IAEAInternational Atomic Energy Agency

Biological Effects of Ionizing Radiation

Michael HajekRadiation Safety and Monitoring Section

Division of Radiation, Transport and Waste Safety

Department of Nuclear Safety and Security

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Outline

• Introduction and historical background

• Targets for biological radiation damage

• Deterministic and stochastic effects

• ICRP system of radiological protection

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Ionizing Radiation

• Ionizing radiation− Composed of particles that individually carry enough kinetic energy to liberate

an electron from an atom or molecule− Kinetic energy > 12.4 eV

Non-ionizing radiation Ionizing radiation

Ionization energy of soft tissue12.4 eV or 100 nm

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Discovery of Ionizing Radiation

X-rays (1895) Natural radioactivity (1896)Wilhelm Conrad Roentgen Antoine Henri Becquerel

Nobel Prize in Physics 1901 Nobel Prize in Physics 1903

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First Medical Observations

• Skin-burn attributed to radiation ─ 1901• Radiation-induced leukaemia ─ 1911• Clinical syndrome following exposure to atomic bomb explosions ─

1946P. D. Keller, J. Am. Med. Assoc. 131, 504 (1946).

• Holzknecht’s chromoradiometer related to skin erythema ─ 1902

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Targets for Biological Radiation Damage

• Human tissues are formed from cells that are grouped into organs and systems of the body to perform the many specialized functions

• Each cell is defined by a membrane enclosing− Cytoplasm containing up to 85% water− Structures such a nucleus

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Chromosomes and DNA

• Chromosomes are organized structures of supercoiled deoxyribonucleic acid (DNA) and proteins found in cells

• DNA macromolecules encode genetic information used in development and functioning of all known living organisms

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Structure of DNA

• Double-stranded helices, with nucleobases (G, A, T, C) attached to sugar-phosphate backbones

• Each type of nucleobase on one strand bonds with just one type of nucleobase on the other strand (complementary base pairing)

Hydrogen bond

3.4 nm

1 nm

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Direct and Indirect Radiation Effects

• Indirect action predominant with low-LET radiation (X- and gamma rays)

• Direct action predominant with high-LET radiation (alpha particles)

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Consequences of DNA Damage

DNA damage

MutationCell deathRepair

Stochastic effectDeterministic effectViable cell

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Mechanism of DNA Repair

• DNA damage occurs at a rate of ~ 100,000 per cell per day• Genetic mutations drive evolution

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Deterministic Radiation Effects

• Occur at high doses when enough cells in an organ or tissue are killed or prevented from functioning normally

− Threshold dose, above which effects are clinically observable− Severity increases with dose− Acute effects, non-malignant late effects− Examples: Cataracts, erythema, acute radiation syndromes (ARS)

DOSE

SE

VE

RIT

Y 100%

Thresholddose

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Deterministic Radiation Effects

• Data on deterministic radiation effects come from

− Survivors of atomic bombs on Hiroshima and Nagasaki

− Effects on early radiologists

− Consequences of severe accidents with industrial radiation sources

− Studies of side effects of radiotherapy

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Organ or tissue Acute dose (Gy) Type of effect Time of occurrence

Bone marrow 1 ARS 1 to 2 months

Skin 3 Erythema 1 to 3 weeks

Thyroid 5 Hypothyroidism ≥ 1 year

Lens of the eye 2 Cataract ≥ 6 months

Gonads 3 Permanent sterility Several weeks

Foetus 0.1 Teratogenesis ─

Deterministic Radiation Effects

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Deterministic Effects after Chernobyl

Chernobyl experience− ARS and radiation burns

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Stochastic Radiation Effects

• Occur at all dose levels as a result of damage to the DNA

− Random or non-threshold effects− Probability of occurrence increases with dose− Late effects, often decades after exposure− Examples: Radiation-induced cancers, hereditary effects

DOSE

RIS

K

Linear-no-thresholdhypothesis

Quadraticresponse

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Stochastic Radiation Effects

• Principal sources of information on stochastic effects are

− Epidemiological studies on atomic-bomb survivors

− Patients exposed to radiation for medical treatment or diagnosis

− Some groups of occupationally exposed workers (uranium miners, nuclear industry workers, radium-dial painters)

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Stochastic Radiation Risks

DOSE

RIS

K

Risk factor

Relationship is irrelevant

Backgroundincidence

Backgrounddose

Average 2.4 mSvTypical 10. mSvHigh 100. mSv

Increment of dose

Increment of probability

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ICRP Nominal Risk Coefficients

• ICRP detriment-adjusted nominal risk coefficients (10−2 Sv−1) for stochastic effects after exposure to radiation at low dose rate

Combined detriment due to excess cancer and hereditary effects ~ 5% per Sv

Exposed population Cancer Hereditary effects Total

Publ.103 Publ. 60 Publ.103 Publ. 60 Publ.103 Publ. 60

Whole 5.5 6.0 0.2 1.3 5.7 7.3

Adult 4.1 4.8 0.1 0.8 4.2 5.6

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ICRP System of Radiological Protection

… to contribute to an appropriate level of protection for people and the environment against the detrimental effects of radiation exposure ...

• Justification− Any decision that alters the radiation exposure situation should do more good

than harm.

• Optimization (ALARA)− The likelihood of incurring exposure, the number of people exposed, and the

magnitude of their individual doses should all be kept as low as reasonably achievable, taking into account economic and societal factors.

• Limitation− The total dose to any individual from regulated sources in planned exposure

situations other than medical exposure of patients should not exceed the appropriate limits specified by the Commission.

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Philosophy of Individual Dose Limitation

• Prevention of deterministic effects− Dose limits lower than threshold

• Reduction of stochastic effects to acceptable level− Comparison with risks for other occupations− Ethical judgment

Annual dose limits

• Occupational exposure → 20 mSv (whole-body exposure)

→ 20 mSv (lens of the eye), 500 mSv (extremities)

• General public → 1 mSv (whole-body exposure)

→ 15 mSv (lens of the eye), 50 mSv (skin)

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Further Information

• IAEA Safety Standards− No. GSR Part 3 (Interim) “Radiation Protection and Safety of Radiation Sources:

International Basic Safety Standards” (2011)

• IAEA Safety Standards Series− No. SF-1 “Fundamental Safety Principles” (2006)− No. RS-G-1.1 “Occupational Radiation Protection” (1999)

• Practical Radiation Technical Manuals− “Health Effects and Medical Surveillance” (2004)− “Personal Protective Equipment” (2004)

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Thank you for your kind attention!

Michael HAJEK | External Dosimetry Specialist | Radiation Safety and Monitoring Section | Division of Radiation, Transport and Waste Safety | Department of Nuclear Safety and Security | International Atomic Energy Agency | Vienna International Centre, PO Box 100, 1400 Vienna, Austria | Email: M.Hajek@iaea.org | T: (+43-1) 2600-22712 | F: (+43-1) 26007-22712 | Follow us on www.iaea.org

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