ZOM703_Radiation_Toxicity.ppt

Ionizing Radiations and Their
Biological Effects
1
ZOM703

2
What is Radiation?
•Form of energy
•Emitted by nucleus of atom or orbital electron
•Released in form of electromagnetic waves
or particles

3
The Electromagnetic Spectrum
Waveform of Radiation
NONIONIZING IONIZING
Radio
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays

4
Difference between ionizing and
nonionizing radiation
•Energy levels:
Ionizing radiation has enough energy to break
apart (ionize) material with which it comes in
contact (knock off e-)
Non ionizing radiation does not

5
Types of Ionizing Radiation
• Important in healthcare:
• Diagnosing - X-Rays, PET
Scans, Nuclear Medicine
• Therapy - Radiation
Treatment, Nuclear
Medicine

6
Sources of Radiation Exposure
•Naturally occurring sources – ground, atm
•Environmental radiation – power plants
•Medical procedures (patient) – x-ray, chemo
•Occupational sources (worker) - airports

7
• Penetrating electromagnetic waves – can cause
internal damage
• Can pass through soft tissue, but not bone
• Originate in outer part of atom
• Used in medical procedures (diagnostic, CT, fluro)
• Energy inversely proportional to wavelength
The shorter the wave, the stronger the energy
X-Ray

Exposures to Radiation
•Tanning beds/sun tanning
•X-ray
•Mammogram
•CT scan
•Nuclear medicine
•Dental X-ray
•Bone scan
•Angioplasty
8

9
Biological Effects of Radiation
• Somatic
Affects cells originally
exposed (cancer)
Affects blood, tissues,
organs, possibly entire
body
Effects range from
slight skin reddening to
death (acute radiation
poisoning)
• Genetic
Affects cells of future
generations
Keep levels as low
as possible (wear
lead)
Reproductive cells
most sensitive

10
Units of Measurement
•Effect of ionizing radiation is determined
by:
Energy of radiation
Material irradiated
Length of exposure
Type of effect
Delay before effect seen
Ability of body to repair itself

11
Radiation Units of Measurement
Roentgen (R) - expression of exposure to x-
rays/gamma rays
Radiation Adsorbed Dose (rad) – amt of energy
released to / absorbed by matter when radiation
comes into contact with it
Radiation Equivalent Man (rem) - Injury from
radiation (depends on amt of energy imparted to
matter)

12
Permissible exposure radiation doses
Body Part
Exposed
Permissible Dose
(rem per quarter)
Whole body 1.25
Hands, forearms, feet,
ankles
18.75

13
Purple and yellow
Radiation Symbol

14
Monitoring Instruments
•Personal monitoring:
Film badges, bracelet, rings
Pocket dosimeter

15
Basic Safety Factors
•Keep exposures As Low As Reasonably
Achievable (ALARA)
Time - Keep exposure times to a minimum
Distance - Inverse square law: by doubling
distance from a source, exposure is dec by a
factor of 4
Shielding – wear lead, use lead wall

16
Basic Safety Factors
•Shielding

17
Laser Radiation
•Sun and lasers are nonionizing radiation
•Eyes are very susceptible to damage from
laser light
•Laser emits either:
Infrared (IR) light
Ultraviolet (UV) light

18
Eye Safety Factors
•Safety glasses are made for a specific wave
length of laser light
•IMPORTANT: Use only the appropriate safety
glasses for laser that you are exposed

Biological effects ionizing
radiations
19
ZOM703

Dominion Dental Journal, 1897
Excerpts: “Danger in X-rays”
“So as to better diagnose the dental troubles
of which Miss Josie McDonald of New York
complained, Drs. Nelson T. Shields and
George F. Jernignan a month ago decided
to have an X-ray photograph taken of the
young woman’s face.
The picture was taken by Mr. J. O’Connor,
and as a result of the exposure to the strong
mysterious light, Ms. McDonald is now
suffering from burns.

A few days after being photographed.
The skin on the young woman’s face,
neck, shoulder, left arm and breast
became blistered and finally peeled off.
One ear swelled to three times its
natural size and it is said there has been
no hearing in it since.

The first picture taken of the young
woman, O’Connor admits, was
unsatisfactory, and a second and
successful attempt was made. The first
exposure lasted eight minutes and the
last one thirteen minutes. Besides the
burns, large patches of Miss
McDonald’s hair have fallen out”

Biological Effects
•First case of radiation-induced human
injury was reported in the literature in
1896.
•Who discovered X rays and when?
• First case of X-ray induced cancer was
reported in 1902

Biological Effects
 X-radiation energy is transferred to the
irradiated tissues primarily by Photoelectric
and Compton’s processes which produce
ionizations and excitations of essential cell
molecules such as DNA, enzymes, ATP,
coenzymes, etc.
 The functions of these molecules are altered.
 The cells with damaged molecules can not
function normally.

Biological Effects
 The severity of biological effect is
related to the type of molecule
absorbing radiation.
 Effect on DNA molecule is more harmful
than on cytoplasmic organelles

Mechanism of Action
• Two mechanisms of radiation damage, mostly
on DNA:
• Direct action: Damage or mutation occurs at the
site where the radiation energy is deposited.
• Indirect action: The radiation initially acts on
water molecules to cause ionization. The water
is abundantly present in the body (approx. 70 %
by weight)
• Indirect effect accounts for 2/3rd of the damage,
direct effect is responsible for the remainder.

Indirect Action
•The ions, H2O+ and H2O-, are very
unstable and break up into free radicals.

Indirect Action
•Free radicals:
highly reactive atoms and molecules
react with and alter essential
molecules that come in contact with
them.
•These altered molecules have
different chemical and biologic
properties from the original molecules.
This translates to biologic damage.

Indirect Action
•Free radicals may also combine with
each other to produce hydrogen
peroxide
OH• + OH•-------> H2O2
•Hydrogen peroxide is a cell poison
which may contribute to biological
damage

Radiation Effects at Cellular
Level
•Point mutations: Effect of radiation on
individual genes is referred to as point
mutation.
•The effect can be loss or mutation in a
gene or a set of genes.
•The implication of such a change is that
the cell may now exhibit an abnormal
pattern of behavior.

Radiation Effects at Cellular
Level
• Chromosome alterations: Several kinds of
alterations in the chromosomes have been
described. Most of these are clearly visible
under the microscope.
• The effect upon chromosomes can result in
the breaking of one or more chromosomes.
The broken ends of the chromosome seem to
possess the ability to join together again after
separation.

Chromosome Breaks
•Such damage may be repaired rapidly
in an error-free fashion by cellular repair
processes (restitution) using the intact
second strand as a template.
•However, if the separation between
broken fragments is great, the
chromosome may lose part of its
structure (deletion).

Chromosome Breaks
•If more than one break, the broken
fragments may join in different
combinations.
•inversion of the middle segment followed
by recombination

Chromosome Breaks
•Double-strand breakage: when both
strands of a DNA molecule are
damaged. Sections of one broken
chromosome may join sections of
another, broken chromosome.

Chromosome Breaks
•A large proportion of damage will result
in misrepair which can result in the
formation of gene and chromosomal
mutations that may cause malignant
development.

Arrested Mitosis
• Ionizing radiations also affect cell division,
resulting in arrested mitosis and,
consequently, in retardation of growth. This
phenomenon is the basis of radiotherapy of
neoplasms.
• The extent of arrested mitosis varies with the
phase of the mitotic cycle that a cell is in at
the time of irradiation. Cells are most
sensitive to radiation during the last part of
resting phase and the early part of prophase.

Cytoplasmic Changes
•Cytoplasmic changes probably play a
minor role in arrested mitosis and cell
death.
•Swelling of mitochondria and changes
in cell wall permeability have been
observed.

Radiation Effects at Tissue Level
•Two types of biological effects may
appear in tissues after exposure to
ionizing radiation.
•Somatic effects
•Genetic effects

•Somatic effects include responses of all
irradiated body cells except the germ
cells of the reproductive system.
•Somatic effects are deleterious to the
person irradiated.
•Somatic effects may be stochastic or
deterministic.

•Genetic effects. Include responses of
irradiated reproductive cells.
•Genetic effects become primarily
important when they are passed on to
future generations.
•Genetic effects are of no consequence
in persons who do not procreate or who
are in the post-reproductive period of
life.

Somatic Effects
•Somatic tissues do not always react to
doses of ionizing radiation so as to give
immediate clinically observable effects.
There may be a time-lapse before any
effects are seen.
•Basically, somatic effects are classified
in two categories:
 Acute or immediate effects
 Delayed or chronic (latent) effects

Acute Somatic Effects
•Appear rather soon after exposure to a
single massive dose of radiation or after
several smaller doses of radiation
delivered within a relatively short period
of time.
•In general, effects which appear within
60 days of exposure to radiation are
classified as acute effects.

Delayed Somatic Effects
•Delayed effects may occur anywhere
from two months to as late as 20 years
or more after exposure to radiation. The
time lapse between the exposure to
radiation and the appearance of effects
is referred to as the "latent period."
•In radiobiology, the term “latent period”
is usually used only in relation to
stochastic effects (malignancy)

Variables in Somatic Effects
•The magnitude of somatic effects
depend on the following variables:
Individual
Species
Cellular and tissue
Extent of exposure (full or partial body)
Total dose
Dose rate

•Individual Variability. Certain
individuals are more sensitive or
resistant than others in their response to
radiation.
•The expression, “LD50 (30 days)”, is
frequently used in radiobiology which
means that a certain dose kills 50% of
the exposed animals within 30 days.
•The 50% who survive are due to the

•Species variability. The phenomenon
of species variability is well known. The
reason is not well-understood.

•Cellular and tissue variability. In 1907
Bergonie and Tribondeu advanced the
first generalization in radiobiology by
stating that "cells are sensitive to
radiation in proportion to their
proliferative activity and in inverse
proportion to their degree of
differentiation.“
•Simply stated, it means that the rapidly
dividing cells are more sensitive to
radiation than more differentiated,

Bergonie and Tribondeu’s Axiom
•One of the most notable exceptions to
this generalization is the lymphocyte,
not capable of proliferative activity, is
a differentiated cell, and is one of the
most radiosensitive cells in the body.

•Total-body vs localized-area
exposure. A single radiation dose of
4.5-5.0 Gy may produce only erythema
of the skin if given to a localized part of
the body.
•However, if the same dose is given to
the entire body, it will cause the death of
50 percent of the people exposed.
•This quantity of radiation is identified as
LD50, the lethal dose for 50 percent of
the people thus exposed

•Specific area protection

• Total dose: The higher the dose of radiation,
the greater is the probability and severity of
occurrence of biological effects.

• Dose rate dependence: radiation dose that
would be lethal if given in a short time, such as
a few hours, may result in no detectable effects
if given in small increments during a period of
several years.
• This is due to the ability of somatic cells to
repair damage caused by exposure to
radiation. However, tissues do not return to
their original state following radiation damage,
as there are some irreparable alterations
produced.

Variables-Dose Rate
•In general, it may be stated that four-
fifths of somatic damage is repaired. But
the irreparable damage is cumulative.
When this cumulative damage reaches
a high level, clinical manifestations may
appear.

Variables-Dose Rate
•Local somatic effect (Alexander, p.149
Revised Edition)

Dose-effect Relationships
•Threshold response: An increase in
radiation dose may not produce an
observable effect until the tissue has
received a minimal level of exposure
called the threshold dose.
•Once the threshold dose has been
exceeded, increasing dose will
demonstrate exceeding observable
tissue damage.
•Cataract and erythema of skin are well-

•Linear response: A linear dose-
response suggests that all exposure
carries a certain probability of harm and
that the effects of multiple small doses are
additive.
•The dose response curve for most
radiation-induced tumors is linear which
implies that there is no "safe" dose, i.e.,
no dose below which there is absolutely
zero risk.

• Linear-quadratic response (curve)
A linear-quadratic response implies
lesser risk at lower dose rate than linear
response or when the exposure is
fractionated. However, there is no safe
dose.

• Age.
"The radiosensitivity is very high in new-born
mammals; it decreases until full adulthood is
reached and then remains constant; old mice
(about 600 days) are again more
radiosensitive." (Bacq and Alexander, P.299)
"The embryo is . . . most sensitive during the
period of most active organ development,
which lasts from the second to the sixth week
after conception." (Alexander, p. 156 Revised
Edition)

•Sex
The female is more radioresistant in
some species possibly due to high
levels of estrogens, some of which have
radioprotective properties. (Arena, p.
463)

•Metabolism. The lower the metabolic
rate and the lower the state of nutrition,
the higher the resistance of the
organism to the effects of radiation.
Higher metabolic rate seems to magnify
the radiation effect.

•Linear Energy Transfer (LET)
The dose required to produce a certain
biological effect is reduced as the LET
of the radiation increases. Thus alpha
particles are more efficient in causing
biological damage than low LET
radiations.

•Oxygen effect
The radioresistance of many biological
tissues increases 2 to 3 times when
irradiation is conducted with reduced
oxygen (hypoxia).

Types of Biological
Responses
•Chronic deterministic effects:
•These effects are observed after large
absorbed doses of radiation. Doses
required to produce deterministic effects
are, in most cases, in excess of 1-2 Gy.
•There is usually a threshold dose below
which the effects are not manifested.
•With increasing dose the severity of the
effect increases.

Deterministic Effects
•Skin. Excessive exposure of the skin to
ionizing radiation may result in erythema
or reddening of the skin, which is
produced by dilatation of small blood
vessels beneath the skin.
•The dose of radiation required to
produce erythema of the skin is between
1.65-3.5 Gy.
•Higher doses are associated with
dermatitis.

•Hair. Epilation, or loss of hair, results
from exposure of the skin to 2.0-6.0 Gy.
A latent period of about 3 weeks ensues
before the hair is lost.
•The hair usually grows back in a few
weeks.
•For permanent epilation, considerably
higher doses are required.

• Sterility.
• Sterility results from destruction by X-
radiation of gonadal tissues which produce
mature sperm or ova.
• A single dose of 4.0 Gy to the male gonads is
necessary to produce permanent sterility.
• The dose required to produce permanent
sterility in the female may be 6.25 Gy or
more.

•Cataract. Exposure of the lens of the
eye to radiation can cause cataract
(opacification of the lens).
•The threshold for cataract induction is
2.0-5.0 Gy for a single exposure and
approximately 10.0 Gy or more for
exposures protracted over a period of
months or years.

Therapeutic Radiation to Oral Tissues
•Standard therapeutic radiation dose for
treating cancer is approximately 50 to
60 Gy.
•Administered over a period of 10 to 14
weeks at the rate of approximately 2.5
Gy twice weekly.

Radiation Effect on Oral Tissues : Teeth
• Adult teeth:
very resistant to the direct effect of
radiation exposure.
no effect on the crystalline structure of
enamel, dentin and cementum.
• Radiation caries: in individuals whose
salivary glands have been damaged resulting
in xerostomia. Secondary to changes in
saliva; i.e., reduced flow, pH and buffering
capacity and increased viscosity.

Radiation Effect on Oral Tissues :
Developing teeth
•<10 Gy has very little or no visible
effect.
•Effects to an infant may include:
destruction of tooth bud, tooth
malformation and delay in eruption.

Radiation Effect on Oral Tissues : Bone
• The most serious complication: jaw
osteoradionecrosis.
• This is primarily due to damage to the blood
vessels of the jaw and consequent decreased
capacity of the bone to resist infection.
• Tooth extraction or other injury: possibility of
bone infection and necrosis becomes very
high.
• More common in the mandible than in
maxilla.

Salivary glands
• Xerostomia: marked and progressive loss of
salivary secretion.
• The mouth becomes dry (xerostomia) and
tender.
• The pH of saliva falls below normal (5.5 as
compared to 6.5 in normal saliva).
• The salivary changes influence oral microflora,
and, secondarily contribute to the formation of
radiation caries.
• Whether xerostomia is temporary or permanent
depends upon the volume of glands exposed.

Mucosa
• Mucositis. At 3rd or 4th week, oral mucosa
becomes red and inflamed (mucositis). As the
therapy continues, mucosa forms yellow
pseudomembrane.
• Secondary infection by Candida albicans is a
common complication. Mucositis is most
severe at the end of the treatment period.
• Healing begins soon after treatment and is
usually complete in about two months after
therapy. The mucosa tends to become
atrophic, thin and relatively avascular
permanently. Dentures may frequently cause
oral ulceration.

Radiation Effect on Oral Tissues: Taste
buds
•Taste acuity is reduced or lost in about 4
weeks into the radiation treatment.
•In general, bitter and acid flavors are
more severely affected when posterior
third of the tongue is irradiated and salt
and sweet when anterior third is
irradiated.
•Complete recovery of taste usually
occurs in two to four months following
treatment completion.

•Life span shortening. Life span of small
laboratory animals can be shortened by
exposure to repeated large doses of
radiation.
•If this phenomenon occurs among the
human beings is inconclusive.

•Embryological and developmental
effects. therapeutic doses of radiation
delivered to the pelvic region of a
pregnant woman can result in the death
of the fetus or in the birth of an
abnormal child.
•The developmental effects on the
embryo are strongly related to the stage
at which the exposure occurs.

Embryological and developmental
• The first 2 weeks of pregnancy: most critical
period. If the dose is high, the fetus will die.
The congenital anomalies are rare at this
stage.
• The highest incidence of malformations is the
period of organogenesis (3-8 weeks of
pregnancy).
• The threshold doses are relatively low: 100-
200 mGy for most malformations and 200
mGy for brain damage.

Embryological and developmental
• After organogenesis, effect is at the tissue
and cellular level rather, than at the organ
level; so that gross, congenital anomalies are
not to be expected.
• In general, a dose as small as 100 mGy may
cause gross defects. In Denmark, a
therapeutic abortion is recommended once it
is determined that the fetus has received 100
mGy (or 100 mSv) of radiation.

Acute Radiation Syndrome
• Radiation Sickness.
• Symptom complex that occurs after the
exposure of the entire body, or a major
portion of the body to a large dose of
radiation (above 1.0 Sv) within a short period
of time. The effect may vary from a transient
illness to death.
• A radiation dose of this magnitude is not
expected in any diagnostic procedure,
especially in dentistry.

•Prodromal Syndrome. 1.0 - 2.0 Gy
exposure.
•Individual usually develops G.I.
symptoms such as nausea, vomiting,
weakness, fatigue, and anorexia. These
symptoms usually disappear soon.

•Hematopoietic Syndrome. 2.0 - 7.0 Gy.
•Severe injury to hematopoietic system
of the bone marrow, irreversible
damage to the proliferative capacity of
the of the spleen and bone marrow.
•Rapid fall in the number of circulating
granulocytes, platelets and erythrocytes
•Rampant infection, due in part from
lymphopenia, granulopenia, and
anemia. The death occurs in 10 to 30

• Gastrointestinal syndrome. 7.0 to 15.0 Gy.
• Extensive damage to GI system: anorexia,
nausea, vomiting, severe diarrhea and malaise
in a few hours after exposure. Basal epithelial
cells of the intestinal villi are destroyed.
• Loss of plasma and electrolytes into the
intestines, hemorrhages and ulcerations.
Results in dehydration and loss of weight. The
denuded surface gets rapidly infected;
septicemia and death is an invariable
consequence.

•Cardiovascular and CNS syndrome.
Excess of 50 Gy.
•Death occurs within 1 or 2 days.
Common symptoms are:
uncoordination, disorientation and
convulsions. This is due to damage to
the neurons and brain vasculature.

Stochastic Effects
• The most important effect of ionizing radiation
on human mortality is judged to be neoplasia
and leukemia . Radiation in this regard is
considered a two-edged sword. It cures
cancer and it also causes cancer.
• The probability of carcinogenic effect
increases with dose.
• It is currently judged that there is NO
THRESHOLD below which the effect will not
occur. Severity of the effect is independent
of the radiation dose.

Stochastic Effects
• There is no controversy relative to relationship
of ionizing radiation exposure and neoplasia
production.
• It is universally accepted that such exposure
increases incidence of tumors in a great variety
of tissues and organs.
• It is important to appreciate that in the U.S.,
almost 20 percent of deaths are attributable to
cancer (400,000 annually) and a very small
fraction of this total number is due to radiation
exposure.

Stochastic Effects
•A statistically significant increase in
cancer has not been detected in
populations exposed to doses less than
50 mSv.

Stochastic Effects- Evidence
• The largest group of individuals studied are
the Japanese atomic bomb survivors.
• In the cohort of 86,572, there were 9,335
deaths from solid cancer between 1950 and
1997. Only 440 deaths were estimated to be
excess over spontaneous incidence and were
considered radiation-induced cancer deaths
(NCRP Report # 145).
• During the same period, 87 leukemia deaths
can be attributed to radiation exposure.

•Other studies have followed over
14,000 British patients who received
spinal irradiations for ankylosing
spondylitis between 1935-1954.
•36 cases of leukemia and 563 cases of
cancer of other types have been
reported in these patients.

•Patients receiving repeated fluoroscopic
examinations during treatment of
tuberculosis and women treated with
radiation for postpartum mastitis
between 1930-1956 demonstrated a
higher risk of breast cancer.

•Increased incidence of thyroid cancer
has been observed in children who
received radiation therapy for enlarged
thymus. Breast cancer was also
elevated in these individuals.

• Until the 1950’s, X rays were used to
epilate children with tinia capitis
(ringworm infection of the scalp) in
Israel. Over 10, 000 children were
exposed.
• These children showed a higher
incidence of thyroid cancer as well as
brain tumors, salivary gland tumors,
skin cancer and leukemia.

• Increased incidence of leukemia in
radiologists (as compared to non-
radiologic physicians) who practiced
before the radiation protection
methods were established.
• Bone tumors in radium dial painters.

•Higher incidence of lung cancer in
miners in Saxony who dug out the ore
from which the radium was extracted.
•Higher incidence of lung cancer was
also reported in uranium miners in
central Colorado

•All patients in above studies received
exposures well above diagnostic range.
•The probability of diagnostic-dose
radiation-induced cancer occurrence
can only be estimated by extrapolating
from cancer rates observed following
exposures to larger doses.

Stochastic Effects- Generalizations
• Cancers other than leukemia typically start to
appear 10 years following exposure (5 years
for leukemia) and the increased risk remains
for the lifetime of the exposed individuals.
• The risk from exposure during fetal life,
childhood and adolescence is estimated to be
about 2-3 times as large as the risk during
adulthood.

Stochastic Effects
• Leukemia: The incidence of leukemia (other
than chronic lymphocytic) rises following
exposure of red marrow. Wave of leukemia
appear within 5 years of exposure, and return
to base line rates within 40 years.
• Children under 20 are more at risk than
adults.
• The mortality data for leukemia are
compatible with a linear quadratic dose
response relationship.

Stochastic Effects
•Thyroid cancer: The incidence of thyroid
carcinoma increases following radiation
exposure.
•The susceptibility is greater early in
childhood that later in life.
•Females are 3 times more susceptible
than males to both radiation induced
and spontaneous thyroid cancer.

Stochastic Effects
• Bone cancer: Patients treated for childhood
cancer demonstrate an increasing risk of
bone sarcomas.
• Brain and nervous system cancer: Ionizing
radiation exposure can induce tumors of the
CNS. Most tumors are benign such as
neurilemommas and meningiomas (average
mid-brain dose of 1 Gy). Malignant brain
tumors have also been demonstrated, but
only at radiation therapy doses.

Stochastic Effects
•Esophageal cancer: The data regarding
esophageal cancer is sparse. Excess
cancers are found in the Japanese A-
bomb survivors as well as in patients
treated with X-rays for ankylosing
spondylitis.

Stochastic Effects
•Salivary-gland cancer: An increased
incidence of salivary gland tumors has
been demonstrated in patients
therapeutically irradiated for the
diseases of head and neck, in the
Japanese A-bomb survivors and in
persons exposed to diagnostic levels of
x-radiation (cumulative parotid dose of
0.5 Gy or more).

Stochastic Effects
• Skin: Association between ionizing radiation
exposure and development of basal cell
carcinoma is well documented in the
literature. There is minimal indication of
association with malignant melanoma.
• Other organs: Excess cases of multiple
myeloma as well as malignancy of paranasal
sinuses have also been demonstrated in
patients receiving radiation doses.

Risk Estimation
•Four agencies or bodies
comprehensively review, assess, or
estimate the radiation risk to humans
from exposure to ionizing radiation and
periodically publish their findings in the
form of reports. These agencies are:

Risk Estimation
1. The Biological Effects of Ionizing Radiations
(BEIR) Committee of the U.S. National
Research Council
2. International Commission on Radiological
Protection (ICRP)
3. National Council on Radiation Protection
and Measurements (NCRP)
4. United Nations Scientific Committee on the
Effects of Atomic Radiation (UNSCEAR).

Risk Estimation
• Radiation induced tumors are clinically,
morphologically and biochemically indistinguishable
from those which occur spontaneously.
• This implies that carcinogenic effects of radiation may
be demonstrated on statistical basis only; that is, one
may infer such action by the demonstration of an
excess in the number of cancers in the irradiated
population over the natural incidence.
• Alternately, the probability of the cancer incidence
from a small dose is estimated by extrapolating from
cancer rates observed following exposure to large
doses.
• Risk vs benefit

ZOM703_Radiation_Toxicity.ppt

Recomendados

Recomendados

Mais conteúdo relacionado

Semelhante a ZOM703_Radiation_Toxicity.ppt

Semelhante a ZOM703_Radiation_Toxicity.ppt (20)

Último

Último (20)

ZOM703_Radiation_Toxicity.ppt