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We can define radiation simply as the emission of energy by one body, its
transmission through an intervening medium, and its absorption by another
body.
By this definition, radiation encompasses the entire electromagnetic
spectrum and certain charged particles emitted by radioactive elements.
Alfa particles, such as the radiation emitted by phosphorus-32, and the
beta particles of elements such as tritium and carbon-14 are of immense
use scientifically and diagnostically but pose few hazards for man.
High energy radiation, in the form of gamma or x-rays is the mediator of
most of the biologic effects.
Radiation can be employed in a benign or malignant manner.
On the one hand, medical practice is inconceivable today without the use
of diagnostic and therapeutic radioisotopes, clinical radiographs, and
radiation therapy. On the other hand, nuclear explosions and accidental exposure to radiation
in nuclear power plants have caused injury and death.
Radiation is quantitated in a number of ways.
The
emission of radiant energy from a source is measured in roentgens,
units that refer to the amount of ionization produced in air. The absorption
of radiant energy - biologically, the more important parameter - is measured
in terms of the rad, the unit that defines the ergs of energy
absorbed by a tissue.
A newer international unit is the Gray, which
corresponds to 100 rads.
Because low-energy particles produce more
biologic damage than gamma or x-rays, the rem unit was introduced
to describe the biologic effect produced by a rad of high-energy
radiation.
At the cellular level, radiation essentially has
two effects: a somatic effect, associated with acute cell killing, and its
induction of genetic damage.
Radiation-induced cell death, is attributed to the acute effects of the radiolysis of water.
The production of activated oxygen species results in lipid peroxidation,
membrane injury, and possibly an interaction with macromolecules of the
cell.
Genetic damage to the cell, whether caused by direct absorption of energy
by DNA (the target theory) or caused indirectly by a reaction of DNA with
oxygen radicals, is expressed either as mutation or as reproductive
failure. Both mutation and reproductive failure may lead to
delayed cell death, and the former is incriminated in the development of
radiation-induced neoplasia.
The differential sensitivity of tissues to radiation has been recognized
for years. For
example, the intestine and the hematopoietic bone marrow are far more
vulnerable to radiation than tissues such as bone and brain.
These
differences should not be construed as a reflection of variable
sensitivity to acute cell killing (even though there may be slight
differences in the acute, somatic response of cells to radiation based on
anti-oxidant and other metabolic defenses).
The important consideration
is that the vulnerability of a tissue to radiation-induced damage depends
on its proliferative rate, which in turn correlates with the natural life
span of the constituent cells.
Damage to the DNA of a long-lived, nonproliferating cell does not necessarily pose a threat to its function
or viability because the reproductive and metabolic functions of the cell
are distinct.
By contrast, a short-lived, proliferating cell such as an
intestinal crypt cell or a hematopoietic precursors, must be rapidly
replaced by division of the stem cells and the committed precursor.
When radiation-induced DNA damage precludes mitosis of these cells, the
mature elements are no longer replaced and the tissue can no longer
function.
Except for unusual circumstances, as in the massive irradiation that
precedes bone marrow transplantation, significant levels of whole-body
irradiation result only from industrial accidents or from the explosion of
nuclear weapons.
By contrast, localized
irradiation is an inevitable byproduct of any diagnostic radiologic
procedure, and it is the intended result of radiotherapy.
Acute somatic
cell death occurs only with extremely high doses of radiation, well in
excess of 1000 rads. It is morphologically indistinguishable from the coagulative necrosis produced by other causes. Irreversible damage to the replicative capacity of cells requires far lower doses, possibly as little
as 50 rads.
Whole-Body Irradiation: click here
Localized Radiation Injury Associated with Radiotherapy: click here
Radiation and Cancer: click here
Microwave and Ultrasound Radiation:
Microwave, produced by ovens, radar, and diathermy,
are electromagnetic waves that penetrate tissue but do not produce
ionization.
Unlike x- and gamma radiation, the absorption of microwave
energy produces only heat. Thus exposure to microwave radiation under
ordinary circumstances is highly unlikely to produce any injury.
Ultrasound, the vibrational
waves in air above the audible range, produce mechanical compression but,
again, no ionization. Highly focused and energetic ultrasound devices are
used to disrupt tissue in vitro for chemical analysis and to clean various
surfaces, including teeth. However, there is no reason to believe that
diagnostic ultrasound or accidental exposure to any industrial device
results in any measurable damage.
Radiation syndromes: a new look at
persisting problems of clinical significance.British
Journal of Radiology Supplement_27 (2005),v-vi
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