Radiation Biology

1. RADIATION BIOLOGY
CHAPTER 4
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2. LEARNING OBJECTIVES
LESSON 4.1: RADIATION BIOLOGY
1. Define the terms associated with radiation injury.
2. Describe the mechanisms and theories of radiation
injury.
3. Define and discuss the dose–response curve and
radiation injury.
4. Describe the sequence of radiation injury and list the
determining factors for radiation injury.
5. Discuss the short-term and long-term effects as well as
the somatic and genetic effects of radiation exposure.
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3. LEARNING OBJECTIVES
LESSON 4.1: RADIATION BIOLOGY (CONT.)
6. Describe the effects of radiation exposure on cells,
tissues, and organs and identify the relative sensitivity of
a given tissue to x-radiation.
7. Define the units of measurement used in radiation
exposure.
8. List common sources of radiation exposure.
9. Discuss risk and risk estimates for radiation exposure.
10. Discuss dental radiation and exposure risks.
11. Discuss the risk versus benefit of dental images.
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4. INTRODUCTION
 Radiation biology is the study of the effects of ionizing radiation
on living tissue.
 All ionizing radiations are harmful and produce biologic changes
in living tissue
 Although the amount of x-radiation used in dental imaging is
small, biologic damage does occur
 Purpose
 To describe the mechanisms and theories of radiation injury
 To define the basic concepts and effects of radiation exposure
 To detail radiation measurements
 To discuss the risks of radiation exposure 4

5. MECHANISMS OF INJURY
 Some x-rays do not reach the dental x-ray film; they are
absorbed by the patient’s tissue.
 Absorption is the total transfer of energy from the x-ray photon
to patient tissues
 Chemical changes occur that result in biologic damage.
 Two mechanisms of radiation injury are possible.
 Ionization
 Free radical formation
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6. 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 result in biologic damage.
 The biologic changes may or may not have an effect on cells.
 Little effect if chemical changes do not alter sensitive molecules
 Change may have profound effect on structures of great importance
to cell function (e.g. DNA)
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7. 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
 Radicals are believed to be involved in degenerative diseases and
cancers. 7

8. 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 8

9. DIRECT THEORY
 Cell damage results when ionizing radiation
directly hits critical areas, or targets, within the
cell.
 For example, if x-ray photons directly strike the DNA of
a cell, critical damage occurs, causing injury to the
irradiated organism
 This occurs infrequently
 Most x-rays pass through the cell with little or no damage.
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10. 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
 An indirect injury results because the free radicals combine and
form toxins, not because of a direct hit by x-ray photons
 Occurs frequently because of the high water content of the cells.
 The changes of free radical formation and indirect injury are great
because cells are 70% to 80% water.
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11. THINGS TO REMEMBER
 All ionizing radiations are harmful
 All ionizing radiations produce biologic
changes in tissues
 There is no such thing as “safe” x-rays
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12. DOSE-RESPONSE CURVE
 To establish acceptable levels of radiation exposure, it is useful to plot
the dose administered and the damage produced
 Curve is used to correlate the damage to tissue with the dose of
radiation received.
 A linear, non-threshold relationship is seen.
 The linear relationship indicates that the response of the tissues is directly
proportional to the dose.
 The non-threshold dose-response curve suggests that no matter how small
the amount of radiation received, some biologic damage occurs.
 There is no safe amount of radiation exposure.
 Although the doses received by the patient are low, damage does
occur. 12

13. DOSE-RESPONSE CURVE
 Most of the info used to produce dose-response curves
for radiation exposure come from studying the effects
of large doses of radiation on populations, for example,
atomic bomb survivors
 In the low-dose range, however, minimum information
has been documented
 Instead, the curve has been extrapolated from animal cellular
experiments
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14. STOCHASTIC AND NONSTOCHASTIC
RADIATION EFFECTS
 The deleterious effects of ionizing radiation on human tissues can be
divided into two types:
 1. Stochastic effects
 A direct function of the dose
 The probability of stochastic effects increases with an increase in the absorbed
dose.
 No dose threshold; effects do not depend on the magnitude of the absorbed
dose
 Examples: Cancer and genetic mutations
 2. Nonstochastic (deterministic) effects
 Somatic effects that have a threshold; effects increase in severity with
increasing absorbed dose
 Nonstochastic effects require large doses of radiation to cause serious health
issues.
 Examples: Erythema, loss of hair, cataracts, and decreased fertility
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15. SEQUENCE OF RADIATION INJURY
 Exposure to radiation can bring about changes in body chemicals, cells,
tissues, and organs.
 Varying amounts of time are required for change to alter cells and cellular
functions
 As a result, the observable effects of radiation are not visible immediately
after 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 (can be long or short)
 Period of injury
 A variety of cellular injuries may result. (cell death, changes in cell function)
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16. SEQUENCE OF RADIATION INJURY (CONT.)
 Recovery period
 The last event in the sequence of radiation injury
 Not all cellular radiation injuries are permanent
 Depending on a number of factors, cells can repair the damage caused by
radiation.
 Most of the damage caused by low-level radiation is repaired within the cells
of the body
 Cumulative effects
 Effects of radiation exposure are additive.
 Unrepaired damage accumulates in tissues.
 Repeated radiation exposure can lead to health problems such as cataracts,
cancer, and birth defects
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17. TISSUE AND RADIATION EFFECT
Tissue or Organ Radiation Effect
Bone marrow Leukemia
Reproductive cells (ova, sperm) Genetic mutations
Salivary gland Carcinoma
Thyroid Carcinoma
Skin Carcinoma
Lens of the eye Cataracts
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18. DETERMINING FACTORS FOR RADIATION INJURY
 In addition to understanding the mechanisms, theories, and sequence of
radiation injury, it is important to recognize the factors that influence radiation
injury.
 Total dose
 Quantity of radiation received; more damage occurs when tissues absorb large
quantities of radiation
 Dose rate
 Rate at which exposure to radiation occurs and absorption takes place; more
radiation damage with high dose rates
 Amount of tissue irradiated
 Total-body irradiation produces more adverse systemic effects than if small, localized
areas of the body are exposed
 Cell sensitivity
 More damage occurs in cells that are most sensitive to radiation
 Age
 Children are more susceptible to radiation damage than are adults.
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19. SHORT- AND LONG-TERM EFFECTS
 Short-term effects (not applicable to dentistry)
 Associated with large doses of radiation in a short amount of time
 Effects that are seen within minutes, days, or weeks
 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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20. SOMATIC AND GENETIC EFFECTS
 All the cells in the body can be classified as either somatic
or genetic
 Somatic cells
 All cells in the body except the reproductive cells
 Genetic cells
 The reproductive cells
 Biologic effects of radiation can be classified as somatic
or genetic.
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21. SOMATIC AND GENETIC EFFECTS (CONT.)
 Somatic effects
 Seen in the person irradiated; poor health in the indiviudal
 Not seen in future generations
 Damage to somatic cells only has a chance of being expressed in that person’s
lifetime
 Whether the damage will be expressed depends on an individual’s genetic
makeup and the degree of mutation
 Genetic effects
 Not seen in the person irradiated
 Passed on to future generations
 Genetic mutations are not apparent until the next generation, because the true
function of these cells is to provide similar information from the contributing
parent to the receiving child
 Genetic damage cannot be repaired 21

22. 22

23. RADIATION EFFECTS ON CELLS
 No all cells respond to radiation in the same manner
 A cell that is sensitive to radiation is termed radiosensitive;
one that is resistant is termed radioresistant.
 The response is determined by:
 Mitotic activity: cells that divide frequently or undergo many
divisions over time are more sensitive to radiation
 Cell differentiation: cells that are immature or are not highly
specialized are more sensitive to radiation
 Cell metabolism: cells that have a higher metabolism are more
sensitive to radiation 23

24. RADIATION EFFECTS
ON TISSUES AND ORGANS
 Radiosensitive tissues and organs
 Blood cells
 Small lymphocytes (most sensitive to radiation)
 Immature reproductive cells
 Lymphoid tissue
 Bone marrow
 Intestines
 Radioresistant tissues and organs
 Salivary glands
 Cells of the bone
 Muscle
 Nerve
 Kidney
 Liver
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25. RADIATION EFFECTS
ON TISSUES AND ORGANS (CONT.)
 Critical organ
 An organ that, if damaged, diminishes the quality of a person’s life
 Critical organs exposed during dental radiographic
procedures include:
 Skin
 Thyroid gland
 Lens of the eye
 Bone marrow
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26. UNITS OF MEASUREMENT
 Radiation can be measured in the same manner, as other physical
concepts such as time, distance, and weight
 Such units are used to define three quantities of radiation: 1)
exposure, 2) dose, and 3) dose equivalent
 Traditional (older) units of radiation measurement
 Roentgen (R)
 Radiation absorbed dose (rad)
 Roentgen equivalent (in) man (rem)
 SI (newer) units of radiation measurement
 Coulombs/kilogram (C/kg)
 Gray (Gy)
 Sievert (Sv)
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27. EXPOSURE MEASUREMENT
 Roentgen (Traditional)
 Roentgen measures radiation by determining
the amount of ionization that occurs in air.
 It does not describe the amount of radiation
absorbed.
 No SI equivalent
 Exposure is stated in coulombs per kilogram. 27

28. DOSE MEASUREMENT
 The amount of energy absorbed by tissue
 Traditional unit is the rad (radiation absorbed dose).
 Unlike the roentgen, the rad is not restricted to air
and can be applied to all forms of radiation.
 SI equivalent is the gray.
 1 Gy = 100 rad
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29. DOSE EQUIVALENT MEASUREMENT
 Dose equivalent measurement is used to
compare biologic effects of different kinds of
radiation.
 Traditional unit is the rem (roentgen equivalent man).
 SI equivalent is the sievert.
 1 Sv = 100 rem
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30. RADIATION RISKS
 Sources of radiation exposure
 Background radiation—depends on where a person lives
 Average person is exposed to approx. 3.1 mSv of background
radiation per year
 Man-made radiation—3.1 mSv per year
 Medical radiation—accounts for nearly half o the annual total exposure
received
 Risk and risk estimates
 In dental imagine, risk is the likelihood of an adverse effect,
specifically cancer induction, occurring from exposure to ionizing
radiation
 The potential risk of dental imaging inducing a fatal cancer in an
individual has been estimated to be approximately 3 in 1 million
 The risk of a person developing cancer spontaneously is much higher
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31. RISK AND RISK ESTIMATES (CONT.)
 1 in 1 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 per day
 Risk estimates suggest that death is more likely to occur
from common activities than from dental imaging
procedures and that cancer is much more likely to be
unrelated to radiation exposure.
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32. DENTAL RADIATION AND EXPOSURE RISKS
 Extremely high doses would be necessary to
cause damage to any of these organs.
 Risk estimates
 Thyroid gland
 Bone marrow
 Skin
 Eyes
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33. PATIENT EXPOSURE AND DOSE
 The amount of
exposure a patient
receives is
dependent on the
use of these four
things.
 Film speed
 Collimation
 Technique
 Exposure factors
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34. RISK VERSUS BENEFIT OF DENTAL IMAGES
 Dental images should be prescribed for a patient only
when the benefit of disease detection outweighs the
risk of biologic damage.
 When dental images are properly prescribed and
exposed, the benefit of disease detection far outweighs
the risk of damage.
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35. QUESTIONS?
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