IT COVERS DIFFERENT RADIOBIOLOGICAL MODELS WITH DERIVATIONS WHEREVER NEEDED

1. BIOLOGICAL BASIS OF
RADIOTHERAPY
By :SALONI CHAWLA

2. RADIOBIOLOGY
•Radiobiology is the study of the action of ionizing radiations on
living things.
•During the passage through living matter, radiation loses
energy by interaction with atoms and molecules of the matter.
•Hence, causing ionization and excitation.
•The ultimate effect is the alteration of the living cells.
History
•The first recorded biologic effect of radiation was due to
Becquerel, who left a radium container in his vest pocket.
•He described the skin erythema that appeared two weeks later
and the ulceration that developed and required several weeks to
heal.
• From these early beginnings, the study of Radiobiology began.

3. CELL
•The cell is the building unit of living matter and consists of two
primary components: the nucleus and the cytoplasm
•All metabolic activities are carried out in the cytoplasm under
the guidance of the nucleus.
•The nucleus contains chromosomes, which have a threadlike
structure of two arms connected by a centromere .
•Chromosomes are formed of genes, which are the basic units of
heredity in the cells of all living species.
•Genes are composed of deoxyribonucleic acid (DNA) molecules.
•The sequence of genes in the chromosome characterizes a
particular chromosome.
•Four important organelles—ribosomes, endoplasmic reticula,
mitochondria, and lysosomes—that carry out the METABOLIC
FUNCTIONS

4. PICTURE OF A CELL

5. DNA
DNA: It is an extremely long chain of molecules that contains all
the information necessary for the life functions of a cell.
Each nucleotide consists of a sugar (deoxyribose) bound on one
side to a phosphate group and bound on the other side to
a nitrogenous base.
The individual molecules that make up DNA are
called nucleotides. They are Adenine (A), Thymidine (T),
Guanine (G), and Cytosine (C).
Chromosomes: The chains of nucleotides in human DNA are
wound up and compacted into 46 chromosomes (two sets of
23) that are found in the nucleus of a cell. The DNA is held
together by proteins called histones which help to keep the
shape of the chromosomes.
Genes: Genes are the functional regions of the genome, they
are a blueprint for all the useful bits and pieces in our cells.
Genes contain the information that tells the cell how to make a
particular protein. Proteins themselves are just chains of amino
acids. A gene gives the instructions for making the amino acid
chain. Proteins (including all enzymes) run our cells - deciding

7. TIME SCALE OF EVENTS
Irradiation of biological system leads to a series of events which
differ enormously on the time scale
1.Physical phase
2.Chemical phase
3.Biological phase

8. PHYSICAL PHASE
The physical phase consists of interactions between the
charged particles and the atoms with which the tissue is
composed.
As radiation passes through the tissues ,it takes 10-13 seconds
to pass through DNA strand leading to either ionisations or
excitations.
If sufficiently energetic they lead to ejection of an orbital
electron or raising others to higher energy levels.
In case of ionisations sometimes secondary electrons can also
be produced giving rise to cascade of other reactions

9. CHEMICAL PHASE
This phase describes the period in which these damaged
atoms and molecules react with cellular components in rapid
chemical reactions.
Ionisation and excitation lead to the breakage of chemical
bonds and formation of free radicals which are highly reactive
.It takes 10ˆ-12 to 1 second
Free radical reactions are complete within approx 1millisecond
of radiation exposure
Important characteristic is the competition between
scavenging reactions and fixation reactions leading to stable
chemical changes.

10. BIOLOGICAL PHASE
In this phase the DNA aberrations and lesions are either
repaired or mutations may occur and cell death.
The period between the breakage of chemical bonds and the
expression of the biologic effect may be hours, days, months,
years, or generations, depending on the consequences involved
If cell killing is the result, the biologic effect may be expressed
hours to days later, when the damaged cell attempts to divide.
 If the radiation damage is oncogenic, its expression as an
overt cancer may be delayed 40 years.
If the damage is a mutation in a germ cell leading to hereditary
changes, it may not be expressed for many generations.

11. DIRECT ACTION OF RADIATION
If radiation interacts with the atoms of the DNA molecule, or
some other cellular component critical to the survival of the
cell, it is referred to as a direct effect.
Such an interaction may affect the ability of the cell to
reproduce and, thus, survive.
 If enough atoms are affected such that the chromosomes do
not replicate properly, or if there is significant alteration in the
information carried by the DNA molecule, then the cell may be
destroyed by “direct” interference with its life-sustaining
system.
The probability of the radiation interacting with the DNA
molecule is very small since these critical components make up
such a small part of the cell

14. A dose of 2 Gy of X-rays is equal to an energy of 2 J/kg.
Since 1 J/kg is equal to 6.25 × 1018 eV/kg, 2 Gy is equal to 12.5 ×
1018 eV/kg.
Since the minimum energy required for ionization is 33 eV, the
number of ions per kilogram is calculated by dividing 12.5 × 1018
eV/kg by 33 eV, which yields ~4 × 1017 ions/kg.
If we apply two doses to the whole body (we know that there are 9.5
× 1025 atoms/kg in the human body), the number of atoms in the
whole body ionized by a dose of 2 Gy can be found by dividing the
ions/kg by the atoms/kg.
The result is nearly 1 × 10−8 (one in a hundred million), which
means that the direct effect of X-rays in terms of DNA damage in
tissue is relatively minor.

15. INDIRECT ACTION OF RADIATION
A cell is mostly made of water. If a cell is exposed to radiation,
there is a higher probability of radiation interacting with the
water that makes up most of the cell’s volume.
When radiation interacts with water, it may break the bonds
that hold the water molecule together, producing fragments
such as hydrogen (H) and hydroxyls (OH).
These free radicals can then attack critical targets such as the
DNA .
 They are able to diffuse some distance in the cell, the initial
ionization event does not have to occur so close to the DNA in
order to cause damage.
These fragments may recombine or interact with other
fragments or ions to form compounds, such as water, which
would not harm the cell.
However, they could combine to form toxic substances, such as
hydrogen peroxide (H2O2), which can contribute to the
destruction of the cell

17. RADIOLYSIS OF WATER
First step in radiolysis of water is the absorption of radiant
energy that can cause ionisation or excitation.
IONISATION OF WATER MOLECULE
H2O H2O+o + e-
This reaction needs 13 eV of energy. Radical ions formed are
short lived, with life of about 10-10sec and will soon decay to
form uncharged free radical.
H2O+o H+ + OHo

18. EXCITATION OF WATER MOLECULE
H2O H2O*
The excited water molecules are not stable and soon undergo
radiolysis giving rise to Ho and OHo radicals.
H2O* Ho + OH0
The OHo free radical have a life time of 10-5 sec. They are very
powerful oxidising agent.
The ejected energetic electrons loose their energy by collision and
are finally captured by water molecules, forming aqueous
electrons(hydrated).
e- + H2O e- aq
These aqueous electrons are strongly reducing agents and can cause
dissociation of water molecules forming free radicals.
H2O + e- aq Ho + OH0

19. H0 +OH0 H2O
Ho +Ho H2
OH0 +OH0 H2O2
This reaction depends on LET And more frequent with high LET
radiation
Ho + O2 HO2
o (HYDROPEROXY RADICAL)
HO2
o is less reactive than OH0 but has a longer life span and
hence diffuse longer distances
OHo +H2O2 H2O2 +HO2
o
HO2
o + HO2
o H2O2 + O2
Free radicals are formed within microseconds after irradiation.
These free radicals interact with the biological molecules and
cause damage to them through these reactions:
R-H + OHo Ro + H2O
R-H + Ho Ro + H2
R-H + HO2
o ROo + H2O

21. DNA DAMAGE
SSB:A single Ionisation cause the break in one of the strands of
DNA molecules. The break can occur either at the bond
between sugar and phosphate or between sugar and the base.
Since most of the SSB’S are because of free radicals, hence low
LET radiations induce more SSB because energy deposited per
unit path length is less and hence causing excitation . About 3-
4 times more SSB are produced in a well oxygenated system, as
compared to hypoxic conditions
DSB: If the breaks in the two strands are opposite to one another
or separated by only a few base pairs <5 DSB may occur. DSB
can formed either by single ionizing event or by two
independent events occurring in complementary strands of
DNA.

23. RADIATION DOSE RESPONSE FOR
SSB AND DSB
Low doses of X-rays produce 10-20 times more SSB than DSB.
Amount of SSB has been shown to increase linearly with dose
whereas DSB varies linear- quadratically with Radiation dose.
The yield of SSB decreases with LET, Whereas yield of DSB
increases with LET.
It is because as the LET increases, the probability of target
getting more than one hit also increases, thus increasing the
chance of DSB.
Radiation can also induce BASE DAMAGE, in purines and
pyrimidines bases. Indirect effect of radiation through OH0
radicals causing hydro peroxidation of the bases is the most
common interaction leading to base damage.
Thymine is most sensitive and hence most affected of the
bases, while guanine is least.

24. WHY CELLS DIE WHEN THEY ARE
IRRADIATED
Exposure of biological tissues to ionising radiation immediately
leads to ionisation and excitation of their constituent atoms.
The molecules where these atoms reside then tend to fall apart,
resulting in so-called free radicals.
Water is the most prevalent molecule within the cell so most of
the free radicals are produced by the radiolysis of water.
Free radicals are highly unstable. They react with other nearby
molecules, thereby transferring chemical damage to them.
These free-radical processes are usually complete within a
millisecond under physiological conditions.
All components of the cell will be damaged in this way:
proteins, enzymes, membrane components, etc.

25. However, such molecules are present in vast numbers in every
cell and damage to a few of them has little impact on the cell’s
viability; they will be quickly regenerated.
But there is one cellular component which is almost unique:
DNA.
DNA is a very long double-helix molecule consisting of a
repeated sequence of bases, and every chromosome has
approximately 200 million bases.
Groups of bases form the genes that contain instructions for
proteins and thus for all aspects of cellular function. There is
usually some duplication of genes, but even so there is a
serious risk that radiation damage may lead to the loss (or
modification) of some genes and thus to a loss of specific
functions (some of which may be essential for survival).
This is the reason why DNA is the most vulnerable part of a cell
to radiation damage.

26. DNA DAMAGE
When normal cell DNA is damaged by radiation provided in the
kinds of doses normally used in radiotherapy, the cell cycle is
stopped by the protein p53.
The DNA is repaired; the cell then re-enters the cell cycle and
continues to proliferate.
If the DNA cannot be repaired, the cell enters apoptosis – the
programmed cell death pathway.
At high radiation doses , the molecules utilized by the DNA
repair mechanisms are damaged, so repair is not possible, the
cell loses its ability to divide, and it subsequently dies.

27. CHROMOSOMAL ABERRATIONS
Breaks induced by DNA may remain unrepaired or re-join incorrectly
to form abnormal configurations known as chromosomal aberrations
.
They are of two types :
Structural Aberrations : These occur due to a loss or genetic
material, or a rearrangement in the location of the genetic
material. They include: deletions, duplications, inversions, ring
formations, and translocations
Numerical Aberrations : The number of chromosomes show a change
from normal diploid state .It results from non disjunction of
chromosomes during mitosis causing unequal distribution in
daughter nuclei .eg trisomy monosomy
DOWN’S SYNDROME : Trisomy of chromosome 21
KLINEFELTER SYNDROME:

28. STRUCTURAL ABERRATIONS
(HALL)If cells are irradiated with x-rays, double-strand breaks are
produced in the chromosomes.
The broken ends appear to be “sticky” because of unpaired
bases and can rejoin with any other sticky end.
Once breaks are produced, different fragments may behave in a
variety of ways:
RESTITUITON
DELETION: Associated with carcinogenesis if the lost genetic
material includes Tumor Suppressor Gene.
REASSORT
They are of two types
Chromosomal aberration : IRRADIATION IN EARLY METAPHASE
Chromatid aberration : IRRADIATION IN LATE INTERPHASE

29. LETHAL
ABERRATIONS(Reproductive Death
)DICENTRIC: It involves an interchange between two separate
chromosomes. If a break is produced in each one early in interphase
and the sticky ends are close to one another, they may re-join This is
replicated during the S PHASE , and the result is a chromosome with
two centromeres (dicentric)and also two fragments with no
centromere (acentric fragment)
RING :A break is induced by radiation in each arm of a single
chromatid early in the cell cycle. The sticky ends may re-join to form
a ring and a fragment.. The fragments have no centromere.
ANAPHASE BRIDGE: It results from breaks that occur late in the cell
cycle (in G2), after chromosomes replication Breaks may occur in both
chromatids of the same chromosome, and the sticky ends may re-
join incorrectly to form a sister union. At anaphase, when the two
sets of chromosomes move to opposite poles, the section of
chromatin between the two centromeres is stretched across the cell
between the poles, hindering the separation into two new progeny

30. PICTURE

31. CELL DEATH
Cell survival, or its converse, cell death, are different things in
different contexts.
 For differentiated cells that do not proliferate, such as nerve,
muscle, or secretory cells, death can be defined as the loss of a
specific function.
 For proliferating cells, such as stem cells in the hematopoietic
system or the intestinal epithelium, loss of the capacity for
sustained proliferation—that is, loss of reproductive integrity—
is an appropriate definition.
 This is sometimes called reproductive death
 The successful use of radiation to treat cancer results primarily
from its ability to cause the death of individual tumour cells.
 Quantification is complicated by the fact that cells die at
various times after irradiation, often after one or two trips
around the cell cycle, and among surviving cells that continue
to proliferate
A cell survival curve describes the relationship between the
radiation dose and the proportion of cells that survive.

32. Cells may die by different mechanisms:
Mitotic death : Death while attempting to divide, that is, mitotic
death, is the dominant mechanism following irradiation.
Apoptosis: For some cells, programmed cell death,is important.
Whatever the mechanism, the outcome is the same
“The cell loses its ability to proliferate indefinitely, that is, its
Reproductive integrity.”

33. QUANTIFYING CELL KILLING
P.E =
𝑁𝑜.𝑜𝑓 𝑐𝑜𝑙𝑜𝑛𝑖𝑒𝑠 𝑐𝑜𝑢𝑛𝑡𝑒𝑑∗100
𝑁𝑜.𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 𝑠𝑒𝑒𝑑𝑒𝑑
S.F=
𝐶𝑜𝑙𝑜𝑛𝑖𝑒𝑠 𝑐𝑜𝑢𝑛𝑡𝑒𝑑
𝐶𝑒𝑙𝑙𝑠 𝑠𝑒𝑒𝑑𝑒𝑑
∗
100
𝑃.𝐸

34.  2,000 cells seeded and exposed to 8 Gy (800 rad).
 plating efficiency is 70%, 1,400 of the 2,000 cells plated
would have grown into colonies if the dish is not irradiated.
 In fact, there are only 32 colonies on the dish; the fraction of
cells surviving the dose of x-rays is thus ;
32
1400
=0.023
This process is repeated so that estimates of survival are
obtained for a range of doses. .
 Too few reduces statistical significance; too many cannot be
counted accurately because they tend to merge into one
another.
The survival curve that results, does not distinguish the mode
of cell death, that is, whether the cells died mitotic or apoptotic
deaths.

35. TARGET THEORY
The number of DNA or critical target cells “hit” by the radiation
depends on random events in target theory, and has no direct
relation to the ionizing radiation dose
Therefore, there is no threshold at which the effects of the
radiation are observed. Whatever the delivered radiation dose,
there is always a chance of it hitting DNA or cells and producing
harmful effects.
The phenomenon where the effects of the radiation do not
depend on dose is known as the “stochastic effect.”
Target theory explains the cell damage caused by radiation
based on the principles of probability.
It assumes that there are certain critical molecules or critical
targets within cells that need to be hit or inactivated by the
radiation to kill the cell.
The simple target theory assumes that each event occur at
random in an irradiated system .There is a statistical chance
that any particular be hit .

36. ASSUMPTIONS OF TARGET
THEORY
As a first approximation, the simple target theory assumes
that events occur at random in an irradiated system.
Degree of effect is not influenced by the dose rate.
Experimental condition during irradiation and after irradiation
are not of importance.
Although these assumptions are not justified in most of the
circumstances, however this theory is of great Significance in
explaining the kinetics of dose response.

37. TARGET THEORY
Target theory is simply the model when the biological effects
meet certain criteria in its relation to dose.
Biological effects may be cell death or its ability to grow and
divide.
Various target theory to observe the radiation effects are.
 Simple target theory
 Multitarget theory
 Multihit theory
 Multitarget Multihit theory

38. NEED
It was put forward to derive a mathematical relationship
between the number of microorganisms that are killed and
radiation dose they received.
Proposed by Crowther and explained by Lee
The two common terminologies in this will be hits and target
Hits : the production of an effective event in the target is called
hits
Target : it is that part of a cell or a cell as whole which when
hit will give a required effect i.e cell death or inability to grow .

39. SINGLE TARGET SINGLE HIT
Taking the natural log
of (1),
We get lnS= (-D/D0)
 It was assumed that SINGLE hit can result in an inactivation of cell
 Each cell has a single target.
 Inactivation of the target kills the cell.
The survival curve is exponential (i.e. a straight line in a semi-
logarithmic plot of cell survival against dose).
No : number of organisms initially present
N : number of organism surviving after dose D
If an increment of dose(dD) is added, N will be decreased by dN in
an amount which is proportional to number present N.
• - dN/dD = kN, k is proportionality constant
- dN/N= kdD
After integration, ln(N/No)= -kD
so, N/No= e-kD
If fraction of surviving cells is S, then S= e-kD

40. For Do= dose that gives an average of
one hit per target.
 Proportionality constant , k=1/Do
,survival fraction S=e-D/D
0
When dose D has been given such that,
D=Do,
S=e-1 i.e. S=0.37
Therefore, when there is average one
hit per Target( number of hits is equal
to number of targets), 37% of original
number of organisms still survive.
X Axis : dose (Gy ), Y Axis : S=N/N0
As shown the number of organism
killed by successive increment of
dose are not equal, but each dose
increment kills the same proportion
of number of organism that have
survived until then.So, the survival
curve is exponential in nature

41. LOGARITHMIC CELL
KILLING
ASSUMPTION :
1. At each dose of 2 Gy, 50%of
the cells are killed.
2. Probability of survival is same
Therefore, by the end of the
course of radiation, very few
cells are killed with each
individual dose

42. EFFECTIVE HITS
At small radiation doses the
number of targets hit and
affected will be directly
proportional to the amount of
radiation
If the dose is doubled twice as
many as targets will be hit.
Hence twice the target will be
affected.
With the increasing dose some
of the events will occur within
targets that have been already
hit .
These hits will be wasted so
the effective hits will decrease

43. POISSON DISTRIBUTION
All calculations of hit probability are governed by Poisson
statistics, where the probability of n events is given by
P(n) =
(e−x )(xn)
𝒏!
x = the average number of events and n = the specific number
of events
• If each “hit” is assumed to result in cell inactivation, then the
probability of survival is the probability of not being hit, P(0).
P(0) =
(e−1 )(10)
𝟎!
= e-1=37% {From the Poisson relationship,
where x = 1, and n=0}
• For this reason, D0 is often called the mean lethal dose, or the
dose that delivers, on average, one lethal event per target.

44. LIMITATIONS OF SIMPLE TARGET
THEORY
According to this theory, effect of given dose is independent on the
rate at which it is delivered. However, it is not true in practice.
Cellular environment such as oxygen status,moisture and presence of
chemicals seems to have influence on cell survival responses.
For a given organism, biological factors such as cell cycle stage,
repair capability etc are also the deciding factor for the responses.

45. MULTITARGET THEORY
Simple target theory was not successful in calculating the target
size for large viruses, so Multi target theory was proposed.
ASSUMPTIONS OF THE THEORY
1. Each cell contains n distinct and identical targets
2. Each target can be inactivated by the passage of a charged
particle (a hit).
3. Inactivation of a target is a sub lethal event.
4. All n targets must be inactivated to kill the cell.
5. For a dose Do there is on average one hit per target

46. Now, the surviving fraction gives the probability of target not
being hit.
P(not hit)= S = e-kD
P(hit)= 1- e-kD
So, the probability of all the targets ( say, n ) being hit =
(1- e-kD )n
Now, for the unit to be surviving, all the targets must not be
receive the hit.
Survival fraction,
S= 1- (1- e-kD )n
For large value of n, (1- x)n = (1- nx) (BINOMIAL THEOREM )
S= 1- (1- n e-kD )
S= n e-kD
this can be written in logarithm form as,
ln(S)= ln(n) - kD

47. MULTI TARGET
MODEL
Survival curve corresponding to this
theory start with less sensitive
region at low doses and show
exponential behaviour at large doses
i.e. shoulder region in the
beginning.
 They are also known as Shoulder
type Survival Curves.
Such curves are obtained when
mammalian cells are irradiated with
low LET radiation e.g. X-rays
SHOULDER represents cells in which
fewer than n targets have been
damaged after receiving a dose D
i.e. cells have received SLD which
can be repaired.

49. MULTIHIT THEORY
This theory postulates that some systems contain a single
target which must be hit ‘m’ times in order to inactivate the
system.
from Poisson’s distribution, probability of receiving
0 hit = e-kd
1 hit =kd e-kd
2hit =(kd)2e-kd /2!
j hits =(kd)je-kd /j!
(M-1) hits =(kd)(M-1) e-kd /(M-1)!
So, the surviving units are all those receiving less than ‘m’ hits
S= e-kD (1+kD+(kD)2/Г2+(kD)3/Г3……….)

50. S = e-kD(∑(kD)t/Гt)
where t varies from 0 to ‘m-1’.
The curve resembles to survival curve for multitarget theory, as
when plotted on semilog paper, linear portion will have slope –k
and extrapolation on the vertical axis corresponds to ‘m’, the
number of hits required to inactivate the target.

51. MULTITARGET MULTIHIT THEORY
According to this theory, each irradiated unit contains ‘n’
targets, each of which must be hit ‘m’ times to be inactivated.
surviving fraction, S= 1-(1-e-kD(∑(kD)t/Гt))n
where t varies from 0 to ‘m-1’.

52. CELL SURVIVAL CURVES
The survival curves of mammalian cells can be used to obtain
direct information on their response to radiation.
It describes the relationship between the radiation dose and
the proportion of cells that survive.
Cell survival, or its converse, cell death, may mean different
things in different contexts.
For differentiated cells that do not proliferate, such as nerve,
muscle, or secretory cells, death can be defined as the loss of a
specific function.
For proliferating cells, such as stem cells in the hematopoietic
system or the intestinal epithelium, loss of the capacity for
sustained proliferation—that is, loss of reproductive integrity—
is an appropriate definition.
This is sometimes called reproductive death.
CELL survival curve doesnot distinguish the mode of death i.e
apoptotic or mitiotic

53. There are two reasons why cell
survival curves are more usually
plotted on a logarithmic scale of
survival:
1. If cell killing is random then
survival will be an exponential
function of dose, and this will be a
straight line on a semi-log plot.
2. A logarithmic scale more easily
allows us to see and compare the
very low cell survivals required to
obtain a significant reduction in
tumour size, or local tumour
control.

54. LINEAR QUADRATIC MODEL
 This model was developed by Douglas and Fowler in 1972.
ASSUMPTIONS :
 The frequency of chromosomal aberration is a linear
quadratic function of dose Hence The aberrations are the
consequence of the interaction of two separate breaks
 At low doses both breaks may be caused by the same
electrons .The probability of exchange aberration is directly
proportional to dose
 At higher doses the two breaks are more likely to cause by
separate electrons .HENCE The probability of an exchange
aberration is proportional to square of the dose .

56. ALPHA BETA
RATIO
a → shows the intrinsic cell
radio sensitivity, and it is the
natural logarithm (loge) of the
proportion of cells that die or
will die due to their inability to
repair radiation-induced
damage per Gy of ionizing
radiation.
b → reflects cell repair
mechanisms, and it is the
natural logarithm of the
proportion of repairable cells
due to their ability to repair
the radiation-induced damage
per Gy of ionizing radiation.

57. EFFECT OF LET ON
CELL SURVIVAL
CURVE
Radiosensitivity increases with high-
LET radiation.
• The slope of the survival fraction
(SF) curve (1/D0) is large for high-
LET radiation.
• The slope of the SF curve (1/D0) is
small for low-LET radiation.

58. EFFECT OF DOSE
RATE ON CELL
SURVIVAL CURVE
Cell survival is greater for a
delivered radiation dose if the dose
rate is decreased
 This is due to the proliferation of
undamaged living cells and SLD
repair during radiotherapy.(REPAIR
AND REPOPULATION )
This effect is very important in
brachytherapy applications. The dose
rate in external therapy is 100
cGy/min.
Low dose rates are used in
brachytherapy, and high doses can
be given due to normal tissue repair
and repopulation.

59. OXYGENATIO
N
Soluble oxygen in tissues increases the
stability and toxicity of free radicals.
The increase in the effect of radiation
after oxygenation is defined as the
oxygen enhancement ratio (OER)
OER =
Required dose under hypoxic condition
Required dose under oxygenated conditions
The maximum value of the OER is 3.
Oxygenation can modify the indirect
effect of free radicals. However, the OER
plays no role in the direct effect of high-
LET radiation;
OER is 1 in this case.

60. THERAPEUTIC INDEX
The therapeutic index defines how the tumour control
probability (TCP) relates to the Normal Tissue Complication
Probability (NTCP) for different doses.
Normal tissues may get damaged by the dose required to
control the tumour; on the other hand, the tumour may not
receive an adequate dose if the normal tissues require
protection.
Achieving the optimal balance between TCP and NTCP is a basic
aim of radiotherapy.
TCP and NTCP curves are sigmoid in shape. The purpose of
treatment is to move the TCP curve to the left and the NTCP
curve to the right.
• The therapeutic index (= therapeutic window) increases if the
region the between two curves becomes large, and the
expected benefit from treatment increases.

62. APPLICATION OF SURVIVAL
CURVES
• The existence of a threshold in cell-survival curves implies
that some damage must accumulate before it is fatal to the cell.
• The larger the value of Dq , the more damage that must
accumulate before reproductive death. This damage to cells
prior to cell death is called sublethal damage.
• In radiation therapy it is very important to note that when a
dose is split into two parts separated by enough time, a
threshold is observed for each part of the dose. Thus by
properly spacing treatment, it is possible to reduce the damage
to healthy cells during radiation treatment.

63. CHEMICAL MODIFIERS OF
RADIOSENSTIVITY
The therapeutic outcome can be improved by :
Increasing the radio resistance of normal tissues so that higher
doses for effective tumour control will be tolerated .
Increasing the efficiency of tumour cells so that higher tumour
killing is achieved at conventional doses of radiotherapy .
The ultimate Aim is to achieve complete elimination of tumour
cells keeping normal tissues reaction in acceptable limits .
Radio protectors : The increase in radio resistance of normal
tissues by using chemicals which protect against radiation
damage
Radio sensitizers : The radiation response of tumours can be
enhanced by using chemicals which increase radio sensitivity of
hypoxic cells .

64. RADIOPROTECTORS
TWO MAINS RP’S ARE :
SH-CH2-CH NH2 (CYSTEINE) SH-CH2-CH2-NH2
(CYSTEAMINE)
COOH
Radio protective efficiency of a compound is given by Dose
reduction factor
DRF:
𝐷𝑜𝑠𝑒 𝑜𝑓 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑝𝑟𝑒𝑠𝑒𝑛𝑐𝑒 𝑜𝑓 𝑟𝑎𝑑𝑖𝑜𝑝𝑟𝑜𝑡𝑒𝑐𝑡𝑜𝑟
𝐷𝑜𝑠𝑒 𝑜𝑓 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑎𝑏𝑠𝑒𝑛𝑐𝑒 𝑜𝑓 𝑟𝑎𝑑𝑖𝑜𝑝𝑟𝑜𝑡𝑒𝑐𝑡𝑜𝑟
Mechanism :
1. Radical Scavenging
R-H+OHo Ro +H20 (INDIRECT ACTION )
Ro+RP RH +So (RESTITUTION )
Ro +O2 RO2
o (DAMAGE FIXATION )

65. 2.Radical Repair
RH Ro+H (Radiation )
Ro+S-H RH +S (Hydrogen Donation )
3.Biochemical Repair
SSB’S are readily repaired unlike DSB’S this RP’s protect p53 etc
repair enzymes and hence increases biochemical Repair .

66. RADIOSENSITIZERS
They act at different levels for enhancing the cell killing effect
of radiation
Before, During and After Irradiation
The efficiency of RS’s is given by:
ENHANCEMENT RATIO =
𝐷𝑂𝑆𝐸 𝐼𝑁 𝐴𝐵𝑆𝐸𝑁𝐶𝐸 𝑂𝐹 𝑅𝐴𝐷𝐼𝑂𝑆𝐸𝑁𝑆𝐼𝑇𝐼𝑍𝐸𝑅
𝐷𝑂𝑆𝐸 𝐼𝑁 𝑃𝑅𝐸𝑆𝐸𝑁𝐶𝐸 𝑂𝐹 𝑅𝐴𝐷𝐼𝑂𝑆𝐸𝑁𝑆𝐼𝑇𝐼𝑍𝐸𝑅
Ideally they should increase sensitivity of tumour cells without
significantly enhancing the normal tissue sensitivity DIFFERENT
TYPES ARE :
Nucleotide Analogues :
These are halogenated pyrimidines eg 5-bromodeoxyuridine
.During DNA Synthesis they are incorporated into DNA which
weakens DNA .

67. Hypoxic cell sensitizers
These sensitize hypoxic cells without sensitizing the
oxygenated cells . E.g NITROIMIDAZOLES .
They act by repair inhibition ,free radical fixation enhancing
oxidative damage to DNA
BIOREACTIVE DRUGS :
These are the compounds which under hypoxic conditions get
reduced metabolically to form active cytoxic agents inside the
cells .In tumour cells they are metabolically reduced to form
highly effective cytotoxins . They are preferentially toxic to
hypoxic tumour cells

68. RP’S AND RS’S RP’S

69. FRACTIONATED TREATMENTS
AIM : To do with exploiting the biology of cells so that we can
give enough dose to the tumour, and minimize the damage
done to the healthy tissue surrounding it.
The reason behind fractionation can be explained by the 5 R’s
of Radiobiology.
Repair,
Redistribution,
Reoxygenation,
Repopulations
Radiosensitivity.

70. REPAIR
Repair is one of the primary reasons for fractionated
Radiotherapy. Three types of damage that Ionising Radiation
can cause to cells:
Lethal Damage :Damage which is fatal to the cell
Sub lethal Damage :Damage which can be repaired before the
next fraction of radiation is delivered
Potentially Lethal Damage :Damage which can be repaired
under certain circumstances (usually when the cell is paused in
the cell cycle due to external factors)
Now by splitting the radiation dose into smaller parts
(fractionation) cells are allowed time to repair sub lethal. The
difference between healthy cells and tumour/cancer cells is that
tumour cells don’t always recognize they are being damaged
and don’t repair themselves.
THIS LIMITS THE DAMAGE TO NORMAL CELLS

72. CELL CYCLE AND
RADIOSENSTIVITYIt basically consists of Interphase and Mitosis phase
Interphase consists of G1,S,G2 phase
M phase consists of Prophase, Anaphase, Metaphase Telophase
Most Radiosensitive Phase: G2-phase and mitosis (M-phase)
Least Radiosensitive Phase: Latter part of S-phase (synthesis of
DNA)
Duration of each phase G1 = 1.5–14 h, S = 6–9 h G2 = 1–5 h, M
= 0.5–1 h in the human cell cycle:.
HRR occurs primarily in the late S/G2 phase of the cell cycle,
when an undamaged sister chromatid is available to act as a
template
NHEJ occurs in the G1 phase of the cell cycle, when no such
template exists. NHEJ is error prone

75. REDISTRIBUTION
By NOW we know Cells are the most sensitive to radiation
when they are in G2 and M phases, after both sets of DNA have
been created.
Cells are the least sensitive in the S and G1 phase.
A small dose of radiation delivered over a short time period
(external beam or high dose brachytherapy) will kill a lot of the
sensitive cells and less of the resistant cells.
Over time, the surviving cells will continue to cycle.
If a second dose of radiation is delivered some time later,
some of these cells will have left the resistant phase and be in a
more sensitive phase, allowing them to be killed more easily.
Redistribution occurs during low dose rate treatment.

76. REOXYGENATION
Tumours are basically a bunch of cells invading a tissue
without any sort of objective, they just multiply.
When they grow like this they get to a point where oxygen
from the blood can’t get to the cells in the centre of the
tumour.
These cells without oxygen are in the G1 Phase, which makes
them insensitive to the radiation treatment AND HENCE WILL
NOT DIE
 Fractionation overcomes this by killing the tumour cells at the
edge.
 This allows oxygen to flow to the inner tumour cells
previously blocked by the outer tumour cells.
 Now the cells will have oxygen again they will start to go
through the phase cycle again and hopefully the next radiation
treatment will kill them.

78. REPOPULATION
Repopulation is a tissue’s response to a decreasing number of
cells.
There is a delayed response between cell killing through
radiation and the tissue repopulating the dead cells.
This is different for different tissue types. On average it takes
around 4 weeks for a tissue to start repopulating after radiation
exposure.
Fractionation helps in certain cases where the normal tissues’
response time is shorter than the time to have all treatments.
Suppose normal tissue started repopulating in 4 weeks, and
the total treatment time is 6 weeks. This means that in the last
two weeks the normal tissue is starting to repopulate. This
reduces side effects as the normal tissue as a whole is repairing
itself to some degree.
But, If the tumour cells start to repopulate before all the
treatments have been delivered we might see an accelerated
growth of the remaining tumour towards the end of the
treatment.

79. RADIOSENSITIVITY OF NORMAL
TISSUELAW OF BERGONI AND TRIBONDEAU
Cells which are actively dividing and are undifferentiated with a
long mitotic future are more sensitive than cells which have
stopped division and are differentiated.
LYMPHOCYTES are an exception though they are in GO phase
but are most radio sensitive .
CELLS ARE DIVIDED INTO FOUR CATEGORIES ON BASIS OF
DECREASING RADIOSENSTIVITY
Vegetative intermitotic cells :Actively dividing cells are most
radiosensitive e.g. stem cells
Differentiating intermitotic cells : They divide regularly and are
formed from vegetative intermitotic cells but the cells undergo
some differentiation e.g. spermatocyte

80. Reverting post mitotic cells : They do not divide and are variably
differentiated but have the potential to divide if stimulated e.g.
parenchyma cells of liver
Fixed post mitotic cells : They are non dividing are highly
differentiated they are most radio resistant e.g. muscle and
nerve cells

81. REPOPULATION
• Increases the number of tumour cells to be destroyed → against treatment
• Increases the number of normal tissue cells following irradiation → in favour of
treatment
REDISTRIBUTION AND
REOXYGENATION
• Redistribution and Re oxygenation → more important for tumour tissues than
normal tissues; as more tumour tissue dies, its Radiosensitivity increases.
REPAIR AND
RADIOSENSITIVITY
• REPAIR is important for normal tissue
• Radio Senstivity of tumour Is higher in favour of our TREATMENT

82. RADIOSENSITIVI
TY
 Law of Bergonie and Tribondeau
explain the radiosensitivity
There are sensitive cell types.
These are cells like stem cells,
sperm and egg cells, intestinal cells
and blood cells.
Then there are cells that are not
sensitive to radiation like neuron or
brain cells, and tumour cells like
melanomas.

83. DETERMINISTIC EFFECT
 The intensities of these effects are directly proportional to the
dose.
They have a specific threshold dose.
Effects appear at higher doses than the threshold dose.
There is a relationship between dose and individual effects.
 skin erythema, sterility, radiation myelitis, and fibrosis are all
examples of
deteministic effects.
For example, if the total body irradiation dose is >5 Gy, bone
marrow suppression is observed, but this suppression is not
observed for a dose of <5 Gy.

84. STOCHASTIC EFFECTS
The chronic effects of radiation are known as stochastic effects
.
These are statistically measurable effects.
There is no threshold dose for these effects.
There is no relationship between the dose and individual
effects.
Carcinogenesis, genetic mutations and chromosome
aberrations are all stochastic effects.

85. CONCLUSION
When the ionising radiation reach the target they create
DNA strand breaks within the cancer cells. Cancer cells
have a reduced ability to repair this damage and thus the
damaged DNA causes the death of the cancer
cell. Adjacent normal cells that are in the path of the
radiation can also experience breaks in their DNA. Normal
cells have a more robust DNA repair capability and the vast
majority of this damage is repaired. To take advantage of
the different repair capabilities of normal and cancer cells,
radiation therapy is broken down into smaller daily doses
of treatment. With the smaller dose, normal cells can more
completely repair their DNA in between the daily
treatments and thus keep the likelihood of long term side-
effects low.