by using a cyclotron with a molybdenum-100 target.

3. The use of cobalt-60 may be phased out altogether, as safer alternative technologies take over -- especially since several patients have been injured by cobalt-60 radiotherapy devices due to programming errors in the computerized AECL radiotherapy units; alternative radioisotopes may be found which can be produced in cyclotrons and which can substitute for cobalt-60, or a method may be developed for producing cobalt-60 economically in an accelerator.

4. Arrangements may be made to maintain a number of small reactors in operation solely for the purpose of producing selected radionuclides for use in medicine , industry and scientific research.

I personally believe that in a number of years, we will look back on radioisotope diagnosis and treatment like our modern perspective on blood-letting or, more recently, open-abdominal surgery for gall bladders and uteri, beneficial when we didn't know how to do anything else but "out of date" because it will have been replaced with better, less risky treatment.

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* Brachytherapy:  a form of radiotherapy where a radioactive source is placed inside or next to the area requiring treatment.