Radiation Biology
Rad T 290
Objectives – Radiation Biology

Radiosensitivity

Somatic Effects

Embryonic and
Fetal Risks

Genetic Effects
Patient Interactions
 **Photoelectric**
 Classic Coherent
Scatter
 **Compton
Scattering**
 Pair Production
 Photodisintegration
Interaction in
the body begin at
the atomic level
Atoms
Molecules
Cells
Tissues
Organ structures
X-ray photons can change
cells
Some radiations are energetic enough to rearrange
atoms in materials through which they pass, and can
therefore he hazardous to living tissue.
1913
Interactions of X-rays with matter
• No interaction; X-ray passes
completely through tissue and
into the image recording
device.
• Complete absorption; X-ray
energy is completely absorbed
by the tissue. No imaging
information results.
• Partial absorption with
scatter; Scattering involves a
partial transfer of energy to
tissue, with the resulting
scattered X-ray having less
energy and a different
trajectory. Scattered radiation
tends to degrade image quality
and is the primary source of
radiation exposure to operator
and staff.
Coherent Scattering
 Also called: Classical scattering or Thompson
scattering
 Occurs with energies below 10 keV
 Incident x-ray interacts with an atom of
matter, causing it to become excited.
Immediately the atom releases this excess
energy and the scattered x-ray.
Coherent Scattering
 The wavelength is equal to the incident x-ray
or equal energy.
 The only difference is the direction of travel
 Energy in = Energy out - Only changes is
direction
Coherent / Classical Scatter
Classical (Coherent) Scattering






Excitation of the total
complement of atomic
electrons occurs as a result
of interaction with the
incident photon
No ionization takes place
Electrons in shells “vibrate”
Small heat is released
The photon is scattered in
different directions
No loss of E
Thompson scatter
 Occurs primarily with low energy x-rays.
Classical will occur throughout the diagnostic
range.
 Coherent contributes slightly to film fog and
reduces image contrast.
Compton Effect or Compton
Scattering
 Occurs throughout the diagnostic
imaging range
 The incident x-ray interacts with the
outer electron shell on an atom of
matter, removing it.
 It not only causes ionization but scatters
the incident x-ray causing a reductions
in energy and the change of direction.
COMPTON
SCATTERING –
OUTER SHELL
ELECTRON IN
BODY –
INTERACTS
WITH
X-RAY PHOTON
FROM TUBE
Compton scatter
 A fairly high energy (high kVp) x-ray photon ejects an




outer shell electron.
Though the x-ray photon is deflected with somewhat
reduced energy (modified scatter), it retains most of its
original energy and exits the body as an energetic
scattered photon.
A Compton e- is also released
Since the scattered photon exits the body, it does not
pose a radiation hazard to the patient.
It can, however, contribute to film fog and pose a
radiation hazard to personnel (as in fluoroscopic
procedures).
XXXXX
Compton scatter
 Both the scattered x-ray and the Compton
electron have enough energy to cause more
ionization before loosing all their energy
 In the end the scattered photon is absorbed
photoelectrically
Compton Effect
 The Compton electron looses all of its kinetic
energy by ionization and excitation and drops
into a vacancy in an electron shell previously
created by some other ionizing event
 The probability of Compton effect increases
as photon energy increases, however the
atomic number does not affect the chances of
the Compton effect
Compton Scatter
 Compton is just as likely to occur with soft
tissue as bone. Compton can occur with any
given photon in any tissue
 Compton is very important in Radiography,
but not in a good way.
 Scattered photons provides no useful
diagnostic information
Compton Effect
 Scattered radiation produces a uniform
optical density on the radiograph that reduces
image contrast
 Scattered radiation from Compton contributes
to the majority of technologists exposure,
especially during fluoroscopy
 STAY AWAY FROM YOUR PATIENT !
Photoelectric Effect or Absorption
 Inner-shell ionization
 The photon is not scattered it is totally
absorbed
 The e- removed from the atom of matter is
called a photoelectron, with an energy level
equal to the difference between the incident
photon and the e- binding energy.
Binding Energy is very important

Photoelectric – Absorption
PHOTOELECTRIC ABSORBTION
IN THE PATIENT
(CASCADE OF ELECTRONS)
Photoelectric effect
• A relatively low energy (low kVp) x-ray photon
uses all its energy (true absorption) to eject an
inner shell electron,
• leaving an orbital vacancy.
• An electron from the shell above drops down to
fill the vacancy and, in doing so, gives up
energy in the form of a characteristic ray.
• The photoelectric effect is more likely to occur
in absorbers of high atomic number (eg, bone,
positive contrast media)
• and contributes significantly to patient dose,
• as all the photon energy is absorbed by the
patient (and for the latter reason, is responsible
for the production of short-scale contrast).
Electron transitions
 Are accompanied by the emission of more x-
rays – secondary radiation
 Secondary radiation behaves much like
scatter radiation
 Secondary contributes nothing to the image
 The probability that any given photon will
undergo a photoelectric interaction is
dependent on the photon energy and the
atomic number of the atom
CASCADE
Photodisintegration
Important X-ray Interactions
 Of the five interactions only two are
important to radiology
 Photoelectric effect or photoelectric
absorption
 Compton scatter
Compton scatter
 Contributes to no useful information
 Is independent of the atomic number of
tissue. The probability of Compton is the
same for bone atoms and for soft tissue
atoms
 The probability for Compton is more
dependent on kVp or x-ray energy
Compton Scatter
 Results in image fog by optical densities not
representing diagnostic information
 Photon are Photons
IR is does not know
the difference
Photoelectric Absorption
 Provides information to the IR because
photons do not reach the IR
 This represents anatomic structures
with high x-ray absorption
characteristics; radiopaque structures;
tissue with high atomic number; or
tissue with high mass density
Attenuation – The total reduction in the # of photons
remaining in an x-ray beam after penetration through tissue
 Absorption = x-ray disappears (Photoelectric,
Pair production & Photodisintegration)
 Scattering = partially absorbed, x-ray
emerges from the interaction traveling in a
different direction (sometimes with less
energy)
 Absorption + Scattering = Attenuation
Attenuation
3 Types of x-rays are important for
IMAGE FORMATION
 DIFFERENTIAL ABSORPTION = the
difference between those x-rays absorbed
and those transmitted to the IR
 Compton scatter (no useful information)
 Photoelectric absorption (produces the light
areas on the image)
 Transmitted x-rays (produces the grey/dark
areas on the image)
Differential Absorption
 Increases as the kVp is reduced
 Approximately 1% of photons that interact
with the patient (primary beam) reach the IR.
Of that 1% approximately 0.5% interact to
form the image
Differential Absorption
 The difference in x-ray interactions
 Fundamental for image formation
 Occurs because of Compton Scattering,
Photoelectric absorption, and X-ray
transmission
Differential Absorption
Compton vs. Photoelectric
 Below 80 kVp Photoelectric absorption is
predominant above 80 kVp Compton scatter
begins to increase.
 Dependent on the tissue attenuation
properties
Differential absorption factors
 High atomic number = larger atoms
 Mass Density = how tightly the atoms of
tissue are packed
 Z # for air and soft tissue are about
the same the OD changes are due to
mass density difference
Human Biology
 X-rays are harmful, low energy photons can
cause skin burns, cancer, leukemia
 It is not known for certain the degree of effect
following diagnostic levels of x-radiation
Technologists Responsibilities
 Technologists, Student Technologists,
Radiologists & Medical Physicists have
ethical & professional responsibilities to
produce high-quality x-ray images with
minimal radiation exposure
 What is the acronym for this?
47
CARDINAL RULES
OF RADIATION PROTECTION
•TIME
•DISTANCE
•SHIELDING
49
Natural radiation
• Natural radiation accounts for
approximately 300 millirem (mrem)
• 3 sources of environmental radiation:
cosmic rays, terrestrial radiation and
internally deposited radionuclides. The
largest source of natural radiation is radon.
Biological Response to
Ionizing Radiation
 X-ray interactions with matter (human
tissue) can cause biological changes.
 Technologists must understand cellular
biology and how radiation interacts with
cells in order to protect oneself and the
patient.
 RBE – Relative Biological Effectiveness
THE EARLY YEARS
Early measurement of
Radiation
Skin dryness & erythemia
 Ulcers formed


Late Effects:
Cataracts
 Cancers

Radiobiology
Radiobiology

The study of the effects of ionizing
radiation on biologic tissue
 Most radiobiology research is designed
to develop dose-response relationships
to determine the effect of planned doses
or accidents
Comparsion of Units
REMS
R - ROENTGENS
RADS –
PATIENT DOSE
OCCUPATIONAL EXPOSURE
RADS
REMS

RADS

REMS

GRAYS

SIEVERTS

PATIENT
ABSORBED
DOSE
Employee
(technologists) =

Rad VS. Rem

1 RAD X QF = 1 REM

1 GRAY X QF = 1 SIEVERT

QF FOR X-RAYS = 1

So…… Rads = Rems
TYPES OF RADIATON
(ALL CAUSE IONIZATION)






PARTICULATE
ALPHA
BETA
FAST NEUTRONS
Unit of mesaure is
the curie (Ci) or
becquerel (Bq)
More destructive




ELECTROMAGNETIC
XRAY
GAMMA
(damaged caused by
indirect action = free
radicals – can be
repaired)
QUALITY FACTOR
Qualifies what the damage is from
different types of radiation

Example: QF for X-ray is 1
QF for alpha is 20

Alpha is 20 x more damaging to tissue

Measurement
(Rad + QF = Rem)
RBEMeasures biologic tissue
response to radiation
66
Patient dose
 Is reported in Entrance Skin Exposure (ESE)
REGULATORY AGENCIES
 NCRP – National Council on Radiation
Protection and Measurement ?
 NRC – Nuclear Regulatory Committee ?
 Other regulatory agencies?
REGULATORY AGENCIES
 NCRP – National Council on Radiation
Protection and Measurement
 Reviews recommendation for radiation
protection & safety
 NRC – Nuclear Regulatory Committee
 Makes LAWS & enforces regulations
 California Department of Public Health,
Radiologic Health Branch (CDPH)
Title 17
Human Radiation Response
 The effects of x-rays on human is the result of
interactions at the atomic level
 Ionization or excitation
 The result if a deposit if energy in tissue. The
excess energy can result in a molecular
change that can be measurable if the
molecule involved is critical to metabolic
function
At each stage cell repair is possible
Atom ionization
 Can cause chemical binding property
change. If the atom is part of a large
molecule the ionization may cause
molecule break down or relocation of
the atom within the molecule
Abnormal molecules
 In time may function improperly or cease to
function. This may cause serious impairment
or death of the cell
 This process is reversible by the ionized atom
attracting a free e- and become neutral again
 Cell and tissues can regenerate and recover
from the radiation injury
Cell bombarded with photons
What damage will they cause?
TARGET THEORY
 BIOLOGIC RESPONSE TO IONIZING
RADIATION DEPENTS ON WHERE THE
PHOTON INTERACTS
 CELL STRUCTURE

NUCLEUS & CYTOPLASM
The most at risk area of the cell…….
CHROMOSOMES, WHICH ARE MADE UP OF
GENES.

Cellular Absorption
Direct vs. Indirect Hit
Direct Hit Theory:
 When radiation
interacts with DNA.
 Ionization of a DNA
molecule.
 Break in the bases
or phosphate bonds
 Can injure or kill the
cell
Indirect Hit Theory:
 Occurs when water
molecules are
ionized
 Produces chemical
changes – injury or
cell death
 Vast majority of
cellular damage is
from indirect hit.
Cells
 The most radiosensitive part of the cell is the
deoxyribonucleic acid (DNA)
 Water is the most abundant molecule in the
body. The body is 80% water. Humans are
basically made of structure water.
Basic Cell Structure
Two parts:
1. Nucleus
2. Cytoplasm
Nucleus contains
chromosomes –
genetic info (DNA)
DNA is at risk when a
cell is exposed to
ionizing radiation
Cytoplasm – 80%
water
Tissue response to radiation
 A precise knowledge of various organ
radiosensitivities in unnecessary. However, it
is important to have a general knowledge of
effects of radiation exposure
 A few important general principals are
important to understand
Response of cells to radiation
 CELL SENSITIVITY TO RADIATION
 TYPE OF CELL
 AGE OF CELL
 TYPE OF DAMAGE RECEIVED
 KIND OF RADIATION EXPOSURE
Human cell types
 Two general types:
 Somatic cells
 Genetic cells
MOST CONCERNING EFFECTS OF
RADIATION EXPOSURE
 LATE EFFECTS
SOMATIC
EFFECTS =
INDIVIDUAL EXPOSED
GENETIC
EFFECTS =
FUTURE GENERATIONS
Target Theory = for a cell to die after radiation
exposure, the target molecule must be inactivated
TARGET THEORY
Photons hit master molecule
DNA = cell dies
Or doesn’t hit nucleus – and
just passes through
No essential damage
Hormoresis – repair that can
occur when below 5 rads
of exposure
DNA is the target molecule of
radiation damage
Radiolysis
poison water theory
 The human body is 80% water molecules and
1% DNA molecules
 Irradiation of water represents the principal
radiation interaction in the body
 When water is irradiation, it dissociates into
other molecular products – RADIOLYSIS OF
WATER
Formation if ions & free radicals
 The ion pair may rejoin into a stable water
molecule
 In this case, no damage is done
HOH+ recombine to H2O
Radiolysis
poison water theory
H
2
O molecules
Ejection of electron = free
radical
H2 02 = hydrogen peroxide
Or H O2 = Hydroperoxyl are
formed
Radiosensitivity of Cells
 Bergonie
& Tribondeau (1906) –
method of classifying a cell’s
response to radiation according to
sensitivity.
 Cells
are most sensitive during active
division (primitive in structure &
function).
The Law of
Bergonie & Tribondeaux
 Cells
that are most sensitive to
radiation
 Young – immature cells
 Stem Cells
 Highly dividing (mitotic) cells
 Highly metabolic
Categorizing Radiation
Exposure
Early vs Late effects of Radiation

Early Effect = response that occurs within
minutes or days after exposure

Late Effects = response that occurs within
months or years

**most human responses have been
observed after LARGE doses. To be
cautious we assume even small doses are
harmful**
Predicting Radiation Dose
Responses
Radiobiology

Irradiated tissue response, besides the
cell properties, is determined by the
amount of energy deposited per unit mass

Linear Energy Transfer (LET) = the rate at
which energy is transferred from ionizing
radiation to tissue
LET

The ability of ionizing radiation to produce
biologic response increases as the LET of
radiation increases

When the LET of radiation increases
ionizations increase. When LET is high,
ionizations occur frequently, increasing
the potential for biologic damage
Relative Biologic Effectiveness

As the LET of radiation increases, the
chances of biologic damage also increases

Relative Biologic Effectiveness (RBE) =
standardizes biologic effects of radiation
exposure

RBE for diagnostic x-rays is 1

radiation with lower LET is less than 1,
radiation with higher LET is greater than 1
QUALITY FACTOR
Qualifies what the damage is from
different types of radiation
Example: QF for X-ray is 1

QF for alpha is 20


Alpha is 20 x more damaging to tissue
TYPES OF RADIATON
(ALL CAUSE IONIZATION)





PARTICULATE
(HIGH LET)
ALPHA
BETA
FAST NEUTRONS






More destructive
ELECTROMAGNETIC
(LOW LET)
XRAY
GAMMA
(damaged caused by
indirect action = free
radicals – can be
repaired)
Why did the bunny die??
BUNNY A

Received 200 rads
BUNNY

B
Received 200 rads
Why did the bunny die??
BUNNY A
200 rads x 1 for
X-RAY = 200 RADS
BUNNY B
200 rads x 20 for alpha
=
4000 rads
LET vs RBE
Biologic Factors Affecting
Radiosensitivity

Oxygen Effect – tissue is more sensitive
when the tissue is oxygenated

Age – Humans are most sensitive before
birth, sensitivity decreases until maturity,
after maturity humans are mostly
resistant to radiation effects
Age Radiosensitivity
LD 50/30
HIGH DOSES
RECEIVED
50% OF THE
POPULATION
WOULD DIE
IN 30 DAYS
110
Threshold vs Chance
Deterministic (non stochastic) vs Stochastic
Radiation Dose-Response
Relationships
 Every radiation dose-response relationship
has two characteristics
 Linear or Nonlinear
 Threshold or Stochastic (chance)
Linear Dose-Response Relationships
 Linear dose-response – when radiation dose
is doubled the response to radiation is
likewise doubled
 Nonthreshold dose-response – any dose,
regardless of it size is expected to produce a
response – chance
 Threshold dose-response – a radiation doses
below a certain level no response is expected
Linear nonthreshold = A & B
Linear threshold = C & D
LINEAR RESPONSE TO
RADIATION –
ASSUMES NO PHOTON
IS SAFE
A. DIAGNOSTIC X-RAY - No
Threshold –
LOW DOSE – OVER LONG
EXPOSURE
B. Early Radiology Exposure
Threshold amount needed to
see affect
FIG. 9–7 Graph indicates no-threshold versus threshold response to radiation.
Elsevier items and derived items © 2007, 2003 by Saunders, an imprint of Elsevier Inc.
SOMATIC & GENETIC
STOCHASTIC VS NON STOCHASTIC
 A = STOCHASTIC
 “CHANCE” EFFECTS
 NONTHRESHOLD
GENETIC, LEUKEMIA,
CANCER
DIAGNOSTIC RADIOLOGY
B= NON-STOCHASTIC
THRESHOLD EFFECTS
DETERMINISTIC
SOMATIC EFFECTS
SKIN ERYTHEMA, CATARACTS,
STERILITY
RAD -MALIGNANCIES
Linear vs Non linear
• Linear – direct
response to the dose
and the effects seen
(proportionally)
• Non linear – effects
are not proportional to
the dose received
• S curve – rad therapy,
skin erythema, most
somatic, deterministic
radiation effects.
120
Organ Systems
 Are identified by their rate of cell proliferation
and their stage of development. Each organ
system have different rates
 Immature cells are called undifferentiated
cells, precursor cells or stem cells.
 Stem cells are more sensitive to radiation
than mature cells
Tissue types
 Radiosensitivity of tissue is also dependent
on structural or functional features
 Tissue types include: Epithelium, Connective
(supporting tissues), Muscle and Nervous
 The various organs of the body exhibit a wide
range of sensitivity to radiation. This is
determined by the function of the organ, the
rate at which cells mature in the organ, and
the inherent radiosensitivity of the cell type
Example of cell sensitivity
Organ or Tissue Weighting Factor
Effective Dose

NCRP: report # 116
Total Body Response to Radiation
 Acute Radiation Syndrome – full body
exposure given in a few minutes.
 3 stages of response:
1. Prodromal Stage: NVD stage
(nausea, vomiting, diarrhea)
2. Latent Period: Feels well while
undergoing biological changes
3. Manifest Stage: Full effects felt,
leads to recovery or death
3 Acute Radiation Syndromes
Early Effects
•
•
•
Bone marrow syndrome: results in infection,
hemorrhage & anemia
Gastrointestinal syndrome: results in diarrhea, nausea
& vomiting, fever
Central nervous syndrome: results in convulsions,
coma, & eventual death from increased intracranial
pressure.
CNS least sensitive in ADULTS –
MOST sensitive in the FETUS
Late Effects of Radiation
 Somatic Effects: develop in the individual who
is exposed
Most common: Cataract formation &
Carcinogenesis
 Genetic Effects: develop in future
generations as a result of damage to germ
cells.
SENSITIVITY TO
RADIAITION
 Which (Male or Female) GONADs are
external vs internal
 Which gender is born with all their
reproductive cells?
 Which gender constantly produces new
cells?
 Which GENDER is more sensitive to
radiation at birth? Why?
Response of cells to radiation
 CELL SENSITIVITY TO RADIATION
 TYPE OF CELL
 AGE OF CELL
 TYPE OF DAMAGE RECEIVED
 KIND OF RADIATION EXPOSURE
• What is this called
• What type classification (direct or indirect?)
133
Pg 619
Permissible
Occupational Dose
• Annual dose:
• 5 Rem / year 50 mSv /
year (NOT TO EXCEED 1.25 rem/quarter)
• Cumulative Dose
• 1rem x age 10mSv X age
OCCUPATIONAL EXPOSURES
• 5 REMS / YEAR
BUT NOT TO EXCEED 1.25
REM/QUARTER
• Technologist essentially receive all
exposure during fluoroscopy exams
Occupational Dose
ANNUAL LIMITS
• WHOLE BODY = 5 rems /
5000mrem
• LENS OF THE EYE = 15
rems
• EXTREMITIES = 50 rems
PUBLIC EXPOSURE
• 10 % OF OCCUPATIONAL
•
(MUST BE MONITORED IF ABOVE 10%)
• NON MEDICAL EXPOSURE
• .5 RAD OR 500 MRAD
• UNDER AGE 18 AND
STUDENT
• 100 mrem 1 mSv
GSD
• GENETICALY SIGNIFICANT DOSE
• Takes all of the population into account
• Annual AVERAGE gonadal dose to
population of childbearing age
• 0. 20 mSv or 20 millirem
• *Bushong
• *30
mrem per NRC website
139
Fetus Exposure
 Radiation exposure is most harmful
during the first trimester of pregnancy
 Embryo-Fetus Exposure limit (Monthly)
 0.05
rem or 0.5 mSv
Effects of radiation in utero are time
and dose related
 Effects include:
 Prenatal death, neonatal death,
congenital abnormalities, malignancy
inductions, general impairments of
growth, genetic effects, and mental
retardation.
Irradiation in Utero
 The first trimester is the most
radiosensitive period. After the 2 weeks
of fertilization
 The first 2 weeks of pregnancy may be
of least concern because the response
is all or nothing
After 200 rads delivered at various
times
Declared Pregnant Worker
• Must declare pregnancy – 2 badges
provided
• 1 worn at collar (Mother’s exposure)
• 1 worn inside apron at waist level
Under 5 rad – negligible risk
Risk increases above 15 rad
Recommend abortion (spontaneous) 25
rad
(“Baby exposure” approx 1/1000 of ESE)
Pregnancy & Embryo
Mother –
occupational worker (5 rem)
• Baby – (500 mRem)
• .5 rem/ year
• .05 rem/month
• 5 mSv .5 mSv / month
Pregnant patient
• ALWAYS ASK LMP before exposure
made
• “10-day Rule” No longer used
• “Grace period” of implantation
• What is the State Law for gonadal
shielding?
Pregnant Patients
 Should never knowingly expose a pregnant
patient unless a documented decision to so
has been made
 If you must expose; use precise collimation &
protective shields. Use a high kVp technique
and only the minimal projections
Unsuspected pregnancy
 Always screen female patients for last LMP

don’t assume ages (patient privacy)
 If unsure obtain a blood test or reschedule
exam if possible
PREGNANT PATIENTS
•
•
•
•
•
•
•
•
•
Ascertain LMP - if fetus is exposed
Medical Physicist will need information:
Which x-ray machine used (mR/mAs)
# Of projections (including repeats)
Technique for each exposure
SID
Patient measurement at C/R
Fluoro time & technique used
Physicist will calculate fetal dose
90 % of cell damage will repair.
At each stage cell repair is possible
Protraction & Fractionation
cause less biological effect
If radiation is delivered over a long period
of time rather than quickly, the effect of
that dose is lessened. Allows for
intercellular repair and tissue recovery.
 Protraction



Dose is delivered continuously but at a lower
dose rate
Fractionation

Same dose rate in short doses over a longer
period (occupational exposure)
Biologic Factors Affecting
Radiosensitivity
 Recovery – human cells can recover from
radiation damage. If the radiation dose is not
sufficient to kill the cell before its next
division. Then given sufficient time, the cell
will recover

If a tissue or organ receives a sufficient
radiation dose it responds by shrinking or
atrophy. Cells disintegrate and are carried
away as waste products
Hormesis
Pg. 518
 repair that can occur when below 5 rads
of exposure
 A growing body of radiobiologic
evidence suggests that a little bit of
radiation is good for you. It stimulates
hormonal and immune responses to
other toxic environmental agents
 We still practice ALARA
Why cancer risks at low doses are
uncertain
 It has been difficult to estimate cancer
induction risks, because most of the radiation
exposures that humans receive are very
close to background levels.
 At low dose levels of millirems to tens of
rems, the risk of radiation-induced cancers is
so low,
 that if the risk exists, it is not readily
distinguishable from normal levels of cancer
occurrence.
 In addition, leukemia or solid tumors induced
by radiation are indistinguishable from those
that result from other causes.
156
Always remember….
 IMAGE GENTLY, LIGHTLY & WISELY !!
Objectives – Radiation Biology
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Radiosensitivity

Somatic Effects
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Questions
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Embryonic and
Fetal Risks

Genetic Effects