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Radiation Safety Course: Biological Effects Radiation Protection Service.

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Presentation on theme: "Radiation Safety Course: Biological Effects Radiation Protection Service."— Presentation transcript:

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 Free radicals (indirect effect) Cellular transformations – repair? Mutations? Cell death Molecular changes Cellular level Sub cellular – chromosomes, nuclei, membranes (Direct effect)

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 Moderately radiosensitive Skin Vascular endothelium Lung Kidney Liver Lens (eye) Radiosensitivity of tissues Highly radiosensitive Lymphoid tissue Bone marrow Gastrointestinal epithelium Gonads Embryonic tissues Bone marrow SkinCNS 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 (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 Coronary angioplasty twice in a day followed by bypass graft because of complication Dose 20 Gy (a) (c) (b) (d)(e)

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) Describes the stochastic biological effects of ionising radiation Basis of legislation Dose Effect

15 Linear no-threshold model (LNT) 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 Dose Effect

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 = w T w R D TR –w T is tissue weighting factor –w R is radiation weighting factor –D TR is absorbed dose to tissue T of radiation R

18 Effective Dose

19 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 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: –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

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