1. Radiation Safety Course: Biological Effects
Radiation Protection Service

2. Ionising Radiation
Ionising radiation can be a hazard because it interacts with matter and can produce changes at molecular level
Damage caused by direct or indirect ionisation
DNA is the most important cellular constituent to be damaged by radiation

3. Damage by ionising radiation
Exposure
Ionisation
(Direct effect)
Free radicals (indirect effect)
Molecular changes
Sub cellular – chromosomes, nuclei, membranes
Cellular level
Cell death
Cellular transformations – repair? Mutations?

4. Damage by ionising radiation
Direct effect:
Mean energy dissipated per ionisation event is 33 eV
More than sufficient to break strong chemical bond
Carbon-carbon bond is 4.9 eV

5. Damage by ionising radiation
Indirect effect:
Ionising event can break molecular bonds but effect may manifest elsewhere
e.g. ionisation of water molecules can produce free radicals (molecule with unpaired electron in outer shell).
Highly reactive
Capable of diffusing a few micrometres to reach and damage molecular bonds in DNA

6. DNA Single strand break can be repaired
Double strand breaks more difficult to repair
Mis-repair = mutation

7. Biological effects Evidence based on: Japanese atomic bomb survivors
Medical exposures: therapeutic & diagnostic
Radiation accidents e.g. Chernobyl, Los Alamos
Occupational exposure
Experimental work

8. Biological effects Biological effect will depend on:-
the type of radiation
the tissue or type of cell
the dose
to some extent the dose rate
Effects are classed as either deterministic or stochastic

9. Radiosensitivity of tissues
Bone marrow
Skin
CNS
Highly radiosensitive
Lymphoid tissue
Bone marrow
Gastrointestinal epithelium
Gonads
Embryonic tissues
Moderately radiosensitive
Skin
Vascular endothelium
Lung
Kidney
Liver
Lens (eye)
Least radiosensitive
Central nervous system (CNS)
Muscle
Bone and cartilage
Connective tissue

10. Deterministic effects
Associated with high radiation doses received over a short period of time
Will only occur above a certain dose (threshold)
Above threshold, severity increases with dose
Effects often take time to develop
Occurrence and severity can be predicted
e.g. skin erythema, temporary or permanent sterility, cataracts, tissue necrosis

11. Deterministic effects: tissue necrosis
(b)
(a)
Coronary angioplasty twice in a day followed by bypass graft because of complication
Dose  20 Gy
(c)
(d)
(e)
(a) 6-8 weeks after procedures
(b) weeks
(c) months after the procedures showing tissue necrosis .
(d) Close-up photograph of the lesion shown in (c).
(e) Photograph after skin grafting

12. Deterministic effects

13. Stochastic Effects Associated with low doses, no threshold
Cannot predict occurrence or severity in individuals
Probability of effect increases with dose
Induction of late-expressing health effects of radiation
Cancer
Non-cancer ??
Heritable disease ?

14. Linear no-threshold model (LNT)
Dose
Effect
Describes the stochastic biological effects of ionising radiation
Basis of legislation

15. Linear no-threshold model (LNT)
Dose
Effect
According to LNT model:
however small the radiation dose
there will be an effect
no safe dose
effect is directly proportional to dose at all dose levels
This takes no account of repair processes within the body. Some dose is inevitable from natural and man made sources

16. Quantifying doses to people
Dose from exposure to radioactive material depends on:
Whether the material is inside or outside the body
How long it remains inside the body
Physical half life
Biological half life
Quantity of radioactive material
Type of radiation emitted
Impossible to make direct measurements but estimates can be made

17. Effective Dose
Risk from exposure to ionising radiation quantified in terms of Effective Dose (Sv)
Takes account of type of radiation & radio-sensitivity of different organs
Effective dose =  wT wR DTR
wT is tissue weighting factor
wR is radiation weighting factor
DTR is absorbed dose to tissue T of radiation R

18. Effective Dose

19. Effective Dose
The doses to a number of different organs are used in the calculation of effective dose
Effective dose allows the comparison between whole body irradiation and a radiation dose which is not uniformly distributed.
Measured in Sieverts (Sv)

20. Effective dose - Risk Factors
Risk of cancer induction in general population: 1 in 20 per Sievert
1 mSv gives 1 in 20,000 chance of cancer induction
Hereditary Effects: 1 in 500 per Sievert
(1 in 500,000 per mSv)

21. Summary Deterministic effects: Stochastic effects:
Erythema, cataracts, sterility etc.
Associated with threshold dose
Avoid risk by keeping exposure below threshold
Stochastic effects:
Increased risk of cancer
LNT model: no threshold, no safe dose
Minimise risk