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. To understand it better, we need to know about a few basic things e.g. Cell Death Estimation of Survival / Plating Efficiency Nature of Cell killing etc. A cell survival curve is the relationship between the fraction of cells retaining their reproductive integrity and absorbed dose. Conventionally, surviving fraction on a logarithmic scale is plotted on the Y-axis, the dose is on the X-axis . The shape of the survival curve is important. The cell-survival curve for densely ionizing radiations (α-particles and low-energy neutrons) is a straight line on a log-linear plot, that is survival is an exponential function of dose. The cell-survival curve for sparsely ionizing radiations (X-rays, gamma-rays has an initial slope, followed by a shoulder after which it tends to straighten again at higher doses.

1. Cell Survival Curve
Dr. Vandana, King George’s Medical University, Lucknow

2. 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.
 To understand it better, we need to know about a few
basic things e.g.
 Cell Death
 Estimation of Survival / Plating Efficiency
 Nature of Cell killing etc.

3. Cell Death
Cell death can have different meanings:
 loss of a specific function - differentiated cells (nerve,
muscle, secretory cells)
 loss of the ability to divide - proliferating cells such as stem
cells in hematopoietic system or intestinal epithelium
 loss of reproductive integrity - “reproductive death”
3

4. Relevant Dose
 100 Gy
 destroys cell function in non-proliferating systems (for
example: nerve, muscle cells)
 2 Gy
 mean lethal dose for loss of proliferative capacity for
proliferating cells
4

5. Survival
 Conversely - “Survival” means retention of
reproductive integrity
 the capacity for sustained proliferation in cells that
proliferate
5

6.  Proof of reproductive integrity - the capability of a
single cell to grow into a large colony, visible to the
naked eye
 A surviving cell that has retained its reproductive
integrity and is able to proliferate indefinitely is said to
be clonogenic
6

7. Estimating Survival
 In order to determine the surviving fraction, we
must know the plating efficiency
 PE is the percentage of cells (in control batch) that
grow into colonies
 in other words, those cells that survive the plating
process
7

8. Derivation of Survival Curves
Always will have a control
 Cells have been taken batch to determine PE.
from stock culture and
placed in seed dishes
 Then irradiated (0 Gy
to 6 Gy)and allowed to
grow into colonies for
1-2 weeks
 Colonies have been
counted for survival
data
8

9. Surviving Fraction
 Equal to the fraction of cells that plate
successfully and survive irradiation (without
losing their reproductive integrity) to grow
into colonies
Colonies counted
Surviving fraction
cells seeded PE/100
9

10. 72 colonies
SF(2) = = 0.2
400 seeded x 0.9 plated
1
0.1
Surviving
Fraction 0.01
0.001
2 4 6
Dose (Gy)
10

11.  As Ionizations produced within cells by irradiation are
distributed randomly.
 So consequently, cell death follows random probability
statistics, the probability of survival decreasing geometrically
with dose.

12. Quantization of cell killing
 A dose of radiation that
introduces an average of one
lethal event per cell leaves
37% still viable is called D0
dose.
 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.  A straight line results when cell
survival (from a series of equal dose
fractions) is plotted on a logarithmic
scale as a function of dose on linear
scale.
 The slope of such a semi-logarithmic
dose curve could be described by the
D0, the dose to reduce survival to 37%,
D50, the dose to reduce survival to
50%, the D10, the dose to reduce
survival to 10%.
 D0 usually lies between 1 and 2 Gy
 D10= 2.3 x D0

14. Survival Curve Features
 Simple to describe qualitatively
 Difficulty lies in explaining underlying biophysical
events
 Many models have been proposed
 Steepness of curve represent the radio-
sensitiveness.
14

15. Survival Curve Shape
 general shapes of survival
curves for mammalian cells
exposed to radiation
 Initial portion has a shoulder
and terminal portion become
straight line.
 In low dose region ,some
dose of radiation goes
waste.
 Terminal portion follow
exponential relationship
means same dose increment
result into equal reduction in
surviving fraction.

16. Mammalian Cell Survival Curve
 Shoulder Region
 Shows accumulation of SUB-
LETHAL DAMAGE.
 The larger the shoulder
region, the more dose will
initally be needed to kill the
same proportion of cells.
 Beyond the shoulder region
 The D0 dose, or the inverse of
the slop of the curve, indicates
the relative radiosensitivity.
The smaller the D0 dose, the
greater the radiosensitivity.

17. Two General Survival Models
 Linear-quadratic model
 “dual radiation action”
 first component - cell killing is proportional to dose
 second component - cell killing is proportional to
dose squared
 Multi-target model
 based on probability of hitting the “target”
 widely used for many years; still has merit
17

18. L-Q Model

22. Linear Quadratic Model
2
 S= e-( D + D )
 where:
 S represents the fraction of cells surviving
 D represents dose
 and are constants that characterize the slopes of the
two l portions of the semi-log survival curve
 biological endpoint is cell death
22

23. Linear Quadratic Model
 Linear and quadratic contributions to cell
killing are equal when the dose is equal to the
ratio of to
 D = / or
 D = D2
 component is representative of damage caused
by a single event (hit, double-strand break,
“initiation / promotion” etc.)
 component is representative of damage caused
by multiple events (hit/hit, 2 strand breaks, initiation
then promotion, etc.)
23

24. and Determination
100
D
Survival
10-1
D2
10-2
0 3 6 9 12
Dose, Gy
24

25. Multi-target Model

28. Multi-target Model
 Quantified in terms of:
 measure of initial slope due to single-event killing,
D1
 measure of final slope due to multiple-event killing,
D0
 width of the shoulder, Dq or n
28

29. D1 and D0 are
1. reciprocals of the initial and
final slopes
2. the doses required to reduce
the fraction of surviving cells
by 37%
3. the dose required to
deliver, on average, one
inactivating event per cell
4. D1,reduces survivivig fraction
to 0.37
5. D0, from 0.1 to 0.037, or from
0.01 to 0.0037 ,and so on.

30. Multi-target Model
 Shoulder-width measures:
 the quasi-threshold dose (Dq)
 the dose at which the extrapolated line
from the straight portion of the survival
curve (final slope) crosses the axis at
100% survival
 the extrapolation number (n)
 This value is obtained by extrapolating
the exponential portion of the curve to
the vertical line.
 “broad shoulder” results in larger value
of n
 “narrow shoulder” results in small value
of n
 n = exp[Dq / D0]
30

31. Multi-Target Model
n
n or Dq represents the size
or width of the shoulder
Dq
100
10-1 Initial slope measure, D1,
Survival
due to single-event killing
10-2
Final slope measure, D0,
10-3 due to multiple-event killing
10-4
0 3 6 9 12
Dose, Gy
31

32. Linear –quadratic model Multi-target model
 Neither the L-Q not the M-T model has any established
biological basis.
 At high doses the LQ model predicts a survival curve
that bends continuosly, whereas the M-T model become
linear.
 At low doses the LQ model describes a curve that bends
more than a M-T curve.

33. Factors affecting cell survival curve
1. LET
2. Fractionation
3. Dose rate effect
4. Intrinsic radiosesitivity
5. Cell age
6. Oxygen presence

34. LET
 Low-LET radiations:
 low dose region
 shoulder region appears
 high dose region
 survival curve becomes linear and surviving
fraction to an exponential function of dose
 surviving fraction is a dual exponential
S = e-( D+ D2)
34

35.  High-LET radiations:
 survival curve is linear
 surviving fraction is a pure exponential function of dose
S = e-( D)
35

36. Survival Curves and LET
 Increasing LET:
 increases the steepness
of the survival curve
 results in a more linear
curve
 shoulder disappears
due to increase of killing
by single-events
36

37. Fractionation
 If the dose is delivered as
equal fractions with sufficient Dq
time ,repair of sub-lethal n = exp[Dq / D0]
damage ocurs 104
103
 Elkind‟ s Recovery takes place 102
between radiation exposure , Dq
101
cell act as fresh target.
100
10-1 D0
 Elkind & Sutton showed that
when two exposure were given 10-2 5 10 15 25
20
few hours apart ,the shoulder Dose (Gy)
reappeared.

38. The Effective Survival Curve: Fractionation
 If the dose is delivered as Showing ~28 Gy
equal fractions with in 14 fractions.
sufficient time between for
repair of the sub-lethal (non-
killing) damage, the shoulder
of the survival curve is
repeated many times.
 The effective survival curve
becomes a composite of all
the shoulder repetitions.
 Dose required to produce the
same reduction in surviving
fraction increases.
 D0 is 3 Gy
38

39. Dose-rate effect
Dose rate determines biological impact
 reduction in dose rate causes reduced cell killing, due to
repair of SLD
 reduction in dose rate generally reduces survival-curve slope
(D0 increases)
 inverse dose-rate effect occurs in some cell lines at „optimal‟
dose rate due to accumulation of cells in G2

40. Dose-Rate Effect in CHO Cells
Broad shoulder to survival
curve
Dose rate effect is more dramatic in CHO than in HeLa Cells

41. Composite survival curves for 40 Human Cell Lines
Less variation Low Dose Rate: Survival
Curves show greater variation,
Greater range in repair times

42. Evidence for Dose Rate Effect in vivo: Crypt Cells
0.54 rad/min
0.92
Crypt cells: rapid dividing
Dramatic dose rate effect
Exposure time is longer than
4.5 Cell cycle (repopulation)
36
274

43. Intrinsic radiosensitivity
Mammalian cells are significantly more radio-sensitive
than microorganisms:
 Due to the differences in DNA content
 represents bigger target for radiation damage
 Sterilizing radiation dose for bacteria is 20,000 Gy

45. Age response:Cell Cycle
Late S—least sens.
M>G2>G1>early S>late S for sensitivity
Difference caused by cell cycle are similar to difference caused by Oxygen effect

46.  Cells are most sensitive to radiation at or close to M
 Cells are most resistant to radiation in late S
 For prolonged G1  a resistant period is evident early G1
followed be a sensitive period in late G1
 Cells are usually sensitive to radiation in G2 (almost as
sensitive as in M)

47. The Oxygen Effect
 Oxygen modifies the biological effects of ionizing radiation
 02 effect does not require that 02 be present during radiation –
just added within 5 msec after generation of free radical
 OER – oxygen enhancement ratio: ratio of hypoxic: aerated
doses needed to achieve the same biological effect
 X-Rays/γ-Rays  at high doses is 2.5-3.5
OER is ~2.5 at lower doses
 OER is absent for high LET radiations like alpha-particles and
is intermediate for fast neutron.
 OER is lower for types of radiation predisposed to killing cells
by single-hit mechanisms

48. Cells are more sensitive to
Radiation in the presence of
Oxygen than in its absence
High dose region of survival
curve
Low dose region of survival
curve

49. Summary
 A cell survival curve is the relationship between the fraction of
cells retaining their reproductive integrity and absorbed dose.
 Conventionally, surviving fraction on a logarithmic scale is
plotted on the Y-axis, the dose is on the X-axis . The shape of
the survival curve is important.
 The cell-survival curve for densely ionizing radiations (α-
particles and low-energy neutrons) is a straight line on a log-
linear plot, that is survival is an exponential function of dose.
 The cell-survival curve for sparsely ionizing radiations (X-
rays, gamma-rays has an initial slope, followed by a shoulder
after which it tends to straighten again at higher doses.

50. Summary
 At low doses most cell killing results from “α-type” (single-hit, non-
repairable) injury, but that as the dose increases, the“β –type”
(multi-hit, repairable) injury becomes predominant, increasing as
the square of the dose.
 Survival data are fitted by many models. Some of them are:
multitarget hypothesis, linear-quadratic hypothesis.
 The survival curve for a multifraction regimen is also an
exponential function of dose.
 The D10, the dose resulting in one decade of cell killing, is related
to the Do by the expression D10 = 2.3 x Do
 Cell survival also depends on the dose, dose rate and the cell
type

51. Thank You