Radiation Safety Course: Biological Effects Radiation Protection Service

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

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?

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

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

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

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

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

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

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

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) 16-21 weeks (c) 18-21 months after the procedures showing tissue necrosis . (d) Close-up photograph of the lesion shown in (c). (e) Photograph after skin grafting

Deterministic effects

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 ?

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

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

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

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

Effective Dose

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)

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)

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