radiation biology short - unideb.hubiophys.med.unideb.hu/sites/default/files/course... · radiation...
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Radiation biology: dosimetry, target and molecular theories direct and indirectmolecular theories, direct and indirect action of radiation, radiation sensitivity
• Dose concepts (II/4.1)
• Dose dependence of radiation effects, target theory (Poisson distribution), molecular
theory (II/4.4‐4.5)
• Factors influencing radiation sensitivity (II/4.6)
• Indirect effect of radiation theory of activation of water dilution effect (p 182 +
1/20
Indirect effect of radiation, theory of activation of water, dilution effect (p. 182 +
lecture material)
• Radiation sickness (II/4.5‐4.6)
Physical dose concepts 1
• Only the absorbed fraction of radiation leads to physical, chemical or biological effects.
• This is characterized by dose: the energy absorbed by the material during the interaction with radiation divided
by the mass of the absorbing material.
1. Absorbed dose:
• the energy absorbed by a body of unit mass:
aED
m
• unit: J/kg=gray (Gy)
• principally the easiest way to measure Da is to detect the temperature increase induced by the absorbed
energy (E), but: the absorption of 8 J/kg of energy is lethal in humans
38 J 2 10kJ 4 1 kgkg K
EE c m T T Kc m
• it is difficult to measure such a low temperature increase an alternative dose concept is required
d f /k l d b l l h d h b d b
kg K
2/20
• since a dose of 8 J/kg leads to severe biological consequences, the damage has to be generated by
molecular events, and not by heat transfer
Physical dose concepts 2
2 Exposure: the amount of positive or negative charges generated by X ray or gamma radiation in a body of unit2. Exposure: the amount of positive or negative charges generated by X‐ray or gamma radiation in a body of unit mass during electron equilibrium:
C, unit: kg
QXm
• electron equilibrium: the number of electrons entering and leaving the detected volume.g
detected volumechamber wallchamber wallenvironment
3/20
Physical dose concepts 3
3. Kinetic energy released in material (KERMA):
• in the case of indirectly ionizing, high‐energy radiation a fraction of the evoked electrons loses its energy
in the environment, outside the absorbing mass of ‘m’
ionization radiation damage
a fraction of the primary electrons giving off their
absorbent
ionization radiation damage
a fraction of the primary absorbed dose results in heating
of the environment of the absorbing mass of ‘m’
energy in the environment of mass ‘m’ also contribute to the radiation damage of
mass ‘m’
secondarya concept characterizing the amount of the
primarily absorbed energy is required
secondary radiation
electrons, some of which gives off its energy in the
environment
• definition of KERMA: the ratio of the total initial kinetic energy of all charges particles and the mass of
the absorbing material. In the case of high‐energy radiation: KERMA > absorbed dose
4/20
• unit of KERMA: gray
Biological dose concepts 11. Equivalent dose:
• the physical properties of radiation (type (electromagnetic, corpuscular, what kind of particle), energy, LET)the physical properties of radiation (type (electromagnetic, corpuscular, what kind of particle), energy, LET) influence the extent of biological damage
• this is taken into account by a weighing factor which used to be called quality factor (QR), but recently its name is radiation weighing factor (wR)g g f ( R)
• definition of equivalent dose (HT) :
• unit of equivalent dose: sievert=J/kg (Sv),T R T R
R
H w D , where wR – radiation weighing factorDT,R – the dose absorbed by a given tissue
from a given type of radiation
Radiation and energy range wR
Photons 1
Electrons 1
Neutrons (EN<10 keV) 5
Neutrons (10 kev<EN<10 keV) 10
Neutrons (100 kev<EN<2 MeV) 20
Neutrons (2 Mev<EN<20 MeV) 10N
Neutrons (EN>20 MeV) 5
Protons, EP>2 MeV 5
a particles heavy nuclei 20
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a particles, heavy nuclei 20
Biological dose concepts2. Effective dose:
• different tissues and organs exhibit different radiation sensitivity and contribute differently to the overall• different tissues and organs exhibit different radiation sensitivity and contribute differently to the overall
radiation damage of the organism
• this is taken into account by a tissue‐specific weighing factor (wT)
• definition of effective dose (E) :
,,
T T T R T RT T R
E w H w w D
equivalent dose
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Dose dependence of radiation effects, dose‐effect (dose‐response) curves
ff h f f ( d d d) d d l ( b ) f fDose‐effect curve: the fraction of surviving (i.e. non‐inactivated, not damaged) individuals (objects) as a function of
dose. It is often called survival curve.
0
N – surviving objectsN0 – number of all objects
1
N/N
0
often plotted on a
dose
plogarithmic scale
Two models have been created to explain the shape of the curves:
target theory: molecular theory:
• generation of radiation damage is stochastic
• the interaction between the radiation and the
biological object is not described molecularly
• generation of radiation damage is stochastic
• radiation damage is described molecularly; the major
determinant of radiation damage is DNA double strand
7/20
• but it appropriately described the dose‐effect
curves of molecules
break
• it is appropriate to describe radiation damage of cells
Inactivation of molecules according to the target theory in the case of one target 1
• There is one target in the molecules The volume of the target is V• There is one target in the molecules. The volume of the target is V.• The distribution of hits in volume V follows a Poisson distribution with parameter (mean value) of Vi (i – number of hits in unit volume)
• The probability that the target with a volume of V receives n hits:• The probability that the target with a volume of V receives n hits:
nn
Vin
ViP e e
nVD
• Since i D, with correct choice of the unit of D the equation can be written in the following form:
! !n n n
!
VDn
VDP e
n
• If k hits are required to inactive the target:q g
……..
000 hit,
0!VDVD
P e 1
11 hit, 1!
VDVDP e
1
11 hits, 1 !
kVD
k
VDk P e
k
k or more hits
inactivated molecules
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the fraction of non‐inactivated molecules:
1
00 !
nkVD
n
VDNe
N n
Inactivation of molecules according to the target theory in the case of one target 2
N/N
0
N/N
0
dosedose
The width of the shoulder of the curve increases with the number of hits required for inactivation: at low doses no molecules are inactivated (because
the probability that a single target is hit k times is negligible (if k >> 1).
The simplest case: one target which is inactivated by a single hit
Only those molecules are not inactivated which are not hit by any radiation. Therefore, the
fraction of surviving targets is 0
0!VD VDVDN
e eN0 0!N
If VD=1 (the number of expected hits in the radiosensitive volume is one), then
1 0.37N
eN
This dose is called D37, because 37% of the objects survive.
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0N37
37 371
1 VD DV
In the case of the one target‐single hit model D37is the reciprocal of the radiosensitive volume.
Molecular theory of radiation damage 1
n, N/N
0
(A) HeLa, (B) CHO, (C) T1 cells
viving
fractio
surv
dose (Gy)
• The curves cannot be interpreted using the target theory.
A d l i d hi h i h di i d f li ll b DNA d• A new model was required which interprets the radiation damage of mammalian cells by DNA damage.
• Evidence for the key role of DNA damage in radiation damage:
• in simple organisms there is a quantitative relationship between DNA damage and radiation damage
• in eukaryotic cells the loss of biological function correlates with single and double strand breaks in
DNA
• DNA repair is correlated with radiation sensitivity:
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p y
• cells lacking DNA repair mechanisms exhibit extreme radiation sensitivity
• agents inhibiting DNA repair increase radiation sensitivity
Molecular theory of radiation damage 2: The modelThe key event leading to radiation damage is DNA double strand break.
radiation
radiation‐generated free radicals
joint effect of two
(see indirect effect of radiation)
formation of DSB induced by a single particlejindependent events
2D DNS
a and – empirical constants (a and
Explanation of the term D2: the probability of the
Molecular or linear‐quadratic model:
n, N/N
0
0
S eN
characterize DSB generated in one and two steps, respectively.
p p yjoint occurrence of two independent events. The probability of the independent events is proportional to dose (D):
2P SSB D P SSB SSB Dviving
fractio
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, P SSB D P SSB SSB D
Surv
Dose
Direct and indirect effects of radiation
Direct effect of radiation:ff f
• the biological molecule is directly hit and inactivated by the radiation
• it is the only mechanism taking place when irradiating dried samples
it b bilit i h ll th th t f hitti l t l l h i di ti l ti• its probability is much smaller than that of hitting a solvent molecule when irradiating solutions.
Indirect effect of radiation:
• In dilute aqueous solutions the
probability that the radiation hits aprobability that the radiation hits a
water molecule is much larger than the
probability of hitting a target (e.g.
enzyme molecule).
• Radiation leads to the generation of
free radicals from water which reach
and inactivate the target.
water radical
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Generation of radicals from water (radiolysis of water)
Radical: an atom or molecule possessing an unpaired electron.
+ ‐2 2ionization of water: H O H O +e
waterhydrated electron (e )
Radical: an atom or molecule possessing an unpaired electron.
+ +2H 0 H +OH ‐
2e +H 0 H OH
*2 2excitation of water: H O H O H•+OH•
The most important radicals generated: H , OH , ewater p g water
Reactions of radicals:
2H• + OH• H O2
2
R‐H + H• R• + HR‐H + H• R‐H •
2R‐H + OH• R• + H OR‐H + OH• R‐HOH•
2
2
2 2
H• + H• HOH• + OH• H O
damage of biological molecules (R)recombination: the reactive radicals react with each other leading to
harmless (or less harmful) molecules.these processes
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( )these processes compete with each other
Enzymes can be inactivated with a lower dose in aqueous solutions
Dried: the molecule is only inactivated if the target is directly hit.
ity (%
)
dried (water‐free)
enzyme activ
i
Aqueous solution: radicals generated from water molecules surrounding the enzyme reach and inactivate the target The target gets “bigger”inactivate the target. The target gets bigger .ribonuclease
5 mg/ml l tisolution
dose (kGy)
14/20
Factors influencing radiation sensitivity 1
A. Quality of radiation
1. Ionization density (LET)
2. Penetrability
B. Biological variation
1. Cell cycle
2. Differentiation
C. Time factor
1. Fractionation, the role of DNA repair
b li dD. Metabolism and temperature
E. The effect of oxygen
15/20
Factors influencing radiation sensitivity 2
A. The quality of radiationq y
‐ the extent of radiation damage depends on ionization density (LET). This is characterized by relative biological effectiveness (RBE), a constant similar to quality factor (QR) and radiation weighing factor (wR).
t bilit l h d b t di ti t t t th ki th l t t i ff t if
Relative biological effectiveness (RBE)
‐ penetrability: alpha and beta radiation cannot penetrate the skin they can only generate systemic effects if they can into the organism
RBE
g ff ( )The ratio of a dose of X‐ray with 250 keV energy (DR) to the dose of the test radiation (DX) required to cause the same biological effect :
R
X
DRBE
D
RBE is similar, but not identical to quality factor (QR) and radiation weighing factor (wR).
LET (keV/m)1 10 100
16/20
Factors influencing radiation sensitivity 3B. Biological variability
1 cells display different radiation sensitivity in different parts of the cell cycle (implications for radiation
cstart of the cycleM:mitosis
1. cells display different radiation sensitivity in different parts of the cell cycle (implications for radiation therapy of cancer: in cancer a higher fraction of cells is in the M phase than in normal tissue).
G1: cell growthG2: preparation for mitosis
M:mitosis
Largest radiation sensitivity: M and G2 phases
S li ti f DNA
Lowest radiation sensitivity: S phase
S: replication of DNA
2. the less differentiated the cells are, the higher their radiation sensitivity is (implications for radiation therapy of cancer: cancer cells are less differentiated than normal ones)
The radiation sensitivity of tissues based on the dependence of radiation sensitivity on cell cycle and differentiation:
tissue tissue
1 lymphatic tissue 6 blood vesselsy p
2 white blood cells, immature erythrocytes in bone marrow
7 glands, liver
3 mucous membrane of stomach and intestine 8 connective tissue
17/20
4 gametes 9 muscle tissue
5 proliferating cell layer of the skin 10 nervous tissue
Factors influencing radiation sensitivity 4
C. Time factor
• If a certain dose is given in fractions, a part of the radiation damage can be repaired between fractions the extent of radiation damage is reduced.
• Repair: primarily DNA repair, repair of double strand breaks.Repair: primarily DNA repair, repair of double strand breaks.
1
dose given in two fractionssurviving fraction if the dose would have been given in a single fraction
0.1
0 01ng fractio
n
have been given in a single fraction0.01
0.001
survivi
1 2 3 4dose (Gy)dose (Gy)
D. Metabolism and temperature
• Cells with a higher metabolic rate usually have higher radiation sensitivity.
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• Since the rate of metabolism increases with temperature, a temperature increase usually leads to higher radiation sensitivity.
Factors influencing radiation sensitivity 5E. The effect of oxygen
• in the presence of O2 the amount of radiation‐generated radicals increases higher radiation sensitivity
• OER (oxygen enhancement ratio): the ratio of doses generating an arbitrary, equal surviving fraction under hypoxic and normoxic conditions.
1 17501
0.1
ng fractio
n
17502.5
700OER
0.025hypoxic
0.01
0.001
survivin
normoxic
400 800 1200 1600dose (cGy)
2000
• cancer therapy:• malignant tumors are often hypovascularized hypoxia• radiation therapy of a hypoxic tumor is not efficient• radiation therapy of a hypoxic tumor is not efficient
normoxiahypoxiaanoxia
hypoxic tumors were cured less efficiently more patients died
19/20Source: The Oncologist, 9(Suppl. 5), 31‐40; Medscape
Radiation sickness (radiation poisoning)
Ionizing radiationradiation sickness
accumulation of mutations
development of tumors
damage of offsprings (in the
1‐2 Gy 2‐6 Gy 6‐8 Gy 8‐30 Gy >30 Gy
d i h i i h i i i i l i i l l
case of damage of gametes)Symptoms of radiation exposure:
dominantaffected organ system
hematopoietic hematopoietic gastrointestinal gastrointestinal central nervous system
latency 28‐31 days 7‐28 days < 7 days none noney y y y
leading symptoms
white blood cell count (leukopenia),
leukopenia,bleedings, infections,
severe leukopenia,fever, nausea,
high fever, nausea, vomiting diarrhea,
seizures, ataxia tremor
fatigue epilation vomiting, diarrhea, electrolyte disturbance,
electrolyte disturbance, shock
hypotension
mortalitywithout medical care
0‐5% 5‐100% 95‐100% 100% 100%
20/20
care
mortality with medical care
0‐5% 5‐50% 50‐100% 100% 100%
Source: Merck Manual