Cell survival curves describe the relationship between radiation dose and the fraction of cells that survive that dose. The shape of the curve is influenced by many factors including: 1) The proliferative capacity of cells, with rapidly dividing cells being more radiosensitive. 2) The presence of oxygen, with hypoxic cells being more radioresistant. 3) Fractionation, with fractionated doses allowing more time for repair between exposures and resulting in a higher surviving fraction compared to a single dose. 4) Dose rate, with low dose rate irradiation resembling continuous fractionation and having less cell killing compared to an acute high dose rate. Cell survival curves take different shapes for early-responding normal tissues dominated

1. CELL SURVIVAL CURVE
Ankita Pandey
Junior Residant,
Dep. Of Radiotherapy
BHU
1

2. What is cell survival curve?
 It describes relationship between
radiation dose and the fraction of
cells that “survive” that dose.
 This is mainly used to assess
biological effectiveness of
radiation.
2

3. Cell death
Cell death can have
different meanings
loss of a specific
function
loss of reproductive
integrity -
“reproductive death
3

4. In our body we have 2 types
of cell
Non proliferating cells proliferating cells
Nerve
Muscle cells
Secretory cells
Cell death = loss of a specific
function
Stem cells in hematopoietic
system or intestinal epithelium
Cell death = loss of
reproductive integrity -
“reproductive death
4

5. A surviving cell that has retained its
reproductive integrity and is able to
proliferate indefinitely to produce a large
clone or colony is said to be clonogenic
1. Mitotic cell death
2. Apoptotic cell
death
Cells loses its
ability to proliferate
either immediately
or after few mitosis
5

6. Radiation sterilizes cells, meaning that
they do not die immediately, but at the
time of the next cell division, or a few
divisions later.
An important factor is the repair
occurring in cells between irradiation and
the next cell division.
This repair in cells, i.e., recovery in
tissues, depends on the turnover rate of
renewing tissues—a day or so for rapidly
proliferating tissues and most tumors, but
6

7. Survival
 “Survival” means retention of reproductive
integrity
 the capacity for sustained proliferation in cells
that proliferate
7

8. In vitro survival curve
 The capability of a single cell to grow into a
large colony that can be seen easily with the
naked eye is a convenient proof that it has
retained its reproductive integrity
8

9. Plating efficiency
PE is the percentage of cells that
grow into colonies
 in other words, those cells that survive
the plating process
9

10. Specimen from a tumor , chop it into small
pieces, and prepare a single-cell
suspension
Trypsin dissolves and loosens the cell
membrane
10

11. Surviving fraction
 fraction of cells that plate
successfully and survive
irradiation (without losing their
reproductive integrity) to grow
into colonies
11

12. Shape of cell survival curve
12

13. Shape of cell survival curve
 Cell killing follows exponential relationship.
 A dose which reduces cell survival to 50% will, if
repeated, reduce survival to 25%, and similarly to
12.5% from a third exposure.
 This means Surviving fraction never becomes zero
13

14. Models for survival curve description
14

15. Single target- single hit theory
 It states that- the objective of treating a
tumor by radiotherapy is to damage
every single potentially malignant cell to
such an extent that it cannot continue to
proliferate.
15

16. 16

17. 17

18. 18

19. Multiple target model
 initial slope D1 resulting from single-event killing
 final slope, D0, resulting from multiple-event killing
 n or Dq to represent the size or width of the shoulder
of the curve.
19

20. The quantities D1 and D0
are the reciprocals of the
initial and final slopes.
Dq defined as the dose at
which the straight portion
of the survival curve,
extrapolated backward,
cuts the dose axis drawn
through a survival fraction
of unity.
20

21. 21
Both the multitarget model and the linear-quadratic model aim to
explain the shape of the cell survival
curve, which can be determined using in vitro experiments. In the
multitarget model, D1 is the initial slope
of the curve and represents cell kill from single-hit events. D0 is
the final slope of the cell survival curve
and represents cell kill from multiple-hit events. Both D0 and D1
represent the dose required to reduce the
fraction of surviving cells to 0.37. Since the surviving fraction is
on a logarithmic scale, the dose required
to reduce the cell population by a given factor (0.37) is the same
at all survival levels. The extrapolation
number, n, is a measure of the width of the shoulder of the cell
survival curve. Dq represents the quasithreshold
dose, which is the dose at which the discrete portion of the
survival curve when extrapolated
backwards cuts the dose axis at 100% survival.

22. Linear quadratic model
 assumes that there are two components to cell killing
by radiation
 one that is proportional to dose (Linear)
 one that is proportional to the square of the dose.
(Quadratic)
22

23. We know that there are
different types of DNA
damage that occurs due to
radiations
Lethal damage
Sub lethal damage
Potentially lethal damage
All the types of damage
and cells ability to repair
it is reflected in cell
survival curve
Why two components in curve?
23

24. There are 2
components –
Linear
quadratic
Linear curve represents
lethal cell killing or
unrepairable component
of cell damage
Quadratic curve represents
repairable portion of cell
damage
24

25. Plotting same curve
on the dose survival
graph
Bending of the
curve depends upon
various factors
25

26. Why linear killing at low doses
26
 Linear components are seen in both low and high
doses
 It represents killing of radiosensitive cells
 Even at very low doses, cells that are
radiosensitive are killed which is manifested in
initial portion of cell survival curve with a linear
curve that is proportional to dose
 These radiosentitive cells can be –
1. Rapidly proliferating cells
2. Well oxygenated cells
3. Cells in mitotic phase
 In these conditions, even the low dose is
sufficient to cause lethal cell damage

27. Why quadratic killing at high doses
27
 Not all the cells that are irradiated are killed by
lethal damage due to single electron set in motion
 Some of the damages produced in DNA are
repaired and low doses are insufficient to kill
these type of cells
 This component of curve represents
radioresistance of cell
 At high doses, ionisation increases and damage
in DNA is a result of multiple electrons
 Even the cells that has high capacity to repair are
rendered irrepairable because multiple breaks in
DNA increases the chances of chromosomal

28. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
28

29. Alpha is the intrinsic
radiosensitivity of the
cells, defined as how
many logs are killed
per gray, in a ‘‘non-
repairable’’ way.
Beta is the
repairable portion of
the radiation damage,
requiring 6 h or more
for complete repair
29

30. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
30

31. 31

32. 32

33. 33
Alfa cell killing
dominates.
Lethal killing is
more
Dual action cell killing.
Sublethal damages are
repaired

34. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
34

35. Difference in the curve of early and late reacting
tissues
35

36. Why difference in shape of curve?
 The radiosensitivity of a population of cells varies
with the distribution of cells through the cycle
Fast proliferating cells Slowly proliferating cells
S phase occupies major
portion of cell cycle
G1 or G0 phase occupies
major portion of cell cycle
Resistance due to many
cells are in resting stage
Redistribution in rapidly
proliferating cells leads to self
sensitizing
This resistance disappears
at larger doses per fraction
36

37. Early reacting tissues
Means cells with high
proliferating capacity
In dividing cells, lethal
damage are more
common ,i.e, they are
more radiosensitve cells
Alfa or linear
component will dominate
while beta component
will be seen at high
doses
Hence high α/β ratio
37

38. Cell survival curve for early responding
tissues
High ratio indicates that
cell is less sensitive to
change in dose per #
α component dominates for long
even at high doses so changing
dose to 2Gy or 3Gy or 4Gy etc is
not going to increase beta cell
killing instead it will effect late
reacting normal tissues and will
cause increase in late side effects
38

39. What we do to increase tumor control ?
Hyper fractionation
Reduce
overall
treatment
time
Increase total
dose
39

40. Late reacting tissues
Small ratio indicates that
cell is more sensitive to
change in dose per #
 breast cancer
Prostate cancer
40

41. 3Gy
Normal
tissue
Tumor
tissue
41

42. 3Gy
42
2G
y

43. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
43

44. DNA content
It is evident that
mammalian cells are
radiosensitive compared
with microorganisms,
principally because they
have a much larger DNA
content, which represents
a bigger “target” for
radiation damage
Mammalian
cell
44

45. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
45

46. Fractionation
46

47. Linear component
caused by lethal lesion
is same whether dose is
given in single exposure
or fractionated
In single exposue,
breaks due to multiple
track damage forms
dicentric and cells die
when it attempts to
divide
In fractionated regimen,
damage is repaired
before second exposure.
Fewer interactions
between broken
chromosomes to form
dicentric and more cells
survive
47

48. 48

49. 49

50. Repair and radiation quality
For x-rays, dividing the total dose into two equal fractions, separated
from 1 to 4 hours, results in a marked increase in cell survival because of
the prompt repair of SLD. By contrast, dividing the dose into two
fractions has little effect on cell survival if neutrons are used, indicating
little repair of SLD.
50

51. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
51

52. Dose rate effect
52

53.  continuous low-dose-rate (LDR) irradiation may
be considered to be an infinite number of infinitely
small fractions
 the survival curve have no shoulder and is
shallower than for single acute exposures
53

54. 54

55. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
55

56. 56

57. 57

58. Factors influencing cell survival curve
1. LET
2. Proliferating capacity of cell
3. Radiosensitivity due to DNA content
4. Fractionation
5. Dose rate effect
6. Presence of oxygen
7. Mechanism of cell death
58

59. Mechanism of cell death
Asymmetric chromosomal
type aberrations are most
common cause of mitotic cell
death
Characteristic laddering is indicative of
programmed cell death or apoptosis during
which the DNA breaks up into discrete lengths
most cell lines have contributions
from both mitotic and apoptotic
death following a radiation
exposure, in varying proportions
59

60. The most radioresistant
cell lines, which have
broad shoulders to their
survival curves, show no
evidence of apoptosis;
those most
radiosensitive, for which
survival is an exponential
function of dose, show
clear DNA laddering as
an indication of
apoptosis.
60

61. Cell cycle and radiosensitivity
 Cells are most sensitive to RT in G2/M.
 have enough time to repair DNA damage before
dividing.
 DNA damage leads directly to mitotic catastrophe.
 Cells become gradually less sensitive in G1 to early
S, and are least sensitive to RT in late S.
 Homologous recombination repair is most active
during late-S phase, after most of the DNA has been
61

62. There is practically no
shoulder to the survival
curve in G2/M
suggesting that very
little repair takes place.
There is a moderate
shoulder in G1 to early
S.
There is a very large
shoulder in late S
62

63. Estimation of BED by survival
curve
BED is defined in relation to the
initial slope, i.e., the linear
component of damage, it is E/a
BED is the dose that would cause the same log cell kill E
as the schedule with n X d fractions, if it could be
delivered in an infinite number of infinitely small
fractions, or at very low dose rate.
All the beta components
would then have been
repaired, and the straight line
representing the log cell kill
versus total dose would follow
just the initial slope alpha
63

64. Survival curve models for high dose
 The LQ model of cell killing does not appear to be
accurate for very large fractions.
 LQ predicts extremely high quadratic killing, but this
does not correlate with experimental observations.
 At very large fraction size, there may be additional
mechanisms for cell killing and cell survival.
 Many different models exist for predicting cell kill at
very large fraction size.
64

65. Universal Survival Curve (USC) / Single
Fraction Equivalent Dose (SFED):
 Combines a LQ and single-hit (D0) curve so
that it stops curving at very high doses.
 Used for calculating SRS/SBRT doses.
65

66. Lethal-Potentially Lethal (LPL) Model
 Damage is classified as lethal or potentially lethal.
 Potentially lethal damage is either repaired over
time or mis-repaired into lethal damage.
 Multiple potentially lethal hits can become lethal.
 Shape of curve is extremely similar to LQ model,
but LPL can more accurately model dose-rate
and fractionation factors.
66

67. Repair Saturation Model
 Initial damage follows a straight-line exponential
function, but is repaired into a survival curve with a
shoulder.
 At high doses repair becomes saturated and is
unable to keep up with damage, which is assumed to
be linear.
 Shape of curve is very similar to single-hit curve.
67

68. Conclusion
68

69. Conclusion
69

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