The document discusses cell survival curves, which describe the relationship between radiation dose and the proportion of cells that survive. It defines key terms like clonogenic cells and explains the components of in vitro survival curves. It describes exponential and shoulder survival curves based on single-hit and multi-target theories. The mechanisms of cell killing like mitotic death and apoptosis are covered. Factors influencing radiosensitivity like dose rate, oxygen level and genetic mutations are summarized. Comparisons are made between survival curves of different mammalian cell types and microorganisms.

1. CELL SURVIVAL CURVE
PRESENTER :DR.VIJAY.P.RATURI
MODERATOR :- MR.TEERTHRAJ SIR
J.R 2 ,KGMU lucknow

2. DEFINITION
• Cell survival curve describes the relationship
between the radiation dose and the
proportion of cells that survive.
– Cell “Death” : loss of reproductive integrity
• Clonogenic : Survivor able to proliferate
indefinitely to produce a large clone or colony.

3. • Mitotic death : Death while attempting to
divide(dominant following irradiation)
• Apoptosis : Programmed cell death
• In general, a dose of 100 Gy is necessary to destroy cell
function in nonproliferating systems.
• By contrast, the mean lethal dose for loss of proliferative
capacity is usually less than 2 Gy.

4. The In Vitro Survival Curve
• Plating efficiency
– PE = x 100
• Surviving fraction
– SF =
• 100 cells are seeded into an
unirradiated culture, and 10
colonies are formed, then the
PE is 10/100.
• If there are 5 colonies after
a 450 cGy dose of radiation,
the SF is 5/[100 × 10/100] =
1/2. Thus, the SF of 450 cGy
is 50%.
NO IRRIDATION
IRRADIATED

5. The In Vitro Survival Curve

6.  The number of cells in cell lines within cell cultures can
increase in one of two way:
Arithmetically or exponentially (geometrically).
 The number of cells increases linearly (by a constant
number) with each generation in an arithmetic.
 In exponential , the number of cells doubles with each
generation, and so exponential growth is faster than
arithmetic growth

7. If the SF is calculated for various doses, then it can be
presented as a cell–dose plot. Combining the points on the
plot leads to a cell survival curve.
SIGMOID CURVE SEMILOGARITHMIC CURVE

8. EXPONENTIAL SURVIVAL CURVE
Survival curves resulting from the single target–single hit
hypothesis of target theory .They show that cell death dueto
irradiation occurs randomly.
At certain doses with one unit increase, both same number
of cell deaths and same proportion of cell death occur.

9.  D0 = dose that decreases the surviving fraction to 37%.
 This is the dose required to induce an average damage
per cell.
 A D0 dose always kills 63% of the cells in the region in
which it is applied, while 37% of the cells will survive.
 1/D0 = the slope of the survival curve.
 As the value of D0 decreases → 1/D0 increases → slope
→ radiosensitive cell.
 As the value of D0 increases → 1/D0 decreases → slope
→ radioresistant cell.

10. SHOULDERED SURVIVAL CURVES WITH ZERO INITIAL SLOPE
These survival curves are based on the multiple target–single hit
hypothesis of target theory
They are produced by the hypothesis of requiring multiple targets per
cell, and only one of these targets needs to be hit to kill the cell.

11.  Dq: half-threshold dose → the region of the survival
curve where the shoulder Starts
(indicates where the cells start to die exponentially)
(= quasi-threshold dose).
 n: extrapolation number (the number of D0 doses that
must be given before all of the cells have been killed).

12.  Dq → the width of the shoulder region.
 Dq = Do log n 2.7
 If n increases → Dq increases → a wide shouldered curve
is observed.
 If n decreases → Dq decreases → a narrow shouldered
curve is observed.
 If Dq is wide and D0 is narrow, the cell is radioresistant.
 The D0 and Dq values for the tumor should be smaller
than those of normal tissue to achieve clinical success.

13. SHOULDERED SURVIVAL CURVE WITH NON ZERO INITIAL
SLOPE

14. COMPONENTS OF SHOULDERED SURVIVAL
CURVES WITH NONZERO INITIAL SLOPE
• Component corresponding to the single target–single hit
model (blue in the figure)
- This shows lethal damage.
- This shows the cells killed by the direct effect of the
radiation.
- This shows the effect of high-LET radiation.
• Component corresponding to the multiple target–single h
it model (red in the figure)
- This shows the accumulation of SLD.
- This shows the cells killed by the indirect effect of the
radiation.
- This shows the effect of low-LET radiation.

15. SHAPE OF THE SURVIVAL CURVE
• At “low doses” for sparsely ionizing(low LET) radiations,
such as x-rays, the survival curve starts out straight on the
log-linear plot with a finite initial slope.
– The surviving fraction is an exponential function of dose.
• At higher doses, the curve bends.
• At very high doses, the survival curve often tends to
straighten again.
• For densely ionizing (high-LET) radiations, such as α-
particles or low-energy neutrons, the cell survival curve is a
straight line from the origin.

16. THE SHAPE OF THE SURVIVAL
CURVE
A:The linear quadratic model. B:The multitarget model.
A. Good fit to experimental data for the
first few decades of survival.

17. MECHANISMS OF CELL KILLING
• The principal sensitive sites for radiation-induced cell
lethality are located in the nucleus as opposed to the
cytoplasm.
• The evidence implicating the chromosomes,
specifically the DNA, as the primary target for
radiation-induced lethality may be summarized as
follows:

18.  Cells are killed by radioactive tritiated thymidine
incorporated into the DNA. The radiation dose results
from short-range α-particles and is therefore very
localized.
 Certain structural analogues of thymidine, particularly
the halogenated pyrimidines, are incorporated
selectively into DNA in place of thymidine if substituted
in cell culture growth medium. This substitution
dramatically increases the radiosensitivity of the
mammalian cells.

19.  Factors that modify cell lethality, such as variation in the
type of radiation, oxygen concentration, and dose
rate, also affect the production of chromosome damage
in a fashion qualitatively and quantitatively similar.
 The radiosensitivity of a wide range of plants has been
correlated with the mean interphase chromosome
volume, which is defined as the ratio of nuclear volume
to chromosome number. The larger the mean
chromosome volume, the greater the radiosensitivity

20. BYSTANDER EFFECT
• Defined as the induction of biologic effects in cells that
are not directly traversed by a charged particle, but are
in proximity to cells that are.
• Nagasawa and Little, 1992
– Low dose of α-particles, a larger proportion than estimated of cells
showed an biologic change.

21. • The use of sophisticated single-particle microbeams,
which make it possible to deliver a known number of
particles through the nucleus of specific cells.
• The bystander effect has also been shown for protons
and soft x-rays.
• The effect is most pronounced when the bystander cells
are in gap-junction communication with the irradiated
cells.
• For example, up to 30% of bystander cells can be killed
in this situation.

22. • The effect being due, presumably, to cytotoxic molecules
released into the medium.
• The existence of the bystander effect indicates that the
target for radiation damage is larger than the nucleus
and, indeed, larger than the cell itself.
• Its importance is primarily at low doses, where not all
cells are “hit”.

23. APOPTOTIC DEATH
• Apoptosis in Greek word : “falling off”
• Programmed cell death
• Occurs in normal tissues, also can be induced in some
normal tissues and in some tumors by radiation.
• Double-strand breaks(DSBs) occur in the linker regions
between nucleosomes, producing DNA fragments that
are multiples of approximately 185 base pairs. 
Laddering in gels.

24. • Apoptosis is highly cell-type dependent.
• Hemopoietic and lymphoid cells are particularly
prone to rapid radiation-induced cell death by the
apoptotic pathway.
• Apoptosis after radiation seems commonly to be a
p53-dependent process; Bcl-2 is a suppressor or
apoptosis.

25. MITOTIC DEATH
The most common form of cell death from radiation is mitotic death.
– Cells die attempting to divide because of damaged chromosomes.
– The log of the surviving fraction
– The average number of putative “lethal” aberrations per
cell(asymmetric exchange-type aberrations such as rings and
dicentrics)
– Data such as these provide strong circumstantial evidence to
support the notion that asymmetric exchange-type aberrations
represent the principle mechanism for radiation-induced
mitotic death in mammalian cells.

26. RELATION BETWEEN CHROMOSOMAL ABERRATION
& SURVIVAL CURVE

28. SURVIVAL CURVES FOR VARIOUS
MAMMALIAN CELLS IN CULTURE
• First in vitro survival curve for mammlian cells irradiated with x-
rays.
• All mammalian cells studied to date, normal or malignant,
regardless of their species of origin, exhibit x-ray survival curves
similar to those in figure.
Initial shoulder

29. • The D0 of the x-ray survival curves for most
cells cultured in vitro falls in the range of 1 to 2
Gy.
• The exceptions are cells from patients with
cancer-prone syndromes such as Ataxia-
telangiectasia(AT); these cells are much more
sensitive to ionizing radiations, with a D0 for x-
rays of about 0.5 Gy.

30. • In more recent years, extensive studies have been made
of the radiosensitivity of cells of human origin, both
normal and malignant, grown and irradiated in culture.
– In general, cells from a given normal tissue show a
narrow range of radiosensitivity if many hundreds of
people are studied.
– By contrast, cells from human tumors show a very
broad range of D0 values.

31. SURVIVAL CURVES FOR VARIOUS
MAMMALIAN CELLS IN CULTURE

32. SURVIVAL CURVE SHAPE AND
MECHANISMS OF CELL DEATH
Radioresistant
Large dose-
rate effect
Radiosensitive
No dose-rate effect
Laddering
(after 10 Gy)

33. • Characteristic laddering is indicative of programmed
cell death or apoptosis during which the DNA breaks
up into discrete lengths as previously described.
• Comparing Fig.A and B, it is evident that there is a close
and impressive correlation between radiosensitivity and
the importance of apoptosis.
• Increased “laddering” = Increased radiosensitivity

34. • Mitotic death results (principally) from exchange-type
chromosomal aberrations; the associated cell survival
curve, therefore, is curved in a log-linear plot, with a
broad initial shoulder.

35. GENETIC CONTROL OF
RADIOSENSITIVITY
Inherited Human Syndromes associated with sensitivity
to X-rays
• Ataxia-telangiectasia(AT)
• Basal cell nevoid syndrome
• Cockayne syndrome
• Down syndrome
• Fanconi’s anaemia
• Usher syndrome
• Nijmegen breakage syndrome

36. EFFECTIVE SURVIVAL CURVE FOR
A MULTIFRACTION REGIMEN
• The effective survival curve is an exponential
function of dose whether the single-dose survival
curve has a constant terminal slope or is continuously
bending.
• The D0 of the effective survival curve: the dose
required to reduce the fraction of cells surviving to
37%(close to 3 Gy for cells of human origin).
• D10(dose required to kill 90% of the population)
– D10 = 2.3 × D0

37. EFFECTIVE SURVIVAL CURVE FOR A MULTIFRACTION
REGIMEN

38. THE RADIOSENSITIVITY OF MAMMALIAN
CELLS COMPARED WITH MICROORGANISMS
• It is evident that mammalian cells are
exquisitely radiosensitive compared with
microorganisms.
• The most resistant is Micrococcus
radiodurans, which shows no significant cell
killing even after a dose of 1,000 Gy.

39. A, mammalian cells; B, E. coli; C, E. coli B/r; D, yeast; E, phage staph E; F,
B. megatherium; G, potato virus; H, Micrococcus radiodurans.

40. THE RADIOSENSITIVITY OF MAMMALIAN
CELLS COMPARED WITH MICROORGANISMS
 The dominant factor that accounts for this huge range
of radiosensitivities is the DNA content. Mammalian
cells are sensitive because they have a large DNA
content, which represent a large target for radiation
damage.
 E. coli and E. coli B/r have the same DNA content but
differ in radiosensitivity because B/r has a mutant and
more efficient DNA repair system.

41. THANK YOU