Radiation Safety Course: Biological Effects Radiation Protection Service.

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 radiationExposure

Ionisation

Free radicals (indirect effect)

Cellular transformations – repair? Mutations?

Cell death

Molecular changes

Cellular level

Sub cellular – chromosomes, nuclei, membranes

(Direct effect)

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

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 marrowBone marrow SkinSkin CNSCNS

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

(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

Coronary angioplasty twice in a day followed by bypass graft because of complicationDose 20 Gy

(a) (c)

(b)

(d) (e)

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)

• Describes the stochastic biological effects of ionising radiation

• Basis of legislation

Dose

Effect

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

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 1 mSv gives 1 in 20,000 chance of cancer inductioncancer induction

• Hereditary Effects: 1 in 500 per Sievert

(1 in 500,000 per mSv)

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