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The Use of Radioactive Isotope in Medicine

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The Use of Radioactive Isotope in Medicine

After the discovery of isotopy by Frederic Soddy and Kasimir Fajans,
it was found that elements considered to be non-active may possess
radioactive isotopes having almost identical chemical properties [1].
Radioisotopes are atoms with a nucleus that is seeking a more stable
configuration by emitting radiation. Scientists have learned that more
radioisotopes could be created by subjecting certain elements to
radiation inside a nuclear reactor or bombarding them using a particle
accelerator [2].

Nuclear medicine uses very small amounts of radioactive isotopes, or
tracers, to diagnose and treat disease. A tracer having sufficiently
long life span, or half life, should be selected to make it compatible
with the duration of the operations and measurements to be performed,
and also one whose radiation intensity is sufficient to execute the
measurements, but without affecting the chemical state of the system.
This is of particular importance in biological investigations, where
the materials used are usually sensitive to radiation [1]. For
instance, iodine-131 has a half life of 8 days which is long enough
for it to concentrate in the thyroid gland, allowing doctors long
enough to monitor its function, but not so long as to cause damage.

Iodine-131 is one of the most widely used isotopes. It is supplied in
capsules or liquid of a specific activity designed to be swallowed by
patients. Iodine is a naturally occurring element known as Iodide. It
has many uses in nuclear medicine, where it is used as a tracer to
diagnose and treat certain diseases, such as an overactive or
cancerous thyroid gland.

There are only two naturally occurring isotopes of iodine that are I -
127 (stable) and I - 129 (radioactive). Other radioactive iodine
isotopes (e.g., I - 131) do not occur in nature [3,4]. The radioactive
isotope I - 123 is considered the agent of choice for brain, thyroid,
and renal imaging and uptake measurements. I - 125 is used as a cancer
therapeutic, and as a brain, blood, and metabolic function diagnostic.
I - 131 is used as a brain, pulmonary, and thyroid [3]. Iodine
isotopes above 127I decay by beta particle emission, and energy is
shared between the beta particle and the gamma ray. A total of 72% of
uranium fissions and 75% of plutonium fissions lead directly, or by
beta decay, to iodine isotopes. For example, 2.89% of 235U and 3.86%
of 239Pu fission atoms lead to the formation of a series of isobar 131
isotopes, including 131In, 131Sn, 131Sb, 131Te, 131I, and 131Xe. In
90.4% of the decays, a beta particle is emitted. The remaining excess
energy is emitted as either a gamma ray or a pair of gamma rays [6].
Beta decay happens when a proton decays into a neutron, a positron
(the antiparticle of the electron) and a neutrino. The positron and
the neutrino are emitted and the radioactive particle is the positron
(fig.2.a). Gamma decay is high-energy electromagnetic waves, which are
emitted from the nucleus (fig.2.b). These waves are photons. A gamma
decay can happen after an alpha decay or a beta decay [9].

[IMAGE]

[IMAGE]


Fig 2.a. Beta decay [Adapted from 10] Fig 2.b. Gamma decay
[Taken from 10]

Isotopes of mass less than 127 are produced in particle accelerators
(common examples are 123I and 125I), while those with a mass greater
than 127 are formed in neutron generators, such as nuclear reactors
(common examples are 129I and 131I), or cyclotrons (fig.2). The
cyclotron uses high voltages and electrical fields to accelerate
hydrogen atoms through a vacuum chamber. When they collide with a
target substance they produce radioactivity. 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 [7].

[IMAGE]


Fig 2. A Cyclotron [Taken from 8]

The detector most commonly used with radioactive tracers is the Anger
scintillation camera, invented by Hal Anger in the late 1950s. Gamma
radiation causes crystals

of sodium iodide to emit photons of light. This is called
scintillation. The process of obtaining an image from a radioactive
tracer is called scintigraphy. Other imaging techniques (computerized
tomography, CT; magnetic resonance imaging, MRI) give anatomical
information. Scintigraphy gives information on the movement of
compounds through tissues and vessels. Tomography uses computer
technology to convert numerous planar images into a three-dimensional
slice through the object. This data processing is also used with CT
and MRI. With radioactive tracers, it is called emission computed
tomography, which includes single photon emission computed tomography
(SPECT) and positron emission tomography (PET). With SPECT scans,
anger scintillation cameras obtain numerous images by rotating around
the patient (fig.1). Computers then form the images that provide data,
and information, about the area of body being diagnosed or treated
[5].

[IMAGE]

Fig 1. Schematic diagram of an anger scintillation camera.
Illustration by Hans & Cassidy. Courtesy of Gale Group. [Taken from 5]

Iodine-131 can be very useful in medicine, but does not come without
risks. The radioactive iodine that is not taken up by the thyroid is
rapidly eliminated through body fluids such as urine, saliva, and
perspiration. This means that, for a number of days after the iodine
has been administered, everything that the patient touches could
become contaminated with radioactive iodine. In order to prevent the
spread of this contamination, it is necessary for the patient to
remain in a specially prepared room.

References

1. Hurwic, J. (no date). Radioactive Tracers [online]. Available
from: http://www.ccr.jussieu.fr/curie.100/fulltext/hurwic2.html
[Accessed on: 27/11/2005]

2. Office of Public Affairs U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001. (2000). The Regulation and Use of
Radioisotopes in Today's World [online]. Available from:
http://www.nrc.gov/reading-rm/doc-collections/nuregs/brochures/br0217/
[Accessed on: 27/11/2005]

3. Agency for Toxic Substances and Disease Registry. (2004). Public
health statement [online]. Available from:
http://www.atsdr.cdc.gov/toxprofiles/tp158-c1.pdf [Accessed:
23/11/2005]

4. Agency for Toxic Substances and Disease Registry. (2004).
Production, import/export, use, and disposal [online]. Available
from: ]. http://www.atsdr.cdc.gov/toxprofiles/tp158-c5.pdf
[Accessed: 23/11/2005]

5. O'Mathúna, Dónal, P. (2005). “Radioactive Tracers”, Gale
Encyclopaedia of Science [online]. Available from:
http://find.galegroup.com/ips/infomark.do?&type=retrieve&tabID=T001&prodId=IPS&docId=CX3418501903&source=gale&srcprod=EB00&userGroupName=lowcoll&version=1.0
[Accessed: 27/11/2005]

6. Agency for Toxic Substances and Disease Registry. (2004).
Chemical. Physical and radiological information [online].
Available from: http://www.atsdr.cdc.gov/toxprofiles/tp158-c4.pdf
[Accessed: 04/12/2005]

7. Nuclear Issues Briefing Paper 26. (2004). Radioisotopes in
medicine [online]. Available from: http://www.uic.com.au/nip26.htm
[Accessed: 04/12/2005]

8. Jefferson Lab. (no date). Cyclotron [online]. Available from:
http://education.jlab.org/glossary/cyclotron.html [Accessed:
05/12/2005]

9. Czarnecki, L. (2002). Nuclear physics [online]. Available from:
http://www.hpwt.de/Kern2e.htm [Accessed: 14/12/2005]

10. TIS Technical Publications. (2003). Theory: Radioactive decay
[online]. Available from:
http://www2.slac.stanford.edu/vvc/theory/nuclearstability.html
[Accessed: 14/12/2005]

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