7. 7
Radiobiology
The study of the sequence of events following
the absorption of energy from ionizing
radiation, the efforts of the organism to
compensate, and the damage to the
organism that may be produced
8. 8
Interactions of
Radiation and Matter
Direct action
Radiation interacts with the target
Indirect action
Radiation interacts with something else that
eventually causes an interaction with the target
• Typically HOH
• More common than direct
9. 9
Indirect Action
Free radical
An atom or molecule with an unpaired electron
and no charge
Very reactive
10. 10
Free Radical Production
HOH + ionizing radiation HOH+ + e-
Can rejoin without damage
e- can bond with HOH
• HOH + e- HOH-
Both products disassociate
HOH+ H+ + OHl
HOH- OH- + Hl
– l represents a free radical
Typically the H+ and OH- rejoin to form HOH with no
damage
11. 11
Free Radical Production
Interactions of free radicals
Possible results
• Hl + OHl HOH
• Hl + Hl H2
• OHl + OHl H2O2
• Join with other normal molecule
Hl + O2 HO2
12. 12
Linear Energy Transfer (LET)
A measure of the energy transferred or
deposited into a material as an ionizing
particle travels through the material
Low LET
• X and gamma rays
Moderate LET
• Neutrons
High LET
• Alpha particles
13. 13
Relative Biologic Effectiveness
(RBE)
A comparison of doses between a standard
radiation (250 kV, x-rays) and a test radiation
(R) that yield the same biologic result
RBE = D250/DR
As LET increases, RBE increases
14. 14
Oxygen Enhancement Ratio
(OER)
A numeric representation of the dose
comparison for a given biologic effect in
anoxic and aerobic conditions
OER = Danoxic/Daerobic
As LET and RBE increase, OER decreases
16. 16
Radiation Effects on DNA
Repair
Base damage
Loss or change of a base
Single-strand break
Double-strand break
Cross-linking
An abnormal bond between DNA strands or
proteins
18. 18
Radiation Effects on
Chromosomes
Acentric fragment
Two broken ends without a centromere
Dicentric chromatid
Two chromosomes with broken ends join, resulting
in one chromosome with two centromeres
Ring
Translocation
Inversion
Deletion
19. 19
Radiation Effects on Other Cell
Components
Cell membrane
Changes in the permeability
Mitochondria
Lysosome
20. 20
Cellular Response to Radiation
In vivo means in the organism
Can observe the effects of radiation only on skin
and hematopoietic system
In vitro means in glassware
21. 21
Fate of Irradiated Cells
No damage
Division delay or mitotic delay
Cell is held in G2 before entering mitosis
Mitotic overshoot
Interphase death
Dose dependent
Reproductive failure
Cell fails to enter mitosis
23. 23
Cell Survival Curve
Describes the relationship between dose and
the percentage of surviving cells
Based on experimental data
Suggests that there are two mechanisms for
cell death
Lethal single-hit killing
Accumulation of multiple sublethal hits resulting in
death
25. 25
Cell Survival Curve
Straight line portion
As dose doubles, the percentage surviving
decreases by half
Occurs at higher doses
Shoulder
The initial portion of the survival curve (low dose)
does not behave like the straight line portion
Initial slope is much more shallow
26. 26
Target Theory
D1
Sometimes called 1D0
Represents the initial
slope of the curve
D0
Represents the terminal
slope or straight line
portion
D37
Dose required to kill all
but 37% of the cells
Dq
Quasithreshold dose
Extrapolation of D0 to the
100% line
N
Extrapolation number or
target number
Extrapolation of D0 back
to the vertical axis
Thought to represent the
number of targets in the
cell
32. 32
Linear Quadratic Model
Dual radiation action theory
a: Lethal single-hit kills
b: Accumulation of sublethal dose kills
D: Dose
SF = aD +bD2
aD is the linear component
bD2 is the quadratic component
34. 34
Linear Quadratic Model
Can be rewritten to account for fractionation
SF = aD[1 + d/(a/b)]
d is the fraction dose
[1 + d/(a/b)] is the relative effectiveness
a/b is the dose at which single-hit and multihit
killing are equal
SF/a is the biologic effective dose
35. 35
Law of Bergonié and Tribondeau
Cells are most radiosensitive when
Actively proliferating
Highly metabolic
Undifferentiated
Well nourished
36. 36
Law of Ancel and Vitemberger
Describes biologic stress and sensitivity to
radiation
Postulates that all cells have the same
inherent radiosensitivity because all have the
same target
“Radiosensitive” cells are those under
biologic stress, such as the need to divide
38. 38
Clonogenic Assay
Investigate the cell’s ability to divide
In situ assay
Example: Intestinal crypt cells
Measure the number of cell colonies after various
doses
Transplantation assay
Example: Bone marrow
Transplant irradiated cells into a new host
Measure the number of cell colonies after various
doses
39. 39
Functional Assays
Used to assess cells that do not rapidly divide
by measuring function after irradiation
Measure late effects
Results in dose-response curves rather than
cell survival curves
40. 40
Lethality Assays
Measure the number of dead organisms after a
specific dose of radiation to a specific organ
LD50
Dose required to kill 50% of the population
Also known as median lethal dose
LD50/30
• Dose required to kill 50% of population in 30 days
TD5/5
Dose that will cause 5% of the population to have
effect after 5 years
41. 41
Cellular Response
Factors that alter the cellular response to
radiation
Physical factors
Chemical factors
Biologic factors
42. 42
Physical Factors Affecting
Cellular Response
LET and RBE
Higher LET and RBE leads to a decrease in SF
High LET and RBE result in steeper shoulder and
slope
Dose rate
Slower dose rates lead to increase in SF
Slow dose rates result in a more shallow shoulder
and slope
High LET radiation is not affected by changes in dose
rate
43. 43
Chemical Factors Affecting
Cellular Response
Radiosensitizers
Increase the effect of ionizing radiation
Presence of oxygen
• Not well understood
• Theorized to increase the production of free radicals or
prevent the repair of chemical damage following radiation
Radioresisters
Also known as radioprotectors
44. 44
Biologic Factors Affecting
Cellular Response
Cell cycle
Most radiosensitive in G2 and M phases
Least radiosensitive in S
Cell cycle is less important as dose increases
Intracellular repair
Basis for fractionation
Most repair completed within 24 hours
45. 45
Acute vs. Late Changes
Acute effects
The result of the depletion of parenchymal cells
Chronic (late) effects
Primary chronic effects
• The result of the depletion of nonparenchymal cells
Secondary chronic effects
• Consequence of irreversible early changes
46. 46
Tissue Healing
Regeneration
Replacement of a dead cell with a cell with the
same function
Repair
Replacement of a dead cell with a different cell
type
• Example: Scar
Both are tissue type and dose specific
47. 47
Organ-Specific Effects
Bone marrow
Reduction in number of stem cells
Principle of TBI
Blood
Cell type specific
• Circulating RBCs are radioresistant
• Lymphocytes are the most sensitive
48. 48
Organ-Specific Effects
Skin
High doses may lead to atrophy, fibrosis,
pigmentation changes, and/or necrosis
Hair follicles are radiosensitive
Sweat glands are somewhat radioresistant
Skin-sparing effects of high-energy radiation
49. 49
Organ-Specific Effects
Gastrointestinal tract
Moderate doses cause mucositis and esophagitis
Small bowel is the most radiosensitive GI organ
Intestinal crypt cells or cells of Lieberkühn
• Replaced daily
• Extremely high doses lead to intestinal denuding
50. 50
Organ-Specific Effects
Male reproductive system
Most tissue is radioresistant, except testes
Reduction in spermatogonin
• Also known as maturation depletion
• Mature sperm is radioresistant
Temporary sterility occurs after 2.5 Gy
Permanent sterility occurs with doses greater than
6 Gy
Any dose may lead to inheritable chromosome
aberrations
51. 51
Organ-Specific Effects
Female reproductive system
Sterility is age dependent
• Temporary sterility may occur after 6.25 Gy
• Radiation-induced permanent sterility will result in early-
onset menopause
Any dose may lead to inheritable chromosome
aberrations
53. 53
Total-Body Response
Conditions for radiation syndromes
Acute exposure
• Seconds to minutes
Total- or near-total-body exposure
External source of radiation
54. 54
Survival Time
Life span shortening is the major effect of
total-body exposure
Measured by LD50/30
Actual doses will vary by species and
individuals within the species
Small percentage of mammals will die after 2 Gy
Between 2 and 10 Gy, survival decreases as dose
increases
Between 10 and 100 Gy, there is little effect on
survival
Above 100 Gy, survival decreases as dose
increases
55. 55
Radiation Syndromes
Stages of response
All patients, regardless of syndrome, experience
the same stages
• Length of stage varies
Prodromal
• Nausea, vomiting, diarrhea
Latent
• Patient appears to be healthy
Manifest illness
• Specific syndrome presents
56. 56
Hematopoietic Syndrome
Doses between 1 and 10 Gy
Prodromal stage
Begins hours after exposure and persists for days to
weeks (3 weeks)
Pancytopenia can result in infection or
hemorrhage
Death
After 2 Gy in 6-8 weeks in sensitive individuals
After 4-6 Gy is the range of LD50/30
After 10 Gy, all die within 2 weeks unless given bone
marrow transplant
• Rarely successful
57. 57
Gastrointestinal Syndrome
Doses between 10 and 100 Gy
Death is independent of dose
All die at same time
• 3-10 days without medical intervention
• 2 weeks with medical intervention
Death is the result of intestinal denuding
58. 58
Central Nervous System
Syndrome
May occur at doses as low as 50 Gy
Latent period ends 5-6 hours postexposure
Death occurs in 2-3 days
Individual experiences nervousness and confusion
Cause of death is not well understood
Autopsies reveal little cellular damage
59. 59
Embryologic Effects
Most sensitive during first few weeks of
development
Divisions of pregnancy
Preimplantation
• First 8-10 days
Major organogenesis
• Second week to seventh week
Fetus
60. 60
Embryologic Animal Studies
Preimplantation exposure
200 R leads to an embryonic death rate of 80%
and a 5% abnormality rate
Major organogenesis exposure
200 R leads to an embryonic death rate of 25%
and a 100% abnormality rate
• Most abnormalities are skeletal or CNS
Fetal exposure
200 R yields negligible side effects
61. 61
Embryologic Human Studies
Pregnant survivors of the atomic bomb
Doses greater than 2 Gy resulted in 36% of
children born with mental retardation
Doses between 0.5 and 1 Gy yielded a mental
retardation rate of 4.55%
Incidence of mental retardation in general
population is less than 1%
63. 63
Somatic Effects
Effects of radiation that occur in the irradiated
individual and cannot be passed on to future
generations
May occur months to years postexposure
A probability of developing effect exists with
all doses
Probability increases as exposure increases
Example: Smoking and lung cancer
64. 64
Carcinogenesis
Risk associated with doses lower than 1 Gy is
not known
Case studies
Radium dial painters
Thymus irradiation in infants
Early medical radiation personnel
Uranium mine workers
Survivors of the atomic bombs
65. 65
Risk
Absolute risk
Associated with a latent period and a period of
increased risk followed by a return to normal risk
• Example: Leukemia
Relative risk
Continuous risk throughout life
Population must be followed until death
Methods of estimating risk
Linear: Assume all doses have same potential for effect
Linear quadratic: Assume that dose and risk are
proportional
66. 66
Cataractogenesis
Normal lens fibers are transparent
Radiation damages lens cells, resulting in cataract
formation
Dose is species dependent
Dose is patient specific
May be as low as 2 Gy but all after 7 Gy
Fractionated dose threshold is 12 Gy
67. 67
Life Span Shortening
Decrease in average life span documented in
irradiated animal populations
No unique diseases
Earlier onset
Retrospective studies of early radiologists
Life span shortening of 5 years on average
69. 69
Mutations
Spontaneous mutations
Changes in DNA that are not the result of outside
stimuli
Permanent and possibly inheritable
Examples: Down syndrome, hydrocephalus
Mutation frequency
Number of spontaneous mutations in a generation
Mutagens
Source of mutation
Examples: Viruses, chemicals, radiation
70. 70
Measuring Risk
Doubling dose
Unit of measurement for mutation frequency
Dose required to double the percentage of
mutations in a generation
71. 71
Studies on Genetic Effects
Animal
Fruit flies
• Hermann Muller
• Determined radiation does not cause unique mutation but
does increase mutation frequency of spontaneous mutations
• No dose threshold
Mega-mouse experiments
• Russell and Russell
Human
Pregnant atomic bomb survivors
72. 72
Goal of Radiation Therapy
“Treat the tumor, spare the normal tissue”
Damage is random and nonspecific
Equal probability for normal tissue and tumor
Do not typically treat to tumoricidal doses
Probability of damage increases as dose
increases
74. 74
Tumor Cell Characteristics
Group 1 (P cells)
Well oxygenated and actively proliferating
Responsible for growth fraction (GF)
Most radiosensitive
Group 2 (Q cells)
Well oxygenated but not proliferating
In quiescence but may be source of future
recurrence
75. 75
Tumor Cell Characteristics
Group 3 (Q cells)
Hypoxic and not proliferating
Most radioresistant
Group 4
Anoxic and necrotic, dead
Not a source of concern
76. 76
Tumor Growth
Measured in doubling time
Time required to double total number of cells
Cell cycle
General rule: Tumor cells have a shorter cell cycle
than normal cells
Doubling time of 40-100 days vs. 60 days for
normal cells
Growth fraction
GF = # of P cells / (# of P cells + # of Q cells)
As GF increases, doubling time decreases
Cell loss
Result of cell death or metastases
77. 77
Role of Oxygen in Tumor Growth
Tumors eventually outgrow vasculature
Central areas of necrosis if tumor is larger than
100-180 microns
Related to the diffusion distance of oxygen, also
known as oxygen tension
Cells closer to the vessel are more
radiosensitive
78. 78
Tumor Radiosensitivity
Varies when total dose to kill tumor is
considered
Varies by tumor cell type
D0 used as measurement
Some postulate that it is the cell’s repair
capabilities not its radiosensitivity
79. 79
Normal Tissue Tolerance Dose
Dose at which additional radiation would
significantly increase probability of severe
normal tissue reaction
Isoeffect curves
Tolerance doses
TD50/5
• Dose that will cause effect in 50% of population in 5 years
TD5/5
• Dose that will cause effect in 5% of population in 5 years
Based on standard fractionation of 10 Gy/week,
2 Gy/day, and 5 days/week
80. 80
Time-Dose Fractionation
The division of the total dose into equal
smaller parts
First used in 1927
Sterilized ram testes without skin reaction
Less effective than single dose of same size
Also has significantly fewer side effects
81. 81
Factors Affecting Effectiveness
of Fractionation
Redistribution
Synchronization of surviving cell into resistant
mitotic phases
Normal cells tend to remain in resistant phases,
whereas tumor cells enter all phases
Reoxygenation
Death of aerobic tumor cells allows hypoxic cells
to become more oxygenated
Regeneration
Occurs between fractions for highly mitotic cells
Repair
Cellular repair of sublethal damage (SLD)
