Download Chapter 7 Body Systems

Effects of Radiation
Exposure
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
Mechanisms of Injury
 Some
x-rays do not reach the dental x-ray
film; they are absorbed by the patient’s
tissue.

Chemical changes occur that result in
biologic damage.
 Two
mechanisms of radiation injury are
possible.


Ionization
Free radical formation
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Ionization
 Results



when x-rays strike patient tissue
Produced through the photoelectric effect or
Compton scatter
Results in formation of a positive atom and
dislodged negative electron
This electron will interact with other atoms
within the absorbing tissues causing chemical
changes within the cell that results in biologic
damage.
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Free Radical Formation
 Cell
damage occurs primarily through
formation of free radicals.
 Free radicals are formed when an x-ray
photon ionizes water.

Free radical
• An uncharged atom or molecule that exists with a
single, unpaired electron in its outermost shell
• Highly reactive and unstable
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Theories of Radiation Injury
 Damage
to living tissue caused by exposure
to ionizing radiation may result from


A direct hit and absorption of an x-ray photon
within a cell
Absorption of an x-ray photon by water within a
cell accompanied by free radical formation
 Two
theories to describe how radiation
damages biologic tissues


Direct theory
Indirect theory
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
5
Direct Theory
 Cell
damage results when ionizing
radiation directly hits critical areas within
the cell.

This occurs infrequently.
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Indirect Theory

X-ray photons are absorbed within the cell and
cause the formation of toxins, which in turn
damage the cell.


When x-ray photons are absorbed by water within a cell,
free radical formation results.
The free radicals combine to form toxins that damage
cells.
• 80% of body is water, ionization changes H2O to
hydrogen and hydroxyl radicals which the theory
proposes changes to hydrogen peroxide. These
chemicals poison the cells and cause dysfunction
• Also called the poison water theory
• This changing back very quickly which limits the
damage
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Dose-Response Curve
 Curve
is used to correlate the damage of
tissue with the dose of radiation received.
 A linear, nonthreshold relationship is seen.


The linear relationship indicates that the
response of the tissues is directly proportional
to the dose.
The nonthreshold dose-response curve
suggests that no matter how small the amount
of radiation received, some biologic damage
occurs.
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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ALARA CONCEPT
 As
low as reasonably achievable
 Radiation protection community considers
any amount of ionizing radiation exposure
nonthreshold meaning that its produces
damages and should be kept to a
minimum
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Radiation Effects Stochastic and
Nonstochastic
 Stochastic


effects
A direct function of the dose
No dose threshold; effects do not depend
on the magnitude of the absorbed dose
• Examples - cancer and genetic mutations
 Nonstochastic

(deterministic) effects
Somatic effects that have a threshold;
effects increase in severity with increasing
absorbed dose
• Examples: erythema, loss of hair, cataracts,
and decreased fertility
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Sequence of Events Following Radiation Exposure




Latent period

The time that elapses between exposure to ionizing radiation and the
appearance of observable clinical signs

Depends on the total dose of radiation received and the amount of time
it took to receive the dose
Period of injury

Following the latent period; certain effects can be observed

A variety of cellular injuries may result.
Recovery period
 Depending on a number of factors, cells can repair the
damage caused by radiation.
Cumulative effects
 Effects of radiation exposure are additive.
 Unrepaired damage accumulates in tissues.
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Factors Determining for Radiation Injury
 Total
dose
 Dose rate
 Area or amount of tissue irradiated
 Variation in species
 Individual sensitivity
 Cell sensitivity
 Tissue sensitivity
 Age
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Radiation Effects
 Short-
and long-term effects
 Somatic and genetic effects
 Radiation effects on cells
 Radiation effects on tissues and organs
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Short- and Long-Term Effects
 Short-term


effects
Associated with large doses of radiation in a
short amount of time
Acute radiation syndrome (ARS)
• Includes nausea, vomiting, diarrhea, hair loss,
hemorrhage
 Long-term


effects
Small doses absorbed repeatedly over a long
period of time
Effects seen after years, decades, or
generations
• Cancer, birth abnormalities, genetic defects
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Somatic and Genetic Effects
 Somatic

All cells in the body except the reproductive
cells
 Genetic

cells
cells
The reproductive cells
 Biologic
effects of radiation can be
classified as somatic or genetic.
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Somatic and Genetic Effects
 Somatic


Seen in the person irradiated
Not seen in future generations
 Genetic


effects
effects
Not seen in the person irradiated
Passed on to future generations
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Radiation Effects on Cells
A
cell that is sensitive to radiation is
termed radiosensitive; one that is resistant
is termed radioresistant.
 The response is determined by



Mitotic activity
Cell differentiation
Cell metabolism
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Radiation Effects on Tissues and Organs

Law of B and T (Bergonie &Tribondeau):
• states that actively dividing cells, such as
white blood cells are more sensitive than
slowly dividing cells.
• Embryonic and immature cells are more
sensitive than mature cells of the same
tissue.
• The 2nd half of the law states that the more
specialized a cell is the more radioresistant
the cell
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Cells Radiosensitivity










White blood cells
Red blood cells
Immature reproductive
cells
Epithelial cells
Endothelial cells
Connective tissue cells
Bone cells
Nerve cells
Brain cells
Muscle cells

High sensitivity

Low sensitivity
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Radiation Effects on Tissues and Organs
 Critical

organ
An organ that, if damaged, diminishes the
quality of a person’s life
 Critical
organs exposed during dental
radiographic procedures include:




Red bone marrow of the mandible
Lens of the eye
Thyroid gland
Skin
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Units of Measurement
 Traditional
(older) units of radiation
measurement



Roentgen (R)
Radiation absorbed dose (rad)
Roentgen equivalent (in) man (rem)
 Systeme
InternationaIe (newer) units of
radiation measurement



Coulombs/kilogram (C/kg)
Gray (Gy)
Sievert (Sv)
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Exposure Measurement
 Roentgen



 SI


Roentgen measures radiation by determining
the amount of ionization that occurs in air.
It does not describe the amount of radiation
absorbed.
R stands for roentgen
Coulombs per kilogram
Exposure is stated in coulombs per kilogram.
C/kg stands for coulomb per kilogram
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Absorbed Dose Measurement
 Traditional
unit is radiation absorbed
dose
 Rad stands for radiation absorbed dose
 SI equivalent is the gray.

1 Gy = 100 rads
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Dose Equivalent Measurement
 Dose
equivalent measurement is used to
compare biologic effects of different kinds
of radiation.

Traditional unit is roentgen equivalent man
• Rem stands for roentgen equivalent man

SI equivalent is the sievert.
• 1 Sv = 100 rems
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Measurements Used
in Dental Radiography
 Milli

means 1/1000
Used to express the small doses used in
dental radiography
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Sources of Radiation Exposure
 Natural

background radiation
A form of ionizing radiation that is ubiquitous in the
environment
• Cosmic radiation

Stars and sun
• Terrestrial radiation



Radioactive materials in the earth and air
In the United States the average dose of
background radiation received by an individual
ranges from 150 to 300 mrads per year (0.00150.003 Gy).
Varies based on geographic area (p 40)
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Sources of Radiation Exposure
 Artificial

or man-made radiation
Resulting from modern technology
• Includes consumer products, fallout from atomic
weapons, weapons production, and the nuclear
fuel cycle
• Biggest contributor: Medical radiation including
medical radiographic procedures, dental
radiography, fluoroscopy, nuclear medicine, and
radiation therapy
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Risk and Risk Estimates
 The
potential risk of dental radiography
inducing a fatal cancer in an individual has
been estimated to be 3 in 1 million.
 The risk of a person developing a cancer
spontaneously is much higher, or 3300 in
1 million.
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Risk and Risk Estimates
1




in a million risks of a fatal outcome
10 miles on a bicycle
300 miles in an auto
1000 miles in an airplane
Smoking 1.4 cigarettes a day
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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Patient Exposure and Dose

Film Speed



Collimation


Rectangular collimation instead of round reduces the absorbed
dose by 60% to 70%
Technique



Using F speed film instead of D speed reduces absorbed dose
by 60%
Using F speed film instead of E speed reduces absorbed dose
by 20%
Longer source to film distance reduces skin dose
Long cone technique is better
Exposure factors

High kVp reduces skin dose
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Risk versus Benefit of Dental Radiographs

Dental radiographs should be
prescribed for a patient only when the
benefit of disease detection outweighs
the risk of biologic damage.

When dental radiographs are properly
prescribed and exposed, the benefit of
disease detection far outweighs the risk of
damage.
Copyright © 2012, 2006, 2000, 1996 by Saunders, an imprint of Elsevier Inc.
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