Basics of radiobiology

Basics of radiobiology in radiation oncology and radiation physics

1. BASICS OF BASICS OF
RADIOBIOLOGYRADIOBIOLOGY

2. DEFINITIONDEFINITION
Is the study of the action of ionizing radiations Is the study of the action of ionizing radiations
on living things.on living things.
Principles of Radiation BiologyPrinciples of Radiation Biology
The biological effects of ionizing radiations are The biological effects of ionizing radiations are
the manifestations of the energy absorption the manifestations of the energy absorption
within a living system.within a living system.

3. Deposition of radiation energy Deposition of radiation energy
Ionization and excitation are the results of energy deposition in Ionization and excitation are the results of energy deposition in
a biological system.a biological system.

Excitation – the raising of an electron in an atom or molecule to a Excitation – the raising of an electron in an atom or molecule to a
higher level without actual ejection of the electron is called higher level without actual ejection of the electron is called
excitation.excitation.
Ionization – if the radiation has sufficient energy to eject one or Ionization – if the radiation has sufficient energy to eject one or
more orbital electrons from the atom of molecule, the process is more orbital electrons from the atom of molecule, the process is
called ionization , and that radiation is said to be ionizing called ionization , and that radiation is said to be ionizing
radiation.radiation.

4. Excitation and IonizationExcitation and Ionization
Energy
ExcitationExcitation
IonizationIonization

5. Types of ionizing radiationsTypes of ionizing radiations
Electromagnetic – are x and gamma raysElectromagnetic – are x and gamma rays
Particulate- are all charged particles and Particulate- are all charged particles and
uncharged particles( electron, protons, alpha uncharged particles( electron, protons, alpha
particles, heavy ions, Neutrons, Negative particles, heavy ions, Neutrons, Negative
piamesons)piamesons)

6. Ionizing radiation cont.Ionizing radiation cont.
Electromagnetic radiation, in their biological Electromagnetic radiation, in their biological
effects, are considered to be ionizing if they have effects, are considered to be ionizing if they have
a photon energy in excess of 124ev , which a photon energy in excess of 124ev , which
corresponds to a wavelength shorter than about corresponds to a wavelength shorter than about
1010
-6-6
cm. cm.

7. Sparsely ionizing – the spatial distribution of the Sparsely ionizing – the spatial distribution of the
ionizing events are well separated in space and ionizing events are well separated in space and
so these radiations are said to be “ sparsely so these radiations are said to be “ sparsely
ionizing”. eg x and g raysionizing”. eg x and g rays
Densely ionizing – those which produce dense Densely ionizing – those which produce dense
ionizations along the track. eg alpha particles, ionizations along the track. eg alpha particles,
heavy ions.heavy ions.

8. Ionizing radiation contIonizing radiation cont
Directly ionizing – individual particles have sufficient Directly ionizing – individual particles have sufficient
kinetic energy, they can directly disrupt the atomic kinetic energy, they can directly disrupt the atomic
structure of the absorber through which they pass and structure of the absorber through which they pass and
produce chemical and biological changes. eg charged produce chemical and biological changes. eg charged
particles particles
Indirectly ionizing – they produce chemical and Indirectly ionizing – they produce chemical and
biological damage themselves , but when they are biological damage themselves , but when they are
absorbed in the material through which they pass they absorbed in the material through which they pass they
give up their energy to produce fast moving charged give up their energy to produce fast moving charged
particles. eg x and gamma raysparticles. eg x and gamma rays

9. Target DNA Target DNA

10. PhotonsPhotons
X rays may be thought of as a stream of X rays may be thought of as a stream of
photons, or “ Packets of energy”. Each energy photons, or “ Packets of energy”. Each energy
packet contain an amount of energy equal to hv.packet contain an amount of energy equal to hv.
E= hv ,E= hv ,h is Planck constant h is Planck constant
v frequency v frequency
The critical difference between nonionizing and The critical difference between nonionizing and
ionizing radiations is the size of the individual ionizing radiations is the size of the individual
packets of energy, not the total energy involved.packets of energy, not the total energy involved.

11. Radiation effects Radiation effects
Ionizing radiation
interacts at the cellular
level:
1.Physical changes
2.Chemical changes
3.Biological effect
cell
nucleus
chromosomes
incident
radiation

12. Biological effects at cellular levelBiological effects at cellular level
Possible mechanisms of cell Possible mechanisms of cell
death:death:
Physical deathPhysical death
Functional death Functional death
Death during interphaseDeath during interphase
Mitotic delayMitotic delay
Reproductive failureReproductive failure
Cellular effects of ionizing radiation
are studied by cell survival curves
%

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Dose
n = targets
D
q
D
0
(threshold)
(radiosensitivity)
100%

13. Direct and indirect action Direct and indirect action
The biological effects of radiation result principally from The biological effects of radiation result principally from
damage to DNA (Critical target)damage to DNA (Critical target)
Direct action- when radiation is absorbed in biological Direct action- when radiation is absorbed in biological
material, it will interact directly with the critical targets material, it will interact directly with the critical targets
in the cells, the atom of the target itself may be ionized in the cells, the atom of the target itself may be ionized
or excited, thus initiating the chain of events that leads or excited, thus initiating the chain of events that leads
to a biological damage. It is dominant process when to a biological damage. It is dominant process when
radiations with high linear energy transfer, such as radiations with high linear energy transfer, such as
neutrons or alpha particles. neutrons or alpha particles.

14. The radiation may interacts with other atoms or The radiation may interacts with other atoms or
molecule in the cell to produce free radicals that molecule in the cell to produce free radicals that
are able to diffuse for enough to reach and are able to diffuse for enough to reach and
damage the critical targets. This is called indirect damage the critical targets. This is called indirect
action.action.
Free radical – is a free atom or molecule carrying Free radical – is a free atom or molecule carrying
an impaired orbital electron in the outer shell. an impaired orbital electron in the outer shell.
Indirect Action Indirect Action

15. LETLET
LET- is defined as the energy transferred per unit length of the LET- is defined as the energy transferred per unit length of the
track. It is usually expressed in Kev per micron unit of unit track. It is usually expressed in Kev per micron unit of unit
density material. density material.
As most of radiation have wide spectrum of energies, the LET As most of radiation have wide spectrum of energies, the LET
cannot have a single value. Express it as an average quantitycannot have a single value. Express it as an average quantity
1. Track average- obtained by dividing the track into equal lengths 1. Track average- obtained by dividing the track into equal lengths
and find the mean of energy deposited in each length.and find the mean of energy deposited in each length.
2. Energy average- dividing the track into equal energy 2. Energy average- dividing the track into equal energy
increments and then averaging the track length over increments and then averaging the track length over
which these increments deposited. which these increments deposited.

16. LETLET
Value- Co- 60 gamma rays – 0.3Value- Co- 60 gamma rays – 0.3
250kv x ray- 2 kev /um 250kv x ray- 2 kev /um
Neutron 14 Mev- 12 kev/umNeutron 14 Mev- 12 kev/um
Heavy charged particles- 100 – 2000 kev/um Heavy charged particles- 100 – 2000 kev/um

17. Oxygen Enhancement ratioOxygen Enhancement ratio
The ratio of radiation dose required to produce a The ratio of radiation dose required to produce a
given biologic effect under hypoxic condition to given biologic effect under hypoxic condition to
that well aerated conditions.that well aerated conditions.
OER- ratio of hypoxic to aerated doses to OER- ratio of hypoxic to aerated doses to
achieve the same biological effect.achieve the same biological effect.
When this ratio is 1 or equal to 1, it shows When this ratio is 1 or equal to 1, it shows
absence of oxygen effect.absence of oxygen effect.
For x and g rays – 2 -3For x and g rays – 2 -3

18. RELATIVE BIOLOGICAL RELATIVE BIOLOGICAL
EFFECTIVENESS( RBE)EFFECTIVENESS( RBE)
RBE of a test radiation is the ratio of the RBE of a test radiation is the ratio of the
amount (dose) of 250 kv x rays to produce a amount (dose) of 250 kv x rays to produce a
given biological effect in a system to the amount given biological effect in a system to the amount
of test radiation to produce same biological of test radiation to produce same biological
effect in the same biological system.effect in the same biological system.
It depends on LET, radiation dose, mode of It depends on LET, radiation dose, mode of
radiation exposure( fractionation), dose rate and radiation exposure( fractionation), dose rate and
the biological system. the biological system.

19. Relationship of RBE, OER with Relationship of RBE, OER with
LETLET
The increase in LET enhances RBE upto certain value The increase in LET enhances RBE upto certain value
and then start falling . When LET becomes more than and then start falling . When LET becomes more than
100 kev/um the RBE starts falling due to overkilling 100 kev/um the RBE starts falling due to overkilling
effect.effect.
For low LET radiation OER has a value which ranges For low LET radiation OER has a value which ranges
from 2.5 – 3 at high doses and decreases with dose to from 2.5 – 3 at high doses and decreases with dose to
some extent, as LET increases the cell killing is more by some extent, as LET increases the cell killing is more by
single track events and hence the OER will fall.single track events and hence the OER will fall.

20. Relationship between RBE and LETRelationship between RBE and LET

21. Dose response curve and Dose response curve and
therapeutics ratiotherapeutics ratio
The therapeutic ratio is defined as the ratio between the The therapeutic ratio is defined as the ratio between the
tumor lethal dose and tissue tolerance.tumor lethal dose and tissue tolerance.
 Tumor lethal dose- That dose of radiation which Tumor lethal dose- That dose of radiation which
produces complete and permanent regression of the produces complete and permanent regression of the
tumor in vivo in the zone irradiated,tumor in vivo in the zone irradiated,
Tissue tolerance- Denotes the dose which give Tissue tolerance- Denotes the dose which give
acceptable rates of tissue complications.acceptable rates of tissue complications.
This ratio should be more than or at the most equal to This ratio should be more than or at the most equal to
1 for curative radiotherapy.1 for curative radiotherapy.

22. Therapeutic ratio (Holthusen’s curve)Therapeutic ratio (Holthusen’s curve)

23. DOSE RESPONSE CURVES
A plot of a biological effect observed against the
dose given is called a dose response curve. Generally,
as dose increases so does the effect.
Three types of dose response relationship are known:
. Linear,
. Linear quadratic,
. Sigmoid.
Dose response curves may or may not have a threshold.
A threshold dose is the largest dose for a particular
effect studied below which no effect will be observed.

24. Cell cycleCell cycle
Phases of cell cyclePhases of cell cycle
M= MitosisM= Mitosis
S= DNA synthesisS= DNA synthesis
G1G2= periods or gaps G1G2= periods or gaps
of inactivity in of inactivity in
the cell cycle the cell cycle

25. Cell survival curveCell survival curve
Cells from tumors and many normal regenerative tissues Cells from tumors and many normal regenerative tissues
grow and form colonies in vitro.grow and form colonies in vitro.
A survivor that has retained reproductive integrity is said A survivor that has retained reproductive integrity is said
to be clonogenic.to be clonogenic.
A cell survival curve describes the relationship between A cell survival curve describes the relationship between
the radiation dose and the proportion of cells that the radiation dose and the proportion of cells that
survive.survive.
Plating efficiency – The fraction of untreated cells that Plating efficiency – The fraction of untreated cells that
grow when seeded is known as the plating efficiency grow when seeded is known as the plating efficiency
(PE ).(PE ).

26. SURVIVING FRACTIONSURVIVING FRACTION
S = colonies counted/ cell seeded xS = colonies counted/ cell seeded x
( PE/100)( PE/100)
Shape of survival curve- dose plotted on a linear scale Shape of survival curve- dose plotted on a linear scale
and surviving fraction on a logarithmic scale.and surviving fraction on a logarithmic scale.
At low doses- for sparsely ionizing radiations , the At low doses- for sparsely ionizing radiations , the
survival curve starts out straight on the log-linear plot survival curve starts out straight on the log-linear plot
with a finite initial slope; that is surviving fraction is an with a finite initial slope; that is surviving fraction is an
exponential function of dose.exponential function of dose.

27. Shape of survival curve cont.Shape of survival curve cont.
At higher doses, the curve bends. At very high At higher doses, the curve bends. At very high
doses the survival curve often tends to doses the survival curve often tends to
straighten again; the surviving fraction returns to straighten again; the surviving fraction returns to
being an exponential function of dose.being an exponential function of dose.
Densely ionizing- curve is a straight line from the Densely ionizing- curve is a straight line from the
origin ; that is, survival approximates to an origin ; that is, survival approximates to an
exponential function of dose.exponential function of dose.

28. Survival curve Survival curve
Dose
n = targets
D
q
D
0
(threshold)
(radiosensitivity)
100%

30. The exponential nature of survival curve shows The exponential nature of survival curve shows
that each dose of equal fractions will kill the that each dose of equal fractions will kill the
same proportion of cells. This results in a same proportion of cells. This results in a
logarithmic decrease in the number of surviving logarithmic decrease in the number of surviving
cells.( if a dose of 2Gy of a first fractional dose cells.( if a dose of 2Gy of a first fractional dose
reduces the survival to 50% then the survival reduces the survival to 50% then the survival
after two fractions would reduce to 25% and so after two fractions would reduce to 25% and so
on)on)

31. Factors which modify cell Factors which modify cell
survival curvesurvival curve
Physical factors :Physical factors :LET,Dose ,Dose rate LET,Dose ,Dose rate
Fractionation & Hyperthermia.Fractionation & Hyperthermia.
Chemical factors :Chemical factors :presence of O2, presence of O2,
Radioprotector,Radiosensitizer Radioprotector,Radiosensitizer
Biological factors: Biological factors: cell stage, Repair cell stage, Repair
process.process.

32. Target theory and survival curve Target theory and survival curve
Target theory- is a mathematical model which calculates the fraction Target theory- is a mathematical model which calculates the fraction
of cells in a system that survives a given dose of radiation.of cells in a system that survives a given dose of radiation.
Simple target- in this model one hit is sufficient to inactivate the Simple target- in this model one hit is sufficient to inactivate the
target.target.
Multitarget model- survival curve is described in terms of an initial Multitarget model- survival curve is described in terms of an initial
slope, Dslope, D
11,due to single – event killing, ,due to single – event killing,
DD
0 , 0 , final slope, due to multiple event killing,final slope, due to multiple event killing,
some quantity to represent the size or width of the shoulder of the some quantity to represent the size or width of the shoulder of the
curve. ( n or Dcurve. ( n or D
qq))
Extrapolation number n– is a measure of the width of the Extrapolation number n– is a measure of the width of the
shoulder.shoulder.
Quasi-threshold dose DQuasi-threshold dose D
qq – it is defined as the dose at which the – it is defined as the dose at which the
straight portion of the survival , extrapolated backward, cuts the straight portion of the survival , extrapolated backward, cuts the
dose axis drawn through a survival fraction of unity. dose axis drawn through a survival fraction of unity.

33. Linear - quadratic modelLinear - quadratic model
There are two components cell killing by radiation. There are two components cell killing by radiation.
One that is proportional to dose and one thatOne that is proportional to dose and one that
proportional to the square of the dose. Expression for cell survival proportional to the square of the dose. Expression for cell survival
curve – curve –
S= eS= e
-aD-BD2 -aD-BD2

S= the fraction of cells surviving a dose D,S= the fraction of cells surviving a dose D,
a and B are constants.a and B are constants.
The initial slope of the cell survival curve is determined by alpha; The initial slope of the cell survival curve is determined by alpha;
the quadratic component of cell killing , beta causes the curve to the quadratic component of cell killing , beta causes the curve to
bend at higher doses. bend at higher doses.
The ratio a/b is the dose at which linear and quadratic The ratio a/b is the dose at which linear and quadratic
components of cell killing are equal.components of cell killing are equal.

35. Rule of thumb for a/b ratiosRule of thumb for a/b ratios
Large a/b ratiosLarge a/b ratios
a/b = 10 to 20a/b = 10 to 20
Early or acute reacting Early or acute reacting
tissuestissues
Most tumorsMost tumors
Small a/b ratioSmall a/b ratio
a/b = 2a/b = 2
Late reacting tissues, eg Late reacting tissues, eg
spinal cordspinal cord
potentially prostate cancerpotentially prostate cancer

36. R’s of radiobiology R’s of radiobiology
Influence on time between fractions= tInfluence on time between fractions= t
overall treatment time= T overall treatment time= T
Repair of sublethal damage= needs minimum t Repair of sublethal damage= needs minimum t
for normal tissuefor normal tissue
Redistribution of cells within cell cycle- needs minimum Redistribution of cells within cell cycle- needs minimum

tt
Repopulation of cells following a treatment- needs to Repopulation of cells following a treatment- needs to
reduce Treduce T
Reoxygenation – needs minimum T.Reoxygenation – needs minimum T.

37. Repair Repair
All cells repair radiation damageAll cells repair radiation damage
This is part of normal damage repair in the DNAThis is part of normal damage repair in the DNA
Repair is very effective because DNA is damaged Repair is very effective because DNA is damaged
significantly more due to ‘normal’ other influences (eg. significantly more due to ‘normal’ other influences (eg.
temperature, chemicals) than due to radiation temperature, chemicals) than due to radiation
The half time for repair, tThe half time for repair, t
rr, is of the order of minutes to , is of the order of minutes to
hourshours

38. Repair Repair
It is essential to allow normal tissues to repair all It is essential to allow normal tissues to repair all
repairable radiation damage prior to giving another repairable radiation damage prior to giving another
fraction of radiation.fraction of radiation.
This leads to a minimum interval between fractions of 6 This leads to a minimum interval between fractions of 6
hourshours
Spinal cord seems to have a particularly slow repair - Spinal cord seems to have a particularly slow repair -
therefore, breaks between fractions should be at least 8 therefore, breaks between fractions should be at least 8
hours if spinal cord is irradiated.hours if spinal cord is irradiated.

39. RRedistributionedistribution
Cells have different radiation sensitivities in Cells have different radiation sensitivities in
different parts of the cell cycledifferent parts of the cell cycle
Highest radiation sensitivity is in early S and late Highest radiation sensitivity is in early S and late
G2/M phase of the cell cycleG2/M phase of the cell cycle
G1
G1
S (synthesis)
M (mitosis)
G2

40. Redistribution cont.Redistribution cont.
The fractionated treatment regime allows them The fractionated treatment regime allows them
to redistribute throughout the division cycle. to redistribute throughout the division cycle.
 The late responding cells or those which are The late responding cells or those which are
static in G0 phase are least radiosensitive or static in G0 phase are least radiosensitive or
relatively radio resistant.relatively radio resistant.

41. Repopulation Repopulation
Cells also grow during radiotherapyCells also grow during radiotherapy
For tumor cells this repopulation partially counteracts the For tumor cells this repopulation partially counteracts the
cell killing effect of radiotherapycell killing effect of radiotherapy
The potential doubling time of tumors, TThe potential doubling time of tumors, T
pp (eg. In head and (eg. In head and
neck tumors or cervix cancer) can be as short as 2 days neck tumors or cervix cancer) can be as short as 2 days
- therefore one looses up to 1 Gy worth of cell killing - therefore one looses up to 1 Gy worth of cell killing
when prolonging the course of radiotherapywhen prolonging the course of radiotherapy

42. Repopulation cont.Repopulation cont.
The repopulation time of tumor cells appears to vary The repopulation time of tumor cells appears to vary
during radiotherapy - at the commencement it may be during radiotherapy - at the commencement it may be
slow (eg due to hypoxia), however a certain time after slow (eg due to hypoxia), however a certain time after
the first fraction of radiotherapy (often termed the the first fraction of radiotherapy (often termed the
‘kick-off time, T‘kick-off time, T
kk) repopulation accelerates.) repopulation accelerates.
Repopulation must be taken into account when Repopulation must be taken into account when
protracting radiation eg due to scheduled (or protracting radiation eg due to scheduled (or
unscheduled) breaks such as holidays.unscheduled) breaks such as holidays.

43. RRe-oxygenatione-oxygenation
Oxygen is an important enhancement for radiation Oxygen is an important enhancement for radiation
effects (“Oxygen Enhancement Ratio”)effects (“Oxygen Enhancement Ratio”)
The tumor may be hypoxic (in particular in the center The tumor may be hypoxic (in particular in the center
which may not be well supplied with blood)which may not be well supplied with blood)
 The fractionated schedule of treatment results in more The fractionated schedule of treatment results in more
damage to tumor cells than the effect produced by the damage to tumor cells than the effect produced by the
same total dose delivered in a single treatment. This is same total dose delivered in a single treatment. This is
because reoxygenation of hypoxic cells takes place because reoxygenation of hypoxic cells takes place
during fractionated treatment and they become aerated during fractionated treatment and they become aerated
and sensitive to radiation , whereas the sublethal and sensitive to radiation , whereas the sublethal
damage to normal cells gets time to recover.damage to normal cells gets time to recover.

44. FractionationFractionation
Tends to spare late reacting normal tissues - the Tends to spare late reacting normal tissues - the
smaller the size of the fraction the more sparing smaller the size of the fraction the more sparing
for tissues with low a/bfor tissues with low a/b
Prolongs treatmentProlongs treatment

54. Mean lethal dose (MLD)Mean lethal dose (MLD)
Dose D0 Will be theoretically able to destroy each and Dose D0 Will be theoretically able to destroy each and
every organism but practically dose D0 will not destroy all every organism but practically dose D0 will not destroy all
the organism because radiation is wasted in organism the organism because radiation is wasted in organism
already in activated .already in activated .
The surviving on dose Do is seen to be 37% Hence D0 is The surviving on dose Do is seen to be 37% Hence D0 is
called mean lethal dose (MLD)called mean lethal dose (MLD)

55. Thank youThank you

56. For completeness, the earlier multitarget
single hit model described the slope of
the survival curve by D0 (the dose to
reduce survival to 37% of its value at any
point on the final near exponential
portion of the curve) and the
extrapolation number n (the point of
intersection of the slope on the log
survival axis). Dq was the quasi-threshold
dose. However, this model dose not have
any current biological basis.

58. The effect of fractionationThe effect of fractionation
0.001
0.01
0.1
1
0 2 4 6 8 10
Dose (Gy)
P
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a
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cell kill (low a/b)
cell kill (high a/b)
fractionated (low a/b)
fractionated (low a/b)

59. The type of radiation influences the shape of the
cell survival curve.
Densely ionizing radiations exhibit a cell survival
curve that is almost an
exponential function of dose, shown by an
almost straight line on the log–linear
plot. For sparsely ionizing radiation, however,
the curves show an initial slope
followed by a shoulder region and then become
nearly straight at higher doses.

60. Dose Dose
responseresponse
Therapeutic window:
Maximum probability
of Complication Free
Tumour Control