Radiobiology (also known as radiation biology, and uncommonly as actinobiology) is a field of clinical and basic medical sciences that involves the study of the action of ionizing radiation on living things, especially health effects of radiation. The action is very complex, involving physics, chemistry, and biology – Different types of ionizing radiation – Energy absorption at the atomic and molecular level leads to biological damage – Repair of damage in living organisms

1. Radiation Biology
Danish Hamid
3rd prof

2. CONTENTS
• Introduction
• Interaction of radiation with biological
molecules
• Radiation effects at cellular level
• Types of biologic effects of Radiation
• Radiosensitivity

3. INTRODUCTION
• Radiation biology is the study of the effects of
ionising radiation on living systems.
• It is also defined as the study of the biological
effect of radiation caused mainly by ionisation
of water molecules within cells which produce
highly reactive free radicals which, in turn,
damage macromolecules ,such as DNA.

4. Interaction of radiation with biological
molecules
Biological effect of radiation occurs through;
a) Direct action, or
b) Indirect action

5. Direct action
• Also called Target action theory
• X-ray radiation directly interacts and ionizes a
biologic macromolecule (DNA) and forms
unstable free radicals.
• These free radicals quickly re-form into stable
configurations by dissociation or cross-linking.

6. .
• X-ray radiation + RH = R.
+ H+ + e-
• Dissociation: R.
= X + Y .
• Cross-linking: R.
+ S.
= RS
• The altered biologic molecules differ
structurally and functionally from original
molecules, this will lead to biologic change.

7. Indirect action
• Also called Poison chemical theory
• X-ray radiation interacts and ionizes the
cellular water releasing highly reactive
radiolytic products, which in turn, react with
cellular DNA causing biologic change.
Fig: Radiolysis of water

8. .
• X-ray radiation + H2O = H.
+ OH.
• R – H + OH.
= R .
+ H2O
• R – H + H.
= R.
+ H2
In the presence of oxygen, hydroperoxyl and
hydrogen peroxide are formed – these are
strong oxidizing agents that contribute
significantly to indirect actions.
• H.
+ O2 = HO2
.
• HO2
.
+ HO2
.
= H2O2 + O2

10. Radiation effects at cellular level
 Nucleic acids
– Breakage of one or both strands of DNA
– DNA-DNA or DNA-proteins cross-linking
– change or loss of base
– disruption of hydrogen bonds between the DNA
strands
 Proteins
– Denaturation
– Inter and intra-molecular cross-linking
– Disruption of secondary and tertiary structure

11.  Chromosomal Aberrancy
–Chromosome aberration is observed in
irradiated cells at the time of mitosis when
DNA condenses to form chromosomes
–Type of damage depends on cell cycle at the
time of irradiation
• Radiation in G2 or mid and late S phases: If
radiation exposure occurs after DNA synthesis,
only one arm of the affected chromosome is
broken, and it is called chromatid aberration

12. • Radiation in G1 or early S phase: If radiation
exposure occurs before the DNA synthesis, then both
arms of the chromosome will be broken, and it is
called chromosome aberration

13. • Chromosome effect could include inability to pass
information, abnormal replication, cell death, etc.
Chromosome aberrations can occur due to incorrect
repairs, such as ring formation, dicentric formation,
translocation and tetracentric exchange

14.  Intracellular
• Nucleus – most sensitive, inhibition of cell
division
• Cytoplasm – Increased permeability to Na and
K ions, swelling and disorganization of
mitochondria, focal cytoplasmic necrosis
• Telomerase is produced – cell becomes
immortal

15.  Cell replication
• Radiation causes damage to cell systems
either by
–Mitotic delay (delay in cellular progression
through cell cycle) or
–Reproductive cell death
• DNA damage
• Bystander effect
• Recovery

16. Types of biologic effects of Radiation
BIOLOGIC
EFFECT
Somatic
Stochastic
effect
Deterministic
effect
Genetic

17. Stochastic effect
• Caused by sub-lethal DNA damage
• There is no minimal threshold level. Effect can
occur by any dose of radiation.
• Severity of effect is independent of dose; ‘ALL OR
NONE’ response – an individual either manifests
effect or does not
• Probability of occurrence of effect is dose
dependent.
• Also seen in diagnostic maxillofacial radiology
• Example: Leukemia, certain cancers

18. Deterministic effect
• Caused by lethal DNA damage causing cell death
• Effect occurs only when threshold dose is
exceeded
• Severity of clinical effects is dependent on dose
of radiation
• Probability of effect is independent of dose
• Not encountered in diagnostic radiology
• Example: Mucositis, Cataract, Osteoradionecrosis

20. Radiosensitivity
• It is defined as the ability of radiations to
biologically change cells or tissues.
• Cells that divide rapidly are more radiosensitive
than those which do not divide frequently.
• Tissues or cells that are least sensitive to
radiation are called radioresistant tissues or
cells
• Tissues or cells which are more sensitive to
radiation are known as radiosensitive tissues or
cells

21. Radiosensitivity of organs
High Intermediate Low
Lymphoid organs Fine vasculature Optic lens
Bone marrow Growing cartilage Mature erythrocytes
Testes Growing bone Muscle cells
Intestine Salivary gland Neurons
Mucous membrane Lungs
Kidney
Liver

22. RADIOSENSITIVITY OF THE CELLS
• The cells having;
– high mitotic rate,
– less differentiated and
– high cellular metabolism
are most radiosensitive.
• Since malignant tumours and cancers have
highly dividing cells, they are more sensitive to
radiation and hence are treated with
radiotherapy

23. Casarett’s categorization of cell types by
radiosensitivity
Radiosensitivity Category Cell division Differentiation
status
Examples
High Vegetative
intermitotic
Rapid Undifferentiated Basal cells of oral
mucosa,
Erythroblasts
Relatively
radiosensitive
Differentiating
intermitotic
Regular Some
differentiation
Myelocytes,
Spermatocytes
Intermediate Multipotent CT Irregular Fibroblasts,
Endothelial cells
Generally
radioresistant
Reverting
postmitotic
Not regular,
but can be
stimulated to
divide
Fully
differentiated
Hepatocytes
Low Fixed
postmitotic
No Highly
differentiated
Neuron , Muscle
cells