Basics of radiobiology in radiation oncology and radiation physics

1. BASICS OFBASICS OF
RADIOBIOLOGYRADIOBIOLOGY

2. DEFINITIONDEFINITION
 Is the study of the action of ionizing radiationsIs 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 areThe biological effects of ionizing radiations are
the manifestations of the energy absorptionthe manifestations of the energy absorption
within a living system.within a living system.

3. Deposition of radiation energyDeposition of radiation energy
Ionization and excitation are the results of energy deposition inIonization 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 aExcitation – the raising of an electron in an atom or molecule to a
higher level without actual ejection of the electron is calledhigher level without actual ejection of the electron is called
excitation.excitation.
Ionization – if the radiation has sufficient energy to eject one orIonization – if the radiation has sufficient energy to eject one or
more orbital electrons from the atom of molecule, the process ismore orbital electrons from the atom of molecule, the process is
called ionization , and that radiation is said to be ionizingcalled 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 andParticulate- are all charged particles and
uncharged particles( electron, protons, alphauncharged particles( electron, protons, alpha
particles, heavy ions, Neutrons, Negativeparticles, heavy ions, Neutrons, Negative
piamesons)piamesons)

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

7. Sparsely ionizing – the spatial distribution of theSparsely ionizing – the spatial distribution of the
ionizing events are well separated in space andionizing events are well separated in space and
so these radiations are said to be “ sparselyso these radiations are said to be “ sparsely
ionizing”. eg x and g raysionizing”. eg x and g rays
Densely ionizing – those which produce denseDensely 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 sufficientDirectly ionizing – individual particles have sufficient
kinetic energy, they can directly disrupt the atomickinetic energy, they can directly disrupt the atomic
structure of the absorber through which they pass andstructure of the absorber through which they pass and
produce chemical and biological changes. eg chargedproduce chemical and biological changes. eg charged
particlesparticles
Indirectly ionizing – they produce chemical andIndirectly ionizing – they produce chemical and
biological damage themselves , but when they arebiological damage themselves , but when they are
absorbed in the material through which they pass theyabsorbed in the material through which they pass they
give up their energy to produce fast moving chargedgive up their energy to produce fast moving charged
particles. eg x and gamma raysparticles. eg x and gamma rays

9. Target DNATarget DNA

10. PhotonsPhotons
X rays may be thought of as a stream ofX rays may be thought of as a stream of
photons, or “ Packets of energy”. Each energyphotons, 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 constanth is Planck constant
v frequencyv frequency
The critical difference between nonionizing andThe critical difference between nonionizing and
ionizing radiations is the size of the individualionizing 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 effectsRadiation 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 cellPossible mechanisms of cell
death:death:
 Physical deathPhysical death
 Functional deathFunctional death
 Death during interphaseDeath during interphase
 Mitotic delayMitotic delay
 Reproductive failureReproductive failure
Cellular effects of ionizing radiation
are studied by cell survival curves
%survivalcells(semilogarithmic)
Dose
n = targets
Dq
D0
(threshold)
(radiosensitivity)
100%

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

14.  The radiation may interacts with other atoms orThe radiation may interacts with other atoms or
molecule in the cell to produce free radicals thatmolecule in the cell to produce free radicals that
are able to diffuse for enough to reach andare able to diffuse for enough to reach and
damage the critical targets. This is called indirectdamage the critical targets. This is called indirect
action.action.
 Free radical – is a free atom or molecule carryingFree 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 ActionIndirect Action

15. LETLET
LET- is defined as the energy transferred per unit length of theLET- is defined as the energy transferred per unit length of the
track. It is usually expressed in Kev per micron unit of unittrack. 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 LETAs 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 lengths1. 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 energy2. Energy average- dividing the track into equal energy
increments and then averaging the track length overincrements 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 /um250kv x ray- 2 kev /um
Neutron 14 Mev- 12 kev/umNeutron 14 Mev- 12 kev/um
Heavy charged particles- 100 – 2000 kev/umHeavy charged particles- 100 – 2000 kev/um

17. Oxygen Enhancement ratioOxygen Enhancement ratio
The ratio of radiation dose required to produce aThe ratio of radiation dose required to produce a
given biologic effect under hypoxic condition togiven biologic effect under hypoxic condition to
that well aerated conditions.that well aerated conditions.
OER- ratio of hypoxic to aerated doses toOER- 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 showsWhen 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 BIOLOGICALRELATIVE BIOLOGICAL
EFFECTIVENESS( RBE)EFFECTIVENESS( RBE)
 RBE of a test radiation is the ratio of theRBE of a test radiation is the ratio of the
amount (dose) of 250 kv x rays to produce aamount (dose) of 250 kv x rays to produce a
given biological effect in a system to the amountgiven biological effect in a system to the amount
of test radiation to produce same biologicalof 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 ofIt depends on LET, radiation dose, mode of
radiation exposure( fractionation), dose rate andradiation exposure( fractionation), dose rate and
the biological system.the biological system.

19. Relationship of RBE, OER withRelationship of RBE, OER with
LETLET
 The increase in LET enhances RBE upto certain valueThe increase in LET enhances RBE upto certain value
and then start falling . When LET becomes more thanand then start falling . When LET becomes more than
100 kev/um the RBE starts falling due to overkilling100 kev/um the RBE starts falling due to overkilling
effect.effect.
 For low LET radiation OER has a value which rangesFor low LET radiation OER has a value which ranges
from 2.5 – 3 at high doses and decreases with dose tofrom 2.5 – 3 at high doses and decreases with dose to
some extent, as LET increases the cell killing is more bysome 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 andDose response curve and
therapeutics ratiotherapeutics ratio
 The therapeutic ratio is defined as the ratio between theThe 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 whichTumor lethal dose- That dose of radiation which
produces complete and permanent regression of theproduces 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 giveTissue 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 toThis 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 gapsG1G2= periods or gaps
of inactivity inof inactivity in
the cell cyclethe cell cycle

25. Cell survival curveCell survival curve
Cells from tumors and many normal regenerative tissuesCells 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 saidA survivor that has retained reproductive integrity is said
to be clonogenic.to be clonogenic.
A cell survival curve describes the relationship betweenA cell survival curve describes the relationship between
the radiation dose and the proportion of cells thatthe radiation dose and the proportion of cells that
survive.survive.
Plating efficiency – The fraction of untreated cells thatPlating efficiency – The fraction of untreated cells that
grow when seeded is known as the plating efficiencygrow 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 scaleShape 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 , theAt low doses- for sparsely ionizing radiations , the
survival curve starts out straight on the log-linear plotsurvival curve starts out straight on the log-linear plot
with a finite initial slope; that is surviving fraction is anwith 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 highAt higher doses, the curve bends. At very high
doses the survival curve often tends todoses the survival curve often tends to
straighten again; the surviving fraction returns tostraighten 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 theDensely ionizing- curve is a straight line from the
origin ; that is, survival approximates to anorigin ; that is, survival approximates to an
exponential function of dose.exponential function of dose.

28.  Survival curveSurvival curve
Dose
n = targets
Dq
D0
(threshold)
(radiosensitivity)
100%

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

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

32. Target theory and survival curveTarget theory and survival curve
Target theory- is a mathematical model which calculates the fractionTarget 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 theSimple target- in this model one hit is sufficient to inactivate the
target.target.
Multitarget model- survival curve is described in terms of an initialMultitarget model- survival curve is described in terms of an initial
slope, Dslope, D11,due to single – event killing,,due to single – event killing,
DD0,0, 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 thesome quantity to represent the size or width of the shoulder of the
curve. ( n or Dcurve. ( n or Dqq))
Extrapolation number n– is a measure of the width of theExtrapolation number n– is a measure of the width of the
shoulder.shoulder.
Quasi-threshold dose DQuasi-threshold dose Dqq – 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 thestraight 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 survivalproportional 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 tothe 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 quadraticThe 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 reactingEarly or acute reacting
tissuestissues
 Most tumorsMost tumors
 Small a/b ratioSmall a/b ratio
 a/b = 2a/b = 2
 Late reacting tissues, egLate reacting tissues, eg
spinal cordspinal cord
 potentially prostate cancerpotentially prostate cancer

36. R’s of radiobiologyR’s of radiobiology
Influence on time between fractions= tInfluence on time between fractions= t
overall treatment time= Toverall treatment time= T
 Repair of sublethal damage= needs minimum tRepair of sublethal damage= needs minimum t
for normal tissuefor normal tissue
 Redistribution of cells within cell cycle- needs minimumRedistribution of cells within cell cycle- needs minimum
tt
 Repopulation of cells following a treatment- needs toRepopulation of cells following a treatment- needs to
reduce Treduce T
 Reoxygenation – needs minimum T.Reoxygenation – needs minimum T.

37. RepairRepair
 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 damagedRepair 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 radiationtemperature, chemicals) than due to radiation
 The half time for repair, tThe half time for repair, trr, is of the order of minutes to, is of the order of minutes to
hourshours

38. RepairRepair
 It is essential to allow normal tissues to repair allIt is essential to allow normal tissues to repair all
repairable radiation damage prior to giving anotherrepairable radiation damage prior to giving another
fraction of radiation.fraction of radiation.
 This leads to a minimum interval between fractions of 6This 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 8therefore, 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 inCells have different radiation sensitivities in
different parts of the cell cycledifferent parts of the cell cycle
 Highest radiation sensitivity is in early S and lateHighest 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 themThe fractionated treatment regime allows them
to redistribute throughout the division cycle.to redistribute throughout the division cycle.
 The late responding cells or those which areThe late responding cells or those which are
static in G0 phase are least radiosensitive orstatic in G0 phase are least radiosensitive or
relatively radio resistant.relatively radio resistant.

41. RepopulationRepopulation
Cells also grow during radiotherapyCells also grow during radiotherapy
For tumor cells this repopulation partially counteracts theFor 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, Tpp (eg. In head and(eg. In head and
neck tumors or cervix cancer) can be as short as 2 daysneck 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 varyThe repopulation time of tumor cells appears to vary
during radiotherapy - at the commencement it may beduring radiotherapy - at the commencement it may be
slow (eg due to hypoxia), however a certain time afterslow (eg due to hypoxia), however a certain time after
the first fraction of radiotherapy (often termed thethe first fraction of radiotherapy (often termed the
‘kick-off time, T‘kick-off time, Tkk) repopulation accelerates.) repopulation accelerates.
 Repopulation must be taken into account whenRepopulation must be taken into account when
protracting radiation eg due to scheduled (orprotracting 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 radiationOxygen is an important enhancement for radiation
effects (“Oxygen Enhancement Ratio”)effects (“Oxygen Enhancement Ratio”)
 The tumor may be hypoxic (in particular in the centerThe 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 moreThe fractionated schedule of treatment results in more
damage to tumor cells than the effect produced by thedamage to tumor cells than the effect produced by the
same total dose delivered in a single treatment. This issame total dose delivered in a single treatment. This is
because reoxygenation of hypoxic cells takes placebecause reoxygenation of hypoxic cells takes place
during fractionated treatment and they become aeratedduring fractionated treatment and they become aerated
and sensitive to radiation , whereas the sublethaland 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 - theTends to spare late reacting normal tissues - the
smaller the size of the fraction the more sparingsmaller 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 andDose D0 Will be theoretically able to destroy each and
every organism but practically dose D0 will not destroy allevery organism but practically dose D0 will not destroy all
the organism because radiation is wasted in organismthe 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 isThe 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)
Probabilityofcellsurvival
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. DoseDose
responseresponse
Therapeutic window:
Maximum probability
of Complication Free
Tumour Control