Study of effects of ionizing radiation on living systems

1. RADIATION
BIOLOGY

2. Radiation biology
Study of effects of ionizing radiation
on living systems

3. HISTORY
✓ First recorded experiment in radiobiology -
performed by Becquerel when he intentionally left
a radium container in his vest pocket.
2 weeks later – erythema - ulceration - several
weeks to heal
✓ Pierre Curie – 1901 – repeated the experiment –
radium burn on his forearm

4. Types of Radiation
1] Photon beam (X-ray, Gama –ray)
2] Electron beam
3] Particle radiation (Neutron )

5. X ray photons
• X rays are produced when electrons decelerate.
• Poor penetration.
• Effective depth – 1cm.
Gamma rays
• Gamma rays are emmited by radioactive isotopes from excited
nucleus itself.
• Oldest – Radium., Caesium & cobalt.
• Cobalt is more widely used : Deep penetration. Low cost and
maintainance.
• Disadvantages: Not precise
Gamma rays are more harm to human body than the X- rays.
Gamma rays have shorter wavelengths than the X-rays.

6. Electron beam
• The subject is directly bombarded with electrons.
• Property :
• The depth of penetration of electron beam depends on energy
level of electron.
• Hence the beam has a depth limit.
• Uses :
a) In head and neck Ca, where spinal cord is to be spared.
b) Primary skin tumour of pinna & cartilage of nose.
c) Anterior placed tumour of frontal & ethmoidal sinus

7. Proton beam
• Heavy positive charged particles.
• Advantages:
a) Depth dose distribution. Small entry & exit point.
b) Does not depend on tissue O2.
• Application:
a) Chondosarcoma of skull base.
b) Chondroma of clivus.
Neutrons
• Mechanism : Heavy energy machines produce heavy uncharged
particles.
• Uses : Parotid gland tumour.

8. Ionizing Radiation
Type of radiation that produces the ejection of an orbital electron
from an atom or molecule and results in the formation of an ion pair.
 Particulate (alphas and betas)
 Waves (gamma and X- rays)
Non-Ionizing Radiation
Deposits insufficient energy and fails to eject electron, but can
induce excitation or molecular vibration and heat.
Examples include UV, microwaves, infrared, visible, radar, radio
waves, lasers, TV
Types of radiation

9. Ionizing Radiation Types

10. Radon
X-Rays
Consumer
Products
Nuclear
Power
Radioactive
Waste
Nuclear
Medicine
Solar Radiation
Cosmic Rays
Terrestrial
Radiation
Food & Drink


Sources of ionizing radiation exposures
Bertha Röntgen's hand,
Jan 18th1986
Potassium-40 (40K)
238U, 226Ra,
210Pb, 210Bi,
210Po

11. Measure of
Amount of
radioactive
material
Ionization in air
Absorbed energy
per mass
Absorbed dose
weighted by type
of radiation
Radiation Units
For most types of radiation 1 R  1 rad  1 rem
Quantity
Activity
Exposure
Absorbed
Dose
Dose
Equivalent
Old Unit
curie (Ci)
roentgen (R)
rad
rem
SI Unit
Becquerel
Coulomb/kg
Gray (J/kg)
Sievert

12. 14
Annual limits of radiation exposure
Occupational workers 20 mSv
General Public 1 mSv
LIMIT ON LIFE TIME DOSE 1 Sv
A simple chest X-ray gives 0.2 mSv !!!
These limits are in addition to exposures from
natural and medical sources

13. Incident X-ray photon
Photon absorption via Compton effect
Formation of free radicals
Radical induced DNA damage
Biological effect (mutation, cancer, etc.)
Sequence of Radiation Effects

14. • Direct Action: When the atoms of the target
itself are ionized by the radiation. It is the
dominant process for high LET radiation.
• Indirect Action: When the radiation
interacts with other cellular molecules (e.g.
water) to produce free radicals that migrate to
and damage the target.
Interaction of radiation with bio-
molecules
For every one DNA
molecule, there are
1.2X107 water
molecules

15. H2O
H2O+
e-
H+
H2O2
OHo
HO2
OH
-
Ho
H2
WATER
Radiation Induced decomposition
of water within a cell
Incoming
Radiation
Production of free radicals within the cell can result in indirect effects
Most abundant molecule within humans: Water (70 -80%)

16. - Sparsely ionizing radiations such as x-rays or gamma- rays
induce damage mainly through indirect effect while densely
ionizing radiations such as neutrons and alpha particles cause
damage primarily through direct effect
- Indirect effects can be altered with use of radioprotectors
during radiotherapy while direct can not be altered

17. Mechanisms of radiation damage
Injury to cells/ tissues results from the transfer of energy to atoms and
molecules.
Atoms and molecules may be exited or ionized depending upon the energy
and these excitation and ionizations can:
ATOMS
MOLECULES
CELLS
TISSUES
ORGANS
THE WHOLE
BODY
• Produce free radicals
• Break chemical bonds
• Produce new chemical bonds and cross linkage
between macromolecules
• Damage molecules that regulate vital
processes (DNA, RNA & Protein synthesis)

18. Biochemical reactions with ionizing radiation
DNA is primary target for cell damage
from ionizing radiation

19. Types of Radiation induced DNA damage

20. The most important types of radiation
induced lesions in DNA
Single-strand breaks
500-1000 per 1 Gy
Double strand breaks
40-50 per 1 Gy
Base damage: 1000-2000 per 1 Gy

21. Genomic
Instability
Sometimes DNA
damage produces
later changes which
may contribute to
cancer.
Gene
Expression
A gene may
respond to the
radiation by
changing its signal
to produce
protein. This may
be protective or
damaging.
Gene Mutation
Sometimes a
specific gene is
changed so that it is
unable to make its
corresponding
protein properly
Chromosome
Aberrations
Sometimes the
damage effects the
entire chromosome,
causing it to break or
recombine in an
abnormal way.
Sometimes parts of
two different
chromosomes may be
combined
Cell Killing
Damaged DNA
may trigger
apoptosis, or
programmed cell
death.
If only a few cells
are affected, this
prevents
reproduction of
damaged DNA
and protects the
tissue.
Effects of DNA Damage

22. Options for irradiated cell
Three
possibilities
Possible cancers,
reproductive
failure or genetic
effects
At the organism level
Irradiated cell may
Repair
Die
Cell survives but
loses some
function
Cell destroyed by
the immune
system
Cell survives but is
dysfunctional
No effect
No effect
No effect
Cell Few cells killed : Organism will
heal & survives
More cells killed: Organism may
be survived with prolonged
symptoms
More cells killed:
Organism will perish
Non repair /
mis- repair

23. The Cell Cycle
Sensitivity
Cells most sensitive – Mitotic and close to
mitosis
Resistance- greatest in latter part of S
For long G1’s, there is an early resistance
period followed by sensitive one at the
end of G1

24. CELL CYCLE CHECK POINTS
progression of
through cell
-To prevent
defective DNA
cycle
- two check points – G1S and
G2M

25. CELL CYCLE CHECK POINTS
CHECK POINT
G 1 - S
POINT
C H E C K -Before a cell makes the final commitment to
replicate, the G1/S checkpoint checks for DNA
damage;
-if damage is present, the DNA-repair machinery
and mechanisms that arrest the cell cycle
-The delay in cell cycle progression provides the
time needed for DNA repair;
-if the damage is not repairable, apoptotic pathways
are activated to kill the cell.

26. G2-M
POINT
CHECK -The G2/M checkpoint monitors the completion
of DNA replication and checks whether the cell
can safely initiate mitosis and separate sister
chromatids.
-This checkpoint is particularly important in
cells exposed to ionizing radiation.
-Cells damaged by ionizing radiation activate
the G2/M checkpoint and arrest in G2; defects in
this checkpoint give rise to chromosomal
abnormalities

27. Cellular radiosensitivity
“The radiosensitivity of a population of cells is directly proportional
to their reproductive activity and inversely proportional to their
degree of differentiation.”
Cells tend to be radiosensitive if they have three properties:
• high division rate
(the time between divisions)
• long dividing future
(immature cells in early cellular life)
• unspecialized
(cells which have a widely diverse future)

28. Lymphocytes
Spermatogonia
Hematopoietic
Intestinal Epithelium
Skin
Nerve Cells
Muscle Tissue
Bone
 Cells most affected:
 Rapidly dividing cells:
(small intestines, bone
marrow, fetus)
 Cells least affected:
 Slowly dividing cells: (brain,
nerves)
Relative Sensitivity of Cells and
Tissue Types

29. BIOLOGIC EFFECTS OF
RADIATION

30. 1. Somatic & Genetic Effects
2. Stochastic & Deterministic Effects
3. Short - term & Long - term Effects
Classification of biologic effects of
radiation

31. 1. Somatic vs. Genetic Effects
Definition Effects Examples
Somatic Harm to person
exposed to the
radiation
Acute — large dose of
radiation absorbed in a
short time - Leukemia
Chronic — small amt of
radiations absorbed
repeatedly over long
time (latent period ≥20
yrs)
Genetic Harm to person
in future
generation due
to ancestor’s
exposure
Radiation to reproductive
organs damage the DNA
of sperms or eggs
No threshold dose
- Congenital
abnormality

32. 2. Stochastic vs. Deterministic Effects

33. 2. Stochastic vs. Deterministic Effects
Traits Effects Example
Stochastic “all or none”
response; any
dose can cause
No threshold dose
Every exposure to
ionizing radiation
carries with it the
possibility of
inducing a
stochastic effect.
 Incidence of
leukaemia &
certain tumours
Deterministi
c
(non
stochastic)
“certainty effects”;
severity of effect
 with 
dose;
Threshold dose
exists below which
there will be no
effect.
Erythema: all will
experience once
minimum dose is
given; gets
worse with
dose

34. 3. Short - term vs. Long - term Effects
Traits Example
Short term
effects
Determined primarily
by sensitivity of its
parenchymal cells
Bone marrow, oral mucosa
- high radiosensitive -
mitosis-linked cell death.
Muscle - low radiosensitive
Long term
effects
Depend primarily on
extent of damage to
fine vasculature
Progressive
fibroatrophy of the
irradiated tissue

35. DETERMINISTIC
EFFECTS OF WHOLE
BODY RADIATION

36. Acute Radiation Syndrome
Dose Levels and Lethal Effects:
>50 Gy: Cerebrovascular syndrome (system collapse)
> 5 Gy: Gastrointestinal death (crypt cells destroyed)
3-5 Gy: Hematopoietic death (stem cell destruction)

37. SPECTRUM OF RADIATION INDUCED SYNDROME
Dose
(Gy)
Name of the radiation
syndrome
Symptoms & consequences
1 - 2 Nausea, vomiting, diarrhea
(NVD) syndrome
Nausea, vomiting, diarrhea, anorexia, giddiness, and
loss of appetite
2 - 6 Haematopoetic syndrome Bone marrow, spleen and thymus gets affected.
Approximate time of death varies between 10- 30 days.
8 - 15 Gastrointestinal (GI)
syndrome
Damage to intestinal crypt takes place resulting in loss
of absorption of nutrient, dehydration, loss of weight,
severe electrolyte imbalance and low blood pressure.
Death occurs usually within 3 – 5 days.
> 25 Central Nervous System
(CNS) syndrome
Irritability, hyper excitability response, epileptic type fits
and coma. Symptoms are irreversible. Death usually
occurs within 48 hrs.

38. LATE DETERMINISTIC
EFFECTS OF WHOLE
BODY RADIATION

39. 1. Skin –
i. Early or acute signs
▪ Increased susceptibility to chapping
▪ Intolerance to surgical scrub
▪ Blunting & leveling of finger ridges
▪ Brittleness & ridging of finger nails
ii. Late or chronic signs
▪ Loss of hair
▪ Dryness & atrophy of skin – sweat glands destroyed
▪ Pigmentations, telangiectasis, keratosis
▪ Indolent type of ulcerations
▪ Possibility of malignant changes in tissue
General effects of radiation

40. 2. Hematopoietic injury – leucopenia, which in some
cases may progress to leukemia, anemia, lymphopenia
3. Eyes
▪
▪
▪
▪
Epilation of eyelashes
Inflammation, fibrosis, & decreased flexibility of eye
lids
Dryness
Ulceration & cataract

41. 4. Ears
▪ edema of mucosa & collection of fluid causing
obstruction of Eustachian tube – Radiation Otitis
Media
▪ Deafness due to rupture of ear drum
5. Testicles
▪
▪
Suppression of germinal activity
Alteration in fertility

42. 6. Oral mucous membrane
▪
▪
▪
▪
▪
▪
Reddening & inflammation (mucositis)
Sloughing & pseudomembrane formation
Secondary infection
Obliteration of vasculature
Fibrosis of underlying C/T
Oral ulcerations

43. 8. Taste buds – alterations in taste
9. Salivary glands – parenchymal component
affected – fibrosis, adiposis, loss of fine
vasculature.
▪
▪
▪
▪
▪
▪
Xerostomia
Composition of saliva altered
Increased Na, Cl, Ca, Mg & proteins
Loses lubricating property
PH decreases – decalcification of enamel
Compensatory gland hypertrophy

44. 10.Teeth
▪
▪
▪
▪
Adult teeth are resistant to radiation
Prior to calcification tooth buds destroy
During calcification defect in cellular
differentiation - ↓sed vascularity, cellularity of
pulp & prone to pulpitis
Radiation caries

45. 11.Bone
▪
▪
▪
▪ Loss of vasculature & hematopoietic
elements
Marrow replaced by fatty marrow & fibrous
connective tissue
Lack of osteoblasts & osteoblastic activity
Osteoradionecrosis following radiation

46. FACTORS MODIFYING
EFFECT OF RADIATION

47. ▪ The severity of deterministic damage seen in irradiated
tissues or organs depends on the amount of radiation
received
▪ All individuals receiving doses above the threshold level
show damage in proportion to the dose
Dose

48. Dose Rate
▪ High dose rate causes more
damage than exposure to the
same total dose given at a
lower dose rate
▪ When organisms are exposed
at lower dose rates, a greater
opportunity exists for repair of
damage, thereby resulting in
less net damage

49. Oxygen
▪ Presence of oxygen alters
how cells process free
radicals
▪ In presence of oxygen free
radicals lead to irreparable
lethal changes

50. OXYGEN ENHANCEMENT RATIO:
• magnitude of the effect of oxygen on radiosensitivity
• Ratio of hypoxic to aerated cells to achieve the same
biologic effect.
• Sparsely ionizing radiation: 2.5-3

51. maximum permissible dose
• Greatest dose of radiation which is not expected tocausedetectable
bodily injury to people at any time during their lifetime.
• Theamount of ionizing radiation aperson may be exposed
to supposedly without beingharmed
• Thelimits of ionizing radiation set for radiation workers and the
general public by the International Commission on Radiological
Protection.
• For radiology workers this limit for the whole body is 50 mSv.

52. Maximum Accumulated Dose
• Occupationally exposed workers must not exceed an accumulated
lifetime radiation dose.
• This is referred to as the maximum accumulated dose (MAD).
• MAD is determined by a formula based on the worker’s age.
• To determine the MAD for an occupationally exposed person, the
following formula is used:
• MAD=(N-18)x5 rems/ year
• MAD=(N-18)x0.05 Sv/ year
• where N refers to the person’s age in years. (Note that the number
18 refers to the minimum required age of a person who works with
radiation.)

53. Median lethal dose
• The amount of ionizing radiation that will kill 50 percent of
a population in a specified time

54. FRACTIONATION

55. ▪ Provides greater tumour destruction than is possible
with a large single dose
▪ Increased cellular repair of normal tissue
▪ Increases the mean oxygen tension in an irradiated
tumour
Rationale of Fractionation in Radiotherapy

56. ▪ Repair of sublethal damage in normal cells
▪ Reoxygenation of tumour bed
▪ Redistribution of cells within the cell cycle
▪ Repopulation of normal cells
4 R’s of Radiobiology

57. Biologic Basis of Dose Fractionation
1. REPAIR OF SUBLETHAL DAMAGE:
- Reflects the ability of cells to recover from damage that does
not cause lethality
- Repair capacity is greater in late responding tissues
- decreasing the dose per fraction will result in greater sparing
of late responding tissues

58. • Studies have shown that although repair can be an ongoing
process, the vast majority of the repair is finished by 6 hours
post irradiation.
• Once repair is complete the remaining cell population will
respond to subsequent dose of radiation as though the
original irradiation had not occurred

59. 2. REOXYGENATION :
- Hypoxic cells are radioresistant
- Preferential elimination of more radiosensitive oxygenated
cells increases oxygen availability to surviving hypoxic cells
- increase tissue sensitivity to radiation

60. • The absorption of radiation
leads to the production of free
radicals .
• The free radicals have life span
of about 10-5 seconds and they
break chemical bonds , produce
chemical changes and initiate
the chain of events that results
in final expression of biologic
damage.
• Oxygen fixes the damage by
free radical to DNA.
Mechanism of Action

61. Mechanism of Reoxygenation
1. Reduction in ratio of total tumor cells to the surface area of
blood vessels.
for example if there are 10 capillaries
supplying to 100 tumor cells the ratio of tumor cells to
capillary is 10 which mean one
capillary supplying 10 cells.
After RT, 80 cells survived then ratio
becomes 8 so now one capillary
supplying to 8 cells

62. Mechanism of Reoxygenation
2. Distance of hypoxic cells from the blood vessels decreases
because of preferential killing and lyses of oxygenated
cells.
RT

63. Mechanism of Reoxygenation
3. Increased radius of oxygen diffusion as total consumption of
oxygen is decreased.
RT

64. 4. As oxygenated cells are depopulated, there is reduction in intra
tumoral pressure permitting re opening of the some compressed blood
vessels.
RT
Mechanism of Reoxygenation

65. Clinical Significance of Reoxygenation
• After RT all oxygenated cells are killed and only hypoxic cells alive.
• So initial 20% becomes 100%.
• Within 6 hours % of hypoxic cells comes down to initial 20%.
• Human tumor re-oxygenate between fractions and it forms one of the
basis of fractionated radiotherapy.
• Timing of re-oxygenation vary from one tumor to other.
• Exact timing of re-oxygenation in human tumor is not known.
• If we know the exact timing of re-oxygenation of a particular tumor then
we can schedule the radiation fractionation accordingly.

66. 3. REDISTRIBUTION:
• During fractionation, after each fraction of RT, cells in sensitive
phase are killed and before next fraction, cells progress through cell
cycle and again come to sensitive phase.
• This process is known as Redistribution

67. Redistribution
Asynchronization:-
The dividing cells are
distributed throughout all
phases of cell cycle
Synchronization:-
If all the dividing cells occupy the
same phase of the cell cycle and then
all cells progress through various
phases of cell cycle simultaneously.
This can be achieved by treating the
cells with hydroxyurea.

68. When hydroxyurea is added, it kills all the cells in S-phase
and impose a block at the end of G1-phase.
Redistribution

69. Redistribution
So by making the cells in synchronization and knowing the time
when they pass through G2, M phase, which is the most sensitive
phase to radiation, we may achieve max kill by scheduling
fractionation accordingly.

70. 4. REPOPULATION :
• As a result of any injury to tissue which causes into depopulation of cells
eg. Physical injury the cells at the edge of the wound will start
multiplying faster in order to replace the dead tissue.
• Same thing happens in injury due to Radiation or Cytotoxic drugs.
• This phenomenon is called Repopulation.

71. Clinical Implications
• Side effects are reduced as normal tissue injury is healed by
accelerated repopulation.
• Effect on tumor is negative as more dose of RT is required to
compensate the accelerated repopulation.
• As the overall treatment time is increased, total dose to get the
same effect will also be increased.
• Once the treatment is started (either by RT or CT), the treatment
should be completed as early as possible.
• All forms of avoidable delay should be avoided.

72. Clinical Implications
• Since late reacting tissues do not show any significant
repopulation, prolonging overall treatment time has little sparing
effect on late reactions but a large sparing effect on early
reactions.
• In head and neck cancers the repopulation starts at the end of 4th
week, so any treatment schedule longer than 4 week require
extra dose to compensate the accelerated repopulation.

73. According to standard therapeutic regimen :
Dose of 200cGY is delivered ; 5 days a week
for 5-6 weeks with total dosage of 50-60 Gy

74. Altered Fractionation Schemes
1. Hyperfractionation
- Smaller fraction size ( 115-120cGy)
- Two to three times in a day
- total dosage is larger ( 74-80Gy)
- treatment duration is same
-Advantages :
- improve therapeutic ratio
- Redistribution of tumor cells into more radiosensitive phases due to
multiple fractions
- differential sparing of late responding normal tissue

75. 2. Hypofractionation :
- Larger fraction size ( 600-800cGy)
- Fractions delivered with gap of several days
- total dosage is lower (2100-3200cGy)
- treatment duration is short
Advantages :
- Evolved for the treatment of malignant melanoma ( conventional fractions
allow tumor cells to recover between fraction intervals , high dose per
fraction will overcome the reparative capacity of tumor cells
Altered Fractionation Schemes

76. 3. Accelerated Fractionation:
- Same fraction size ( 180-200cGy)
- Two to three times in a day
- total dosage is similar
- treatment duration is short
Advantages :
- reduce the opportunity for accelerated repopulation of cells
Altered Fractionation Schemes

77. 1. HALL, E. J., “Radiobiology for the radiologist”, Lippincott, Philadelphia,
Pennsylvania, U.S.A. (2000).
2. NIAS, A.W.,“An introduction to radiobiology”, Wiley, New York, New York,
U.S.A. (1998).
3. STEEL, GG., “Basic clinical radiobiology”, Arnold, London, United
Kingdom (2002).
4. Oral radiology, principles & interpretation: white & pharoah
5. Oral sequelae of head & neck radiotherapy: crit rev oral biol med 2003
6.Prevention & treatment of the consequences of head & neck radiotherapy:
crit rev oral biol med 2003
7.Protocol for the prevention and treatment of oral sequelae resulting from
head and neck radiation therapy: cancer 1992
References

78. THANK YOU