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IAEAInternational Atomic Energy Agency
Basics of Biological Effects ofIonizing Radiation
Lecture
Module 1
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Biological effects of radiation
Ionizing radiations have many beneficial applications,but they also may have detrimental consequences forhuman health and for environment
Since X-rays were discovered in 1895, it was quickly
realized that they may be harmful
To protect people and the environment it is essential
to understand how radiation-induced effects occur
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Absorbing ionizing radiation
What is ionizing radiation?
electromagnetic (X and - rays)
corpuscular (- and -particles and neutrons)
A radiation can be considered as ionizing if depositedenergy is high enough to ionize the traversed material
Types
Each type interacts in its own way with material
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Interactions of ionizing radiation with
matter
Photons
For energies lower than 50 MeV there are
three main processes by which photonsinteract with matter:
Photoelectric effect
Compton scattering Pair production
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Photoelectric effect.:incident photon istotally absorbed and ejects electron from
atom. This effect dominates with low-
energy photons interacting with heavier
elements
InCompton scattering electron is also
ejected, but incident photon survives and is
scattered by losing some of its energy. Inwater or biological tissues, this effect
dominates at energies above 50 keV
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Pair production is process in which its
energy is converted into electron-positron
pair. This interaction starts occurring at
energies higher than 1 MeV. Unlikeelectron, positron will eventually disappear
annihilating one electron of surrounding
material. Positron-electron pair is
converted into two photons with energy ofabout 0.5 MeV
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Neutrons
Neutrons interact with nuclei (elastic and inelastic diffusion,
nuclear reactions, captures), and produce emission ofsecondary charged particles (like protons, alpha particles
or nuclear fragments heavier than carbon, oxygen, nitrogen
or hydrogen) which are responsible for tissue ionization
and for biological effect +
elastic diffusion with
production of proton and
another neutron
+ +
+
-collision with nucleus with the
production of various charged
particles: protons, nuclear
fragments, electrons
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Radioactive decay is process by which atomic nucleus of unstable atomloses energy by emitting ionizing particles (ionizing radiation)
Radioactive decay is stochastic process at level of single atoms and
chance that given atom will decay is constant over time, so that given
large number of identical atoms (nuclides), the decay rate for collection ispredictable to extent allowed by law of large numbers
Important measure is the ACTIVITY
SI unit of activity is becquerel (Bq). 1 Bq is defined as one transformation(or decay) per second. Former unit of radioactivity was curie (Ci):
1 Ci is equal to 3.7 1010Bq
Units of radioactivity
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IAEA Images from:http://www.flickr.com/photos/mitopencourseware
Cobalt-60 decay emitting a
b-
particle
Examples of radioactive decay
Radium-26 decay emitting
an a-
particle
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Number of radioactive atoms
decreases by exponential decay
Image from: http://www.flickr.com/photos/mitopencourseware11
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Quantities used in radiation studies
Amount of radiation producing effect is specified as energydeposited per unit mass in irradiated material. This is
absorbed dose (D)
mD
Where
is energy absorbed in mass
m. This ismeasured as J/kg and SI unit is gray (Gy)
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However, each type deposits its energy in different way
Low-LET High-LET
-, -particles and neutrons and denselyionizing radiations.
The energy is distributed
inhomogeneously
X and -rays are sparsely ionizing radiationsEnergy is distributed homogeneously
Linear energy transfer (LET) is measure of energy transferred by ionizing
particle to traversed material. This measure is typically used to quantifyeffects of ionizing radiation on biological specimens and is usually
expressed in units of keV/m
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High LET radiation types are more efficient in
producing damage
To normalize the Relative Biological Effectiveness is used
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Relationship between RBE and LET
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Equivalent absorbed radiation dose (equivalentdose) - computed average measure of radiation
absorbed by fixed mass of biological tissue
accounts for different biological damage
potential of different types of ionizing radiationon different organs, considering differences in
their RBE
Equivalent dose is a judged quantity for
assessing health risk of radiation exposure
Equivalent Dose
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Equivalent dose cannot be measured directly. Dose for each tissue Tand each type of radiation R (often denoted by HT,R) is calculated by:
HT,R= Q x DT,R
where DT,Ris total energy of radiation absorbed in unit mass of tissue T,
and Q is radiation quality factor that depends on type and energy ofthat radiation. Quality factor is related to relative biological
effectiveness of radiation
SI unit for equivalent dose is severt (Sv) - dose of absorbed radiation, in
Gy, that has same biological effect as dose of one joule of gamma raysabsorbed in one kilogram of tissue
Sv has replaced the previous unit rem (roentgen equivalent man):
100 rem = 1 Sv
Equivalent Dose
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Radiation quality or weighting factors
Radiation Type Energy W (ICRP-60) W (ICRP-92)
Photons all 1 1
Electrons,
muonsall 1 1
Neutrons 100 keV- 2Mev 20 function
Neutrons >2 -20 MeV 10 function
Neutrons >20Mev 5 function
Protons
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Chromosomal structure
Association of DNA andhistones in nucleosome
structure has been
demonstrated in
considerable detail. DNA isexternal to the histone core
of nucleosome. Some
studies support existence of
axial core structure formed
by non-histone proteins ornon-histone protein scaffold
in metaphase chromosome
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Human karyotype
Human karyotype - characteristic complement for humans, and consists of 23
pairs of large linear chromosomes of different sizes, giving total of 46chromosomes in every diploid cell. Human chromosomes are normally
combined into seven groups from A to G plus pair of sex chromosomes X and Y.
Chromosomal groups are: A:1-3, B: 4 and 5, C: 6 -12, D: 13-15, E: 16-18, F: 19
and 20 and G: 21 and 22.
MaleFemale 20
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Energy deposited in and near DNA
Ionizing radiation produces
discrete energy deposition
events in time and space
DNA is damaged directly and
indirectly by generation of
reactive species mainly
produced by radiolysis ofwater
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Water radical cation is very strong acid that loses proton to neighbouring
water molecule and forms OH radical which is oxidizing agent (3, 4), that is
probably the most damaging radical
H2O+ H3O
++ OH (3)+ H2O
H2O+ OH + H+ (4)
Electron becomes hydrated by water (5) and electronically excited watercan decompose into OH and H(6). So, three kinds of free radicals are
initially formed OH , H, and e-aq
+ H2Oe- (5)e- aq
H2O* (6)OH + H
Globally, and after further reactions, radiolysis of water in presence of
oxygen produces: OH, e- aq, H, O2
-, H2O2, H2.
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Low-LETradiation can
produce localized cluster of
ionizations within single
electron track
High-LETradiation produces
somewhat larger number of
ionizations that are closertogether
Damage in DNA
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Estimation of numbers of
radiation - induced
different types of DNA
lesions after 1 Gy
irradiation with low-LET
radiation
Types of DNA lesions
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Cell has complex signal transduction, cell-cycle checkpoint and
repair pathways to respond to DNA damage
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Cell cycle and checkpoints
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DSB are critical DNA lesions. Their mis-repair or non-repair leads to
formation of aberrations likedicentrics.
There are two main mechanisms to repair DSB: Homologous recombination
(HR)and non-homologous end-joining (NHEJ)
Two mechanisms operate in different phases of cell cycle. NHEJ occurs mainly in
the quiescent G0phase and during cell cycle in G1but can also occur in later
phases. HR can occur only when DNA is replicated, in S and G2 phase.
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Non-homologous end joining
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